The periodic table
Chemistry is the branch of science that studies the composition, structure, properties, and changes
of matter. It explores the interactions between different substances at the molecular and atomic
levels.
Here are some ways in which chemistry is used in various aspects of our daily lives and in different
industries:
The atom and atomic structure
An atom is the basic unit of matter. It is made up of three types of subatomic particles:
Protons,
Neutrons,
Electrons.
Protons are positively charged particles that are found in the nucleus of an atom. They have a mass
of approximately 1 atomic mass unit (amu). The number of protons determines the atomic number.
Neutrons are neutral particles that are also found in the nucleus of an atom. They have a mass of
approximately 1 amu.
Electrons are negatively charged particles that orbit the nucleus of an atom. They have a much
smaller mass than protons and neutrons, approximately 1/1836 amu. The number of electrons are
equal to the number of protons in an atom.
How to calculate the number of neutrons
To calculate the number of neutrons in an atom, you can use the following formula:
Number of neutrons =
atomic mass - atomic number
For example, if an atom has an atomic mass of 23 and an atomic number of 11, the number of
neutrons would be:
23 - 11 = 12
So this atom would have 12 neutrons.
Atomic mass unit
AMU is a unit of measurement for the mass of atoms and subatomic particles.
One atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom, which is
approximately 1.66 x 10^-27 kilograms.
The atomic mass of an element is typically measured in atomic mass units. This is the standard
international (SI) unit used to express the mass of atoms, ions and molecules.
Mass vs weight
Mass is a measure of the amount of matter an object contains. Measured in kilos or grams.
It is a scalar quantity, meaning it has only magnitude and no direction.
Mass is an intrinsic property of an object and does not change due to its location.
It will remain the same whether it is on Earth, the Moon, or anywhere else in the universe.
Weight, on the other hand, is the force exerted on an object due to gravity and is measured in
newtons or pounds.
It is a vector quantity, meaning it has both magnitude and direction.
The weight of an object depends on its mass and the strength of the gravitational field it is in and
therefore, can vary on different planets.
Isotopes
Isotopes are atoms of the same element that have the same number of protons but a different
number of neutrons.
This means that isotopes have the same atomic number, but a different atomic mass.
For example, carbon-12 and carbon-14 are isotopes of carbon, they have 6 protons but 6 and 8
neutrons respectively.
Isotopes can be stable or radioactive and have different properties and uses depending on their
stability and atomic mass.
This is why the mass number is not always a whole number, it is the average mass of all isotopes.
The periodic table
Groups - are arranged vertically and have similar chemical and physical properties.
The elements in a group have the same number of valence electrons.
Valence electrons are the electrons in the outermost shell that participate in chemical reactions.
This is why elements in the same group have similar reactivity and form similar compounds.
Group: number of valence electrons
Period: number of electron shells
Periods - are arranged horizontally and have elements with gradually increasing atomic number.
The period relates to the number of electron shells the atom has.
As you move across a period from left to right, the atomic number of the elements increases, and so
does the number of protons, neutrons, and electrons in the atom.
Groups and properties
One simple way to divide the periodic table is into metals, non-metals and metalloids.
The physical properties of metals, non-metals and metalloids can give us information about the
elements
Group 1 - Alkali Metals
Group 1 of the periodic table are known as the alkali
metals and have the following properties:
Highly reactive
Soft
Lustrous
High density
High thermal conductivity (also ductile)
Silvery-white in colour
Example: phone screens/touch screens are ductile to
pick up electrical conductivity
Group 2 - Alkaline earth metals
Group 2 includes the alkali earth metals which
have very similar properties to group 1.
Highly reactive (but less than group 1).
Harder than alkali metals.
Lower density than group 1.
High thermal conductivity.
Metalloids
Lustre: have a shiny appearance
Semi- Conductive: can conduct heat and electricity but not as good as metals
Solid at room temperature
Break easily so they cannot be hammered or pressed
Example: The semi-conductive nature allows electricity in the processor without overloading it
Metals donate or lose electrons in ionic bonding
Non metals gain electrons in ionic bonding
Non-Metals
Non-metals are a diverse group of elements that can be found across the periodic table. They have
a wide range of properties but some common characteristics include:
Less reactive than metals
Poor conductors of heat and electricity
Low density and melting/boiling points.
High electron affinity
Brittle and non-lustrous
Varying physical states (solids, liquids and gases).
Halogens
Halogens are a group of elements found in Group 7 (17) of the periodic table.
They are highly reactive
High ionization energy
High electron affinity
Exist in different physical states
Low melting and boiling points.
Will react with metals
Noble gases
The noble gases consist of elements in group 8 of the periodic table.
They are stable and do not react due to having a complete valence shell
They are colourless/odourless
Exist as gases
Low boiling points (easy to solidify and liquify)
Non-toxic
Non metals
Group VII - Halogens
Group VIII - Noble Gases
Electron configuration
Electron configuration relates to the groups and rows of the periodic table.
1) The group number relates to the number of valence electrons in the final shell.
2) The period number (or shell number) relates to the number of electron shells around the nucleus.
The last shell and final electron(s) in the last shell are referred to as the valence shell and valence
electron(s).
This relates to several key aspects of the periodic table such as their properties (reactivity) and
location on the periodic table.
Each shell can only contain a certain number of electrons and once it is ‘full’, the remaining
electrons will ‘fill up’ another shell.
This relates to the energy state of the atom.
Electron configuration has a specific notation that is directly linked to the electron number of the
atom.
Sodium (Na) has an atomic number of 11 - which means it has 11 protons and 11 electrons.
Therefore, the electron configuration is written as: 2, 8, 1
This means it has filled the first shell with 2 electrons, a second shell with 8 (which gives us a total
of 10 electrons so far) and a third shell with 1 electron. 2 + 8 + 1 = 11.
This means it has three shells or periods and is in group 1, with the following configuration: 2, 8, 1.
Reactivity & Bonding
The three types of bonding
Ionic Bonding - the bonding of a metal and a non-metal to form an ionic compound.
Metallic Bonding - the bonding between two metals which share electrons to create a lattice
structure.
Covalent bonding - the sharing of electrons between non-metal atoms to form molecules.
The properties of atoms determine how they bond and how reactive they are (meaning how likely
they are to react).
Atoms want to “complete” their valence shell to reach a lower energy state.
Ions, Ionic Bonding and Naming Ionic Compounds
Ions
Ions are atoms that have lost or gained electrons which results in the
atom either being positively or negatively charged. This means it has an
unequal number of protons and electrons.
This changes the overall charge of the atom to either positive or negative (depending on if it has
lost or gained an electron).
An atom NEVER gains/loses a proton, only its electron(s).
The loss or gain of electrons to form an ion relates to which group the element is from in the
periodic table, specifically, the group number will determine how many electrons are lost or gained.
Types of anions
Cations
Anions
Cations are positively charged ions formed by
Anions are negatively charged ions formed by
the loss of one or more electrons from an atom.
the gain of one or more electrons by an atom.
They are formed when an atom loses electrons
They are formed when an atom gains electrons
and becomes positively charged. They are
and becomes negatively charged. They are
metals.
non-metals.
An example of a cation is the sodium ion
An example of an anion is the chloride ion
(Na1+), which is formed by the loss of an
(Cl1-), which is formed by the gain of an
electron from a neutral sodium atom (Na).
electron by a neutral chlorine atom (Cl).
Ions, groups and writing Ions
As discussed previously, the ionic form of an atom is linked to its specific group in the periodic table.
Atoms lose or gain electrons to complete their valence shell and become stable.
Atoms in group one have 1 electron in the outer shell - this means they will become a cation because
they want to lose their valence electron. It is much easier to lose one electron than gain seven to
complete an electron shell.
This means elements such as Lithium and Sodium will be written ionically as Li1+ and Na1+
The “1+” symbolises the amount of charge that the ion has. For these two ions, they have lost 1
electron, which means that they now have 1 more proton than electrons.
As a result, they have an overall charge of 1+ as there are more positively charged particles than
negatively charged.
More examples of Ions:
Mg is in group two, which means it will lose 2 electrons and be written as Mg2+
Anions follow the same process but have an “-” to represent a negative charge.
Chlorine is in Group 7, which means it needs to gain 1 electron in order to have a full outer shell.
Thus, its ionic form would be Cl1-.
Ionic bonding
Ionic bonding is the chemical bond that forms between a metal and a non-metal. Ionic bonding is
where one atom loses one or more electrons and the other atom gains one or more electrons.
This results in the formation of ions, atoms or molecules with a net (neutral) electric charge.
An example of ionic bonding is the formation of table salt (NaCl). In this case, the sodium atom (Na)
loses an electron to the chlorine atom (Cl) forming a positively charged sodium ion (Na+) and a
negatively charged chloride ion (Cl-).
The opposite charges attract each other and form an ionic bond. This results in a compound with a
neutral charge → the positively charged ion and negatively charged ion cancel each other out.
Writing ionic compounds
When ionic bonding occurs, the cations and anions are electrostatically attracted to each other
forming a chemical bond - this is known as an ionic bond.
The cations and anions can be from different groups, which means that the ratio of charges between
each atom can vary.
When this happens, a subscript is written next to the atom to represent how many atoms are
required to make the compound neutral.
Example: Mg2+ + Cl1- → MgCl2
Mg has 2 positive charges and chlorine has 1,
this means there needs to be two chlorine
atoms for every one magnesium to make MgCl2
Criss-Cross method
Naming ionic compounds
There are a few key things to note when naming ionic compounds:
The metallic (cation) component will retain its usual name.
The non-metal (anion) component will keep its ‘root’ name, but will will have the suffix ‘-ide’ added
to it.
If there is more than one of a particular atom present within the compound, the following prefixes
are used:
1 = mono (or no prefix)
2 = di
3 = tri
4 = tetra, etc.
Examples:
Na2CO3 = Disodium Carbonate
BCL3 = Boron Trichloride
The important ions that you will encounter that involve oxygen are those below:
CO3 = Carbonate
SO4 = Sulfate
PO4 = Phosphate
NO2 = Nitrite
NO3 = Nitrate
Polyatomic Ions
A polyatomic ion is a group of atoms that are chemically bonded together and have a net electric
charge. These ions are formed by the combination of cations and anions. They are also known as
molecular ions.
Examples of common polyatomic ions include:
Nitrate (NO3-), which is made up of one nitrogen atom and three oxygen atoms.
Carbonate (CO32-), which is made up of one carbon atom and three oxygen atoms.
Sulfate (SO42-), which is made up of one sulfur atom and four oxygen atoms.
Ammonium (NH4+), which is made up of one nitrogen atom and four hydrogen atoms.
Metallic bonding
Metallic bonding is a type of chemical bond that happens between atoms in a metal.
Think of the atoms in a metal like people in a big, crowded room. Just like how people in a crowded
room might bump into each other, the atoms in a metal also bump into each other.
When atoms bump into each other, they can share some of their electrons with each other. This is
what happens in metallic bonding.
The electrons that are shared move freely around the metal, kind of like how people in a crowded
room might move around and bump into different people. This is known as the sea of electrons.
Summary:
Metallic bonding is a type of chemical bond that happens between atoms in a metal.
Atoms bump into each other within the metal and share their electrons with each other.
The electrons that are shared move freely around the metal.
This is known as the ‘sea of electrons’.
This is why metals are good conductors of heat and electricity, because the electrons are free to
move around.
These freely moving electrons continue to bump into each other and the metal atoms, transmitting
energy as they do - which is one way that heat and electricity can travel through a substance.
Metallic Bonding - Malleability
Metals are malleable because the layers of ions in their lattice structure can slide over one another
without breaking the interactions with the delocalised electrons.
The ions are not stuck together, but rather are held in close proximity by their attraction to the sea
of electrons around them.
This same reasoning explains why metals are ductile (can be drawn into thin wires).
Covalent bonding
Covalent bonding is when non-metal atoms share electrons with each other in order to achieve a full
valence shell of electrons and become more stable.
This means atoms can share one or more electrons to become stable.
This type of bonding leads to the formation of covalent compounds, which:
Form both simple and complex molecules.
Are usually gases or liquids.
Have low melting and boiling points.
Are poor conductors of heat and electricity.
Physical Reactions
In a physical reaction no new products are formed. The reactants have changed state. These
reactions can be reversed.
E.g. ice melts to form water.
H2O(s) → H2O(l)
Heat is applied and ice melts, when the heat is removed, ice will reform.
Chemical reactions
In a chemical reaction new products are formed.
Bonds have been broken and reformed. These reactions are difficult to reverse.
E.g. When a piece of sodium is put into water, it fizzes about and a gas is formed.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Word equations
Word equations describe the reactants used and products formed.
Often the state of the compound is described:
solid (s), liquid (l), gas (g) or if dissolved in water (aq) is used as the subscript.
Reactant A + Reactant B
—-->
Product C + Product D
Important definitions
Reactant
Product
The elements which are being combined to form
A new substance produced by combining 2 or
a new substance.
more elements.
Law of Conservation of Mass
The law of conservation of mass states that mass cannot be created nor destroyed during a
chemical reaction, it can only be transferred or transformed. Therefore, the number of atoms on
both sides of the equation must be equal.
So, we place numbers in front of the compounds to balance the number of atoms on the left and
right side of the equation.
For example:
Word equation:
calcium + oxygen —->
Formula equation:
Ca +
O2
calcium oxide
—->
CaO
Worded chemical formulas
We can also use our knowledge about ionic compounds and acids to convert
worded formulas to chemical formulas (and then balance them!).
Example 2:
sodium carbonate + hydrochloric acid → sodium chloride + carbon dioxide + water
Important Acid Formulas:
Hydrochloric Acid = HCl
Sulfuric Acid = H2SO4
Nitric Acid = HNO3
Types of reactions
Combination (Synthesis) Reactions
Combination reactions occur when 2 reactants form 1 product.
X
+
Y
XY
When the ‘pop’ test is used to test for hydrogen, water is formed when the hydrogen reacts with
oxygen.
hydrogen
2H2(g)
+
oxygen
+
water
O2(g)
2H2O(g)
Decomposition Reactions
When a single reactant breaks apart to form several products, the reactant is said to decompose.This may
occur due to a change in pressure (for gases) or a temperature change, for example.
XY= X + Y
Example: When you open a soft drink container, you hear it fizz. The gas is under pressure which means it is
dissolved in the liquid (carbonic acid) and is released when the container is opened
Carbonic acid= carbon dioxide + water
H2CO3(l) =
CO2(g)
+
H2O(l)
Single Replacement Reaction
These reactions involve removing one element or group of elements and replacing them with another
element or group of elements. For example, silver crystals are formed during the single replacement
reaction below:
Copper + Silver Nitrate → Silver + Copper Nitrate
Double replacement reaction
These reactions involve two compounds reacting and swapping ions to form two new compounds. In
the reaction between potassium iodide and lead nitrate, you can see that they have undergone a
‘swapping of ions’, which means it is a double replacement reaction.
Potassium Iodide + Lead Nitrate → Potassium Nitrate + Lead Iodide
Precipitation Reactions
2+
3-
+
When lead nitrate (Pb(NO3)2) solution is added to the same sodium iodide (NA1) solutio , NO , Na and
-
I ions. Lead ions can now react with iodide ions as can sodium and nitrate ions.
The new ionic substances that could be formed are lead iodide (PbI2) and sodium nitrate (NaNO3):
Lead nitrate + sodium iodide → lead iodide + sodium nitrate
Pb(NO3 + 2NaI (aq) → PbI2(5) + 2NaNO2(aq)
Checking the solubility table, lead iodide is insoluble whereas sodium nitrate is soluble.
The precipitate (lead iodide) will be visible.
Predicting the precipitate
To predict what the precipitate will be from a chemical reaction, you can follow the steps bellow
1.
2.
Swap the cations and anions of the reactants to get the two products.
Check the solubility table to see whether either of the products are insoluble - so if, this
product will be the precipitate.
Examples
Potassium sulphate + calcium nitrate → Potassium Nitrate + Calcium Sulphate - Insoluble
precipitate
Copper(I) nitrate + sodium hydroxide → Copper(I) Hydroxide + Sodium Nitrate - Insoluble
precipitate
Ammonium sulphide + Zinc(II) Chloride → Ammonium Chloride + Zinc(II) Sulphide - Insoluble
precipitate
Sodium bromide + ammonium hydroxide → Sodium hydroxide + ammonium bromide - Both
soluble precipitate
Particle collision theory
Particles are constantly moving, some of them with more energy than others.
For a chemical reaction to take place between two reactants their particles must collide first.
For the collision to be effective (meaning. For a reaction to occur), the particles must have the
right amount of energy.
The minimum amount of energy required for an effective collision is called the activation
energy.
What happens when particles collide?
1.
A collision but with no effect
●
reactant particles collide
●
product particles not formed as there is not enough energy.
2.
Collision and a reaction occurs
●
Reactant particles collide
●
But this time there is enough activation energy for products to be formed
Note:
-
The amount of energy required is different for each reaction
- However, every reaction needs some activation energy: they all need a little push to get
started.
Rate of reaction
Practicals that test the impact of four different factors on the rate of reaction
●
Surface area
●
Temperature - the higher the temperature the faster the reaction time
●
Concentration -
●
Agitation - more of the agitation, the more the reaction
Rate Of Chemical Reactions
The rate of chemical reaction is the speed at which the reaction occurs
For example explosions and combustion reactions occur quickly whereas rusting or
ripening of fruit are slow reactions.
Exothermic Reactions
When these reactions occur, the amount of energy stored in
the reactants is greater than that stored in the products.
As a result, energy is released and we call the reaction
exothermic
→ The energy released takes the form of light or heat.
Some examples of exothermic reactions are:
●
Burning candles
●
Freezing of water into ice
●
Rusting of iron
Energy is a the end of the reaction
E.g.
Burning methane in the lab
CH4 + 2O2 —->
CO2 + 2H2O + heat
E.g Respiration
C6H12O6 +
6O2
—--->
6CO2 + 6H2O + energy
This is why you feel warmer after a meal!
Endothermic Reactions
When the products have more energy than the reactants,
energy in the form of heat/light is absorbed, leaving the
surrounding area colder.
Some examples of endothermic reactions are:
-Melting ice cubes
-Photosynthesis
-Chemical cold packs
Energy is at the start of the reaction
Example: Photosynthesis
6CO2 + 6H2O + sunlight —----> C6H12O6 +
6O2
Important reactions with oxygen
Combustion
Combustion describes any chemical reactions
in which a reactant burns in oxygen (from the
air) to produce heat (and light).
This is an exothermic reaction.
General Formula:
Fuel + Oxygen
→ Product(s) + Heat
Two types of reactions
1.
Combustion
Incomplete combustion
Complete combustion
Incomplete combustion occurs when
Fuels such as natural gas and petrol
the supply of air or oxygen is poor,
contain hydrocarbons. These are
which stops the fuel from burning
compounds of hydrogen and carbon only.
Water is still produced, but carbon
●The carbon oxidises to carbon dioxide.
monoxide and carbon are produced
●The hydrogen oxidises to water
instead of carbon dioxide.
(remember that water, H2O, is an oxide of
completely.
When they burn completely:
Incomplete Combustion Reaction:
hydrogen).
hydrocarbon + oxygen
→ carbon monoxide + carbon
Complete Combustion Reaction:
+ water
Hydrocarbon + oxygen → carbon dioxide +
water.
2.
●
Corrosion
Corrosion is the oxidation of metals. It is a slower reaction than
combustion.
●
Once oxidised, metals lose their lustre and become tarnished or dull.
●
Certain conditions speed up the reaction. Metallic surfaces at the beach
oxidise faster than those inland.
○
Can you think what might be the cause? - salt
The general equation for corrosion is:
metal
+
oxygen
→
aluminium + oxygen
→
metal oxide
For example:
4Al
+
3O2
→
aluminium oxide
2Al2O3
When iron corrodes, we call this rusting:
iron
+
oxygen
→
iron(III) oxide
4Fe
+
3O2
→
2Fe2O3
Term 2
Cells, DNA And Chromosomes
Cell theory
1. All living things are composed of one or more cells
2. The cell is the basic structural and function unit of living organisms
3. All cells arise from preexisting cells
DNA
Deoxyribonucleic acid (DNA) is the molecule that stores
and transmits genetic information that determines the
characteristics of all living things. These characteristics
are heritable.
It has a double helix structure and is located in the
nucleus of eukaryotic cells.
James watson and francis crick described its structure in
1953, due to the work of rosalind franklin.
Nucleotide
DNA is made up of nucleotides. A nucleotide consists of
three subunits:
-
A deoxyribose sugar
-
A phosphate group
-
One of four nitrogenous bases (amino acids):
– Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
The phosphate attaches to the sugar of the next
nucleotide, creating a ‘backbone’ of alliteration
phosphates and sugars, forming a strand of DNA
Complementary base pairing
Each of the four bases in a nucleotide have their own
chemical structure that are complementary to another
base.
●
A pairs with T
●
G pairs with C
This is known as complementary base pairing.
This is critical as it allows DNA to replicate itself
accurately (more on this later)
Genes
A gene is a section of DNA along a chromosome. Each
gene will code for a specific protein.
Genes are the basic functional unit of heredity
(transmission of genetic traits from one generation to the
next).
Different genes vary in length from each other (the
number of bases).
Chromosomes
A chromosome is a thread-like structure composed of
DNA and proteins found in the nucleus of a cell.
Chromosomes carry genetic information in the form of
genes, which are segments of DNA that code for specific
traits or functions.
Every cell of an organism has the same number of
chromosomes.
Generally speaking, members of the same species also
have the same number of chromosomes.
Chromosomes - Key terms
Key terms
definition
Homologous
These are a pair of chromosomes that carry the same genes in the same
Autosome
Chromosomes NOT involved in determining the sex of the individual. Carry
chromosomes
order, although they may have different versions (alleles) of those genes.
genetic information such as eye colour, height and blood type. Humans have
44 autosomes (22 pairs).
Centromere
This is the region of the chromosome where the two sister chromatids are
joined together and where the spindle fibres attach during cell division.
Chromosomes number
In humans this is 46, but it is often expressed as 23 pairs
because each chromosome has a partner.
Pairs of chromosomes are the same:
●
Size
●
Shape
●
●
Branding pattern
Provide information for the same characteristics
The number of chromosomes generally determines the
species.
Chromosomes
●
Chromosomes involved in determining the sex of an organism are called non-sex
chromosomes (autosomes).
●
Humans have 44 (22 pairs) of non-sex chromosomes (22 pairs of homologous
chromosomes).
●
Sex chromosomes
○
Two X chromosomes (females)
○
One X chromosome and one Y chromosome (males).
Because x (female chromosomes) is longer compared to y (male chromosomes) meaning female
would have 23 pairs and male 22 pairs
Mitosis and meiosis
DNA Replication
DNA replication is the mechanism of copying (doubling) the DNA. it occurs in the nucleus of the cell.
DNA replication is semi-conservative because each double strand of DNA consists of
●
One old template strand
●
One new complementary strand
DNA replication occurs before cells divide.
This is an important step before mitosis to ensure each new cell (called a daughter cell) has a
complete copy of the genetic material from the parent cell.
This results in the daughter cells containing the same number of chromosomes and the same
amount of DNA as the original cell.
DNA replication can be summarised by the following
●
The enzyme helicase breaks the hydrogen bonds between the complementary bases
joining the two strands
●
‘Each strand serves as a template for making a new strand which is complementary to
the template strand. (remember, semi conductive)
●
DNA nucleotide base-pair to the exposed bases A-T or G-C
●
Enzymes (known as DNA polymerases) link the nucleotides to the newly forming strand
●
Each new double stranded DNA molecule rewinds into a double helix
Mitosis
Mitosis is a type of cell division that occurs in somatic cells (non sex cells). During mitosis, a single
cell divides into two identical daughter cells.
This process is essential for
-
Growth
-
Repair
-
Maintenance
Of tissues and organs in multicellular organisms.
Mitosis
I - Interphase
P - Prophase
M - Metaphase
A - Anaphase
T - Telophase
Interphase
This is the pre-mitosis phase. It is where the parent
cell's DNA is replicated so that two copies exist.
Prophase
This is where the nuclear membrane of the parent breaks
down and the (already copied) chromosomes condense
and become visible under a microscope.
At this stage, each chromosome consists of a pair of
identical chromatids joined by a structure called a
centromere.
Long protein filaments called spindle fibre are formed to
form a structure known as the spindle.
Metaphase
This stage is where the pair of chromatids are gradually
moved to the equator of the cell by the spindle fibres.
F
Anaphase
This is where chromatic pairs are separated to form two
identical sets of daughter chromosomes.
Each daughter chromosome set is then moved to the
poles of the spindle by the spindle fibres.
Telophase
This is where the spindle fibres breaks down and the
chromosomes de-condense and nuclear membrane forms.
Cytokinesis
This stage occurs near the end of telophase. In animal
cells protein fibre in the cell membrane constricts to form
an infolding called a cleavage furrow.
More consriction of the cleavage furrow leads to the
production of two genetically identical daughter cells.
Meiosis
Meiosis is the process of cell division that produces the gametes - sex cells (sperm and eggs)
Meiosis creates daughter cells with exactly half as many chromosomes as the starting cell.
Meiosis is the division process of going from a diploid cell to a haploid cell.
-
In humans the haploid cells are the sperm and eggs. When a sperm fuses with an egg
during fertilisation, the two haploid sets of chromosomes form a diploid set.
- Diploid = cell with two sets of chromosomes (2n)
- Haploid = a cell with a single set of chromosomes (n)
Haploid and diploid
Two haploids gametes (sex cells) fuse to restore the diploid chromosome number in the zygote
(fertilised cell).
Sperm contains genetic material from the father → 22 autosomal and 1 sex chromosome.
Egg contains genetic material from the mother → 22 autosomal chromosomes and 1 sex
chromosome.
Summary of meiosis
Firstly, the chromosomes replicate in preparation for cell division just as they do mitosis.
However, the chromosomes form up into their homologous pairs.
-
Once these pairs are formed, a process called crossing over occurs, where there will be
a random exchange of genes within the pairs of chromosomes.
Once the crossing over process has been completed, the cell begins to divide into two daughter
cells.
During this process, one chromosome from each homologous pair is randomly distributed to each
of the two daughter cells in a process called independent assortment.
Finally, the two new daughter cells undergo further cell division, which produces a total of four
haploid daughter cells.
NOTE:You don't have to be able to draw the diagrams of meiosis. But, you need to be able to
recognise the difference between diagrams of mitosis and meiosis.
Mitosis produces two daughter cells, which
are identical copies of the parent cell.
Meiosis produces four daughter cells, each
containing half of their original parent cell.
Genetic variation
Due to meiosis, sexual reproduction results in nearly infinite possibilities of genetic variation,
which is vital to the survival of species. In other words sexual reproduction results in offspring that
are genetically unique. This occurs mainly due to crossing over and independent assortment.
-
Crossing-over is the exchange of genetic material between homologous chromosomes.
It results in new combinations on each chromosome.
-
This point where homologous chromosomes cross over and exchange genetic material
is random.
-
This results in many different new combinations of genes within each chromosome.
When cells divide during meiosis, homologous
chromosomes are randomly distributed to
daughter cells
This is called independent assortment. It
results in gametes that have unique
combinations of chromosomes (i.e. some from
the father of the original organism, some from
the mother).
Why is the chromosome number always halved in sex cells?
Sex cells, also called gametes, have half the number of chromosomes as somatic cells (body cells)
because of the process of meiosis.
This is necessary for sexual reproduction because when a haploid sperm cell fertilises a haploid
egg cell, the resulting zygote will have the full complement of chromosomes necessary for
development into a diploid organism.
Without meiosis, the chromosome number would double with each generation, leading to a situation
known as polyploidy, which can cause developmental abnormalities and hinder the survival of
offspring.
Chromosomes - Diploid & Haploid
Characteristic
Definition
Represented as
Number of
chromosomes
Found in
Diploid Chromosomes
Two sets of chromosomes (one from each
parent)
Haploid Chromosomes
One set of chromosomes
2n
n
Humans have 46 chromosomes (23 pairs)
Humans have 23 chromosomes
Most somatic (body) cells
Gametes (sex cells)
Formation
Role
Results from the fusion of two haploid
gametes during fertilisation
division that halves the chromosome
number
Determines traits, controls cell functions,
Essential for sexual reproduction, and
Skin cells, muscle cells, blood cells
Sperm cells, egg cells
and enables growth
Examples
Produced by meiosis, a type of cell
the creation of genetic diversity
Mendelian genetics
Gregor Mendel was the ‘father’ of modern day genetics. He pioneered many principles that still
apply today.
1. The inherited traits are determined by genes that are passed from parents to children
2. A child inherits two sets of genes - one from each parent
3. A trait may not be observable, but its gene can be passed to the next generation.
Alleles
Every organism inherits two copies of every gene. A set of genes from the father (paternal) and a
set of genes from the mother (maternal)
The variations in a gene - different traits, are known as alleles.
Alleles are represented by letters
A capital letter represents a trait (gene) that is dominant.
A lowercase letter represents a trait that is recessive. This means it is less likely to be expressed
E.g.
A = brown eyes, a = blue eyes Therefore, it is Aa
Important definitions
Phenotype
Genotype
(observed). These are generally dominant.
inherited from both parents for that gene. it
The phenotype is the allele that is expressed
For example your eye colour is your
The genotype is the genetic information
results in a specific phenotype.
phenotype.
Homozygous
Heterozygous
Homozygous genotypes refers to an individual
Heterozygous genotypes are when the
parents.
from each parent.
And blue eyes = b
And blue eyes = b
that has inherited the same alleles from both
If brown eyes = B
Homozygous genotypes (same) = BB or bb
individual has inherited a different allele
If brown eyes = B
Heterozygous genotypes (different) = Bb
Punnett squares
Punnett squares are a useful way to predict the likely genotypes and phenotypes of an individual.
It can be used to predict the ratio or percentage change of inheriting a trait.
Sex-linked inheritance
Sex linked inheritance refers to traits that arise from genes, which are located on the sex
chromosomes (X or Y).
Remember Males = X, Females =Y
Sex linked traits are more prevalent in males because males inherit all X linked genes from their
mother.
If a male inherits an X linked trait (recessive or dominant) it will be expressed because he has no
other X chromosomes to prevent the gene from being expressed.
An example of this is colour blindness in men, which is much more prevalent as the gene is on the X
chromosome.
Revision
DNA Structure and Nucleotides
Nucleotides are organised in a double helix structure made up of alternating phosphate and sugar
groups
Each of the 4 bases have a different chemical structure that compliment each other, like a jigsaw
puzzle.
The relationship between genes, chromosomes and DNA
Replication
DNA Replication = DNA copying itself
Mitosis = Cell division
Meiosis = Sex cell division
DNA Replication
DNA replication is the mechanism for copying (doubling) the DNA. It occurs in the nucleus of the
cell.
DNA replication is semi- conservative because each double strand of DNA consists of:
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One old template strand
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One new complementary strand
DNA replication occurs before cell division. This is an important step before mitosis to ensure each
new cell (daughter cell) has a complete copy of the genetic material from the parent cell.
This results in the daughter cells containing the same number of chromosomes and the same
amount of DNA as the original cell.
Meiosis
Is a process of cell division that produces the gametes - sex cells (sperm and eggs)
Meiosis creates daughter cells with exactly half as many chromosomes as the starting cell.
Meiosis is the division process of going from a diploid cell to a haploid cell.
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In humans, the haploid cells are sperm and eggs. When a sperm fuses with an egg
during fertilisation, the two haploid sets of chromosomes form a diploid set
- diploid =cell with two sets of chromosomes (2n)
- haploid = a cell with a single set of chromosomes (n)
Exam revision
How to calculate the number of neutrons
Atomic mass - atomic number