DNA structure
and function
DNA – is a type of nucleic acid
DNA = deoxyribose
nucleic acid. The role of
DNA is to pass on
genetic information from
one generation to the
next. It is the chemical
that genes are made out
of.
DNA- Function
• DNA must be found in every
cell (there are exceptions)
• DNA must carry information
• DNA must be stable
• There are coding and noncoding regions found on DNA
• Coding regions code for genes
(proteins)
• Non-coding regions can be
either DNA junk or help
regulate protein synthesis
DNA-structure
• A DNA molecule is
like a twisted ladder
(the double helix).
The 2 strands make up
a DNA molecule
which are the sides of
the ladder. These are
made up of a
phosphate group
alternating with a
deoxyribose sugar.
DNA continued…
• Connecting the two strands, like the rungs
of the ladder, are pairs of bases. There are 4
types of bases in DNA:
A= adenine
T = thymine
C= cytosine
G= guanine
A only bonds with T and C only bonds with G.
These pairs are called complementary bases.
DNA
DNA Base pairing rule - A and T pair together
- C and G pair together
Nucleotides
• DNA is made up of simple repeating units
called nucleotides which consist of:
1) a phosphate group
2) a deoxyribose sugar
3) one of 4 bases (A,G, C, T)
Nucleotides
phosphate
Sugar (deoxyribose)
Base
(A, T, C or G)
Chromosomes are found in
the nucleus, they are made
of DNA. DNA never leaves
the nucleus.
Humans have 23 pairs of
chromosomes (a total of
46) in every cell in our
bodies (except our eggs or
sperm – they have a half
set of only 23
chromosomes.
Chromosomes are long stretches of DNA,
within which lie genes. A single chromosome
may contain hundreds of genes.
Below are the locations of some known genes on
human chromosomes:
Karyotypes
A karyotype is a picture of the chromosomes in an
organism arranged in homologous pairs according
to shape and size.
The Karyotype shows:
Autosome chromosomeschromosomes that carry nonsex genes
Sex chromosomesChromosomes that carry sex
linked genes
Different living things have
different numbers of chromosomes
Genes
Genes are the basic units of heredity in living
cells.
They consist of a length of DNA that contains
instructions ("codes") for making a specific
protein.
Through these proteins, our genes influence
almost everything about us, including how tall we
will be, how we process foods, and how we
respond to infections and medicines.
Genes and alleles
• A gene is a section of DNA that carries the
information for a particular protein
(characteristic) eg. Eye colour, hair colour,
production of a particular enzyme. Now a
sequence of bases on a DNA molecule.
• Alleles are different forms of the same gene
ie. The gene is eye colour, the alleles are
blue, brown and green.
Although most of our cells have the
same genes, not all genes are active
in every cell. Heart cells synthesize
proteins required for that organ's
structure and function; liver cells
make liver proteins, and so on.
In other words, not all the genes
are "switched on" and expressed as
proteins within every cell. Within an
individual cell, the same genes may
be switched on at some times and
switched off at other times.
Different genes are activated in
different parts of the body creating the specific
proteins that give a particular cell type its
character.
.
http://www.accessexcellence.org/AE/AEPC/NIH/gene03.html
Tieing it all together





1. Wrap the 2m of string as tighly and as small as
possible
2. Untangle the string and cut into 8 pieces (4 sets of
pairs)
3. Label and name 2 genes on each chromosome(1
chromosome per set)
4. Write down the base sequence for each gene.
5. Tweak the base sequence for each gene and place
this on the other 4 chromosomes in the set. Name
these alleles.
Problems with genes
We know a lot about the position of genes on
chromosomes by looking at the chromosomes of
people with genetic diseases.
Scientists can work out what the gene sequence
should be like from healthy people and can see
what has gone wrong in someone with a genetic
disease. New discoveries are being made often.
The following picture shows human chromosomes
5, 6, 7 and 8 and the positions of genes that we
know about so far.
Review Questions:
•
•
•
•
1. Define a gene
2. Define an allele
3. When may a cell have to divide
4. What has to be done prior to a cell
starting cell division (there are two things).
Answers
• 1. A gene in a section of
DNA/chromosome that code for a
particular protein.
• 2. An allele is a different form of the same
gene.
• 3. When an organism is growing/repairing
or producing sex cells
• 4. DNA must be replicated and the cell
must grow in size.
DNA replication
• 1. When would cells need to copy DNA?
• 2. What properties of DNA ensures it is
copied accurately?
• 3. Why does DNA need to be copied
accurately?
DNA Replication
DNA replication
Sequence DNA Replication
Put the following sentences into the correct order:
• Enzymes add free nucleotides to complement bases on
both strands of DNA
• DNA is unwound with enzymes
• DNA is unzipped between complementary bases with
enzymes
• Two new strands of DNA are made
• This process as known as semi-conservative because
each section of DNA is made of one older strand and
one newer strand.
• 6. DNA replication occurs prior to cell division (mitosis
and meiosis)
Sequence DNA Replication
• Put the following sentences into the correct order:
• 3. Enzymes add free nucleotides to complement bases
on both strands of DNA
• 1. DNA is unwound with enzymes
• 2. DNA is unzipped between complementary bases with
enzymes
• 4.Two new strands of DNA are made
• 5. This process is known as semi-conservative because
each section of DNA is made of one older strand and
one newer strand.
• 6.DNA replication occurs prior to cell division (mitosis
and meiosis)
What is a chromosome?
 Genetic materials found inside the nucleus
of a cell
 Made of protein (histone) and DNA
 Only seen in a cell during cell division
 Seen as a mess of spaghetti called
chromatin most of the time.
 Each species has a fixed number of chromosomes
in the nucleus of each of its cells
 Chromosomes always exist in pairs in the body
(somatic) cells
 Each human somatic cell has 46 chromosomes

23 pairs of homologous chromosomes
 Each gametic cell will contain half the number of
chromosomes (only one homologous
chromosome)
 Human gametic cells will have 23 chromosomes
• Members of
homologous
chromosomes carry
same genes
• But the genes on the
members of
homologous
chromosomes may be
of different forms
 Alleles
 Each chromosome will
come from one parent.
allele for
green eyes
allele for
blue eyes
Chromosome forms
 Chromosomes can exist in two forms:
 Replicated and unreplicated
A replicated
chromosome has two
arms. They are
genetically identical and
called chromatids
The centromere is a
specialised region of the
chromosome. It keeps
the two chromatids
together.
 22 pairs are identical in both sexes

autosomes
 The 23rd pair is different in male and female

sex chromosomes
Cell division
Mitosis
Meiosis
Purpose To create cells
To create gametes
identical to the original (eggs or sperm) that
cell and to keep the
contain HALF the
chromosome number
original chromosome
the same.
number
Growth and repair
Where
it
occurs
In all body cells for
growth and repair
In the ovaries in
females to make
eggs
In the testes of
males to make sperm
Cell division Mitosis
Mitosis occurs in every
cell in our body – without
mitosis we wouldn’t grow
or heal cuts and injuries.
Mitosis is a cell division
where the chromosome
number is kept the same
and two identical cells
are produced from one.
Cell division
- Meiosis
Meiosis occurs in the
ovaries of females to
make eggs and the
testes of males to
make sperm.
Two divisions occur,
making four cells that
have half the number
of chromosomes that
the original cell had.
Key terms- copy onto describe
map
• Somatic cells- non- sex cells, replicate via mitosis.
• Gametic cells- sex cells, created via meiosis.
• Spindles- proteins that join to chromatids during cell
division.
• Centrioles- organelles that connects to spindles and
separate the chromatids during cell division.
• Haploid- half a set of chromosomes in a cell.
Represented as n.
• Diploid- a full set of chromosomes in a cell. Represented
as 2n.
• Daughter cell- cells that are created as a result of cell
division.
Sequence mitosis and meiosis
Mitosis key terms:
• DNA replication
• Daughter cells
• Genetically identical
• Diploid
• Spindles
• Centrioles
• Chromatids
• Replicated
chromosomes
Meiosis key terms:
•DNA replication
•Daughter cells
•Genetically different
•Haploid
•Spindles
•Centrioles
•Chromatids
•Replicated chromosomes
•Homologous pairs
•Independent assortment
•Crossing over
Sex Determination
 22 pairs are identical in both sexes

autosomes
 The 23rd pair is different in male and female

sex chromosomes
Fertilisation- the process where a
sperm and egg fuse together to produce a
single cell called a zygote
Egg
23 chromosomes
Zygote
Haploid (n)
Sperm
23
46 chromosomes
Diploid (2n)
Sex determination
What are the chances of having a boy or a girl???
Everyone has a pair of chromosomes that determine our sex
XX = female XY = male
X X
X XX XX
Y XY XY
The punnett square on the left
shows us the probabilities that a
man and woman are faced with
each time they have a child.
XX = 50%
XY = 50%
If you have had two boys it does
not mean you will have a girl next,
each time a new zygote is formed
the chances or it being male or
female are 50 : 50.
Sex determination question
• Describe the combination of chromosomes
that determine a male and female
• Complete a punnet square to show the
chances of producing either a boy or girl
• Marriam has three boys with Doug. They
are expecting another baby to arrive soon.
What is the probability of having a girl?
monozygotic (identical) twins
• Twins can occur this
way when the zygote
splits early on.
• Because the twins
come from the same
original cell they will
be genetically
identical
dizygotic ("fraternal") twins
• These twins occur
when the mother
releases more than
one egg during a
menstrual period.
• The eggs will be
fertilised by different
sperm .
• This means they will
be genetically
different
Yr 11 Quiz
•
•
•
•
•
1. Define the term allele
2. Define the term gene
3. Define the term phenotype
4. Define the term genotype
5. State the genotype of a male and
female human
• 6. Complete a punnet square with
phenotype ratio to show the chances of
offspring being male or female
• 7. A cat has black fur (B)and has a
heterozygous genotype. Write down the
genotype
• 8. Complete a punnet square to show if
two heterozygous black cats bred
together. Include a phenotype ratio.
• 9. Define the term haploid
• 10. What two mechanisms in meiosis
creates variation?
• 1. A variation of the same gene.
• 2. A section of DNA that codes for a particular
protein.
• 3. The physical appearance of a trait
• 4. the genetic code for a trait made up of two
alleles.
• 5. Male = XY, Female =XX
• 6. 1:1 OR 50%:50%
X
X
X
XX
XX
Y
XY
XY
• 7. Bb
• 8. 3:1 OR 75%: 25%
B
b
B
BB
Bb
b
Bb
bb
• 9. Half a set of chromosomes
• 10. Independent assortment and crossing
over
Two new key terms:
• Pure breeding- an individual with a
homozygous genotype (either dominant or
recessive).
• Carrier- an individual with a heterozygous
genotype. Can pass down recessive
alleles even though the individual does not
express it.
How do we know is an allele is
dominant
• Think of the black cat and white cat example:
• Breed a black cat and a white cat together. Then
breed two of their offspring together with the
same trait.
• If any of their offspring aren’t the same colour as
the parents (F1), you know that the colour is
dominant.
• This relies on producing many offspring to
minimise chance.
F1 generation (Black cat x White
cat)
A
a
a
Aa
aa
a
Aa
aa
F2 generation (Black cat x Black
cat)
A
a
A
AA
Aa
a
Aa
aa
F1 generation (Black cat x White
cat)
A
a
a
Aa
aa
a
Aa
aa
F2 generation (White cat x White
cat)
a
a
a
aa
aa
a
aa
aa
Interpreting Pedigree trees
What information can you
gather?
• Which allele is dominant/recessive
• What genotype each individual is (or
most).
• What potential offspring may occur
between two parents.
Alleles
• One of two or more forms of a gene at a given position on a
chromosome. They are caused by a difference in the sequence of DNA.
• A gene which controls eye colour in humans may have two alternative
forms – an allele that can produce blue eyes (b), and an allele that
produces brown eyes (B). In a plant that occurs in tall and short forms,
there may be an allele that tends to produce tall plants (T) and an
alternative allele that produces short plants (t).
• The individual genes that form a pair of alleles are located at exactly the
same point along a chromosome. There will always be two genes for a
characteristic in a cell.
Genotype
TT = homozygous dominant
Tt = heterozygous
tt = homozygous recessive
Phenotype
TT = tall plant
Tt = tall plant
tt = short plant
Alleles
T
t
a
a
b
B
C
U
r
y
Z
C
u
r
Y
Z
ZZ =
homozygous
dominant
Uu =
heterozygous
aa =
homozygous
recessive
Monohybrid cross
The study of single-gene inheritance is done
through
monohybrid crosses.
- Capital letters represent dominant alleles
- Lower case letters represent recessive alleles
e.g. coat colour in guinea pigs
Genotype
Phenotype
BB
Black
Homozygous = 2 of the
same alleles eg BB or bb
Bb
Black
Heterozygous = Bb
bb
white
A cross between 2 heterozygous black guinea
pigs ( Bb x Bb ) expressed as a ratio
Possible
fertilisations
B
b
B
b
Place parents alleles at the top
and side of the punnet square
Possible
fertilisations
B
b
B BB Bb
b Bb bb
3 black : 1 white
Contemporary Applications of Genetics
• In the exam the
questions will be
resource based.
• The context will be
selected from
selective breeding,
cloning or genetic
modification.
Selective breeding
Selective breeding is when we
choose animals with good traits for
breeding.
Eg. Bulls are chosen to mate with
many cows to pass on his good
genes.
Selective breeding
• Also called artificial selection
The steps for selective breeding are:
• Select the stock or plants that have the best
characteristics.
• Breed them with each other.
• Select the best of the offspring and combine
them with the best that you already have.
• Continue this process over many generations
until you have the desired traits
Selective breeding - Dogs
Cloning
• Cloning is the creation of an organism that is an exact
genetic copy of another. This means that every single bit
of DNA is the same between the two!
• You might not believe it, but there are human clones
among us right now. They weren't made in a lab, though:
they're identical twins, created naturally. Below, we'll see
how natural identical twins relate to modern cloning
technologies.
• How is cloning done?
• You may have first heard of cloning when Dolly the
Sheep showed up on the scene in 1997. Cloning
technologies have been around for much longer than
Dolly, though. The first animal, a tadpole was cloned in
1952. Mice, pigs, cats, and rabbits have also been
cloned.
Celebrity Sheep Has Died at Age 6
Dolly, the first mammal to be cloned from adult
DNA, was put down by lethal injection Feb. 14,
2003. Prior to her death, Dolly had been suffering
from lung cancer and crippling arthritis. Although
most Finn Dorset sheep live to be 11 to 12 years
of age, postmortem examination of Dolly seemed
to indicate that, other than her cancer and
arthritis, she appeared to be quite normal. The
unnamed sheep from which Dolly was cloned
had died several years prior to her creation. Dolly
was a mother to six lambs, bred the old-fashioned
way.
Image credit: Roslin Institute Image Library, http://www.roslin.ac.uk/imagelibrary/
• Scientists are looking at therapeutic cloning that
can be used to generate tissues and organs for
transplants.
• To do this DNA would be extracted from the
person needing a transplant and inserted into an
egg. Once the egg (with the persons DNA) starts
to divide, the stem cells that can be transformed
into any type of tissue would be harvested.
These stem cells would be used to generate an
organ or tissue that is a genetic match to the
recipient. In theory the cloned organ could then
be transplanted without risk of tissue rejection.
Injection of nucleus into egg cell
during the cloning process
Blunt
end
pipette
Empty cell
Nucleus
put in
by
sharp
ended
pipette
Cloning – any
Advantages?
Genetic modification
• This involves moving sections of DNA
(genes) from one organism to another so
that it produces useful biological products.
• Bacteria is currently used to produce
human insulin for diabetes sufferers.
• It also produces human growth hormone
for children who aren’t growing properly.
Stages of genetic engineering
• The useful gene is “cut out” by enzymes.
• Particular enzymes will cut out particular parts
of DNA
• The DNA of a bacterium is then cut (by
enzymes) and the human gene is inserted.
• This splicing of a new gene is again controlled
by enzymes.
• The bacteria is then cultivated and soon there
are millions of bacteria producing human
insulin.
How it occurs
Genetic modification
Comparison
Selective Breeding
• Some people think that it is wrong to
manipulate nature.
• Is it right to produce cows that would die if we
didn’t milk them, because we have bred them to
produce too much milk.
• Is it right to breed pigs with so much meat on
them that they have trouble standing up.
• Is it wrong to breed tomatoes or potatoes that
are heavy cropping.
Cloning:
• Groups of scientists want to clone human
embryos to get replacement tissues and organs
for people who need them.
• Using organs from embryos cloned from
themselves, would save the lives of people who
would otherwise die because they have
rejected transplanted organs.
• A lot of people argue that to create a life for
spare parts and then kill it is wrong.
• This is the stage that a lot of countries allow
abortion at.
Genetic engineering:
 A big problem is the possibility of making designer
babies – people may want there child to be perfect
and not carry diseases, need to wear glasses, have
the perfect nose etc.
 Changing the genetic make-up of any organisms may
effect ecosystems in ways laboratory tests can’t
predict.
 Large seed companies can make money every year by
selling plants that won’t produce fertile seeds, or by
producing plants that are resistant to their weed
killers.
 G.E. can also be used to produce crops that grow in
places that they wouldn’t normally and this could save
lives
The future
• One day in the not-too-distant future, your health care
provider may talk to you about obtaining a single blood
sample for DNA analysis, the results of which will be
recorded in a computer chip on a wallet-sized plastic card.
This card will contain specific aspects of your genetic
makeup that can be identified as needed. The genetic
information contained there may be used in several ways:
• To predict your risk of developing certain diseases,
allowing their earlier diagnosis or possible prevention.
• To more accurately diagnose the cause of symptoms or
diseases you may experience.
• To help your health care provider more accurately select
the medicine most likely to be of benefit and least likely to
cause you harm.
• To help scientists more efficiently discover and develop
safer, more effective medicines aimed at the root causes of
diseases, not just their symptoms.