Chapter 9: Genes, chromosomes
and patterns of inheritance 1
EL: To introduce genetics, with a
focus on chromosomes
Before we begin..
• Answer questions 1-4 on page 291
Genetic instructions
• Offspring receive genetic instructions from
their parents (or parent cells in the case of
unicellular organisms)
• In humans, these instructions are packaged in
gametes: the egg cells of a female and the
sperm cells of a male
• We will be examining how these instructions
come together to determine the
characteristics of the offspring
Prokaryotic Chromosomes
• In prokaryotes, a single circular chromosome is attached to the
plasma membrane at a specific point.
• When the cell divides by BINARY FISSION
• DNA molecule replicates
• The two copies are separated by the expansion of the plasma membrane
• Plasma membrane and cell wall furrow inwards to divide the cytoplasm
resulting in two daughter cells.
Binary = two, Fission = splitting
DNA
• Deoxyribonucleic acid (DNA)
is found within the nucleus of
eukaryotic cells
• Chromatin is a mass of
uncoiled DNA and associated
proteins called histones.
• When cell division begins,
DNA coils around the proteins
forming visible structures
called chromosomes.
Chromosome structure
Haploid cells
A cell with one set of chromosomes is called haploid (n) –
gamete cells are usually haploid
Diploid cells
A cell with two sets of chromosomes is called diploid (2n) – somatic cells
are diploid
Each matching pair is called “homologous” – they each contain the same
genes – however, the DNA sequence isn’t necessarily the same
Polyploid cells
A cell with more than two sets of chromosomes is called
polyploid. This is usually only found in plant cells.
Chromosome Numbers
• An organism of a particular species always has the same number
of chromosomes (e.g. humans have 46 chromosomes or 23 pairs)
• See table 9.1 page 292
Human chromosomes
• Diploid number = 46, Haploid number = 23
• The 22 matched, homologous pairs of autosomal
chromosomes are distinguished by:
– Relative size
– Position of centromeres
– Patterns of light and dark bands when stained
• The 23rd pair in a diploid somatic cell are the sex
chromosomes (N.B. In males these are NOT
homologous)
Human Karyotype
Karyotype: the display of the number, size and shape of
chromosomes from a cell
HUMAN FEMALE
HUMAN MALE
Autosomal chromosomes
Sex chromosomes
Activity
• In two groups, Complete Part A of activity 9.1
“Karyotypes” – one group will do figure 9.1C
and the other group will do Figure 9.1D.
Answer questions 1-5 i
• Re-visit Chapter 9 quick check questions 1-4
on page 291 (how did you go?) and complete
qu 5-8 on page 301, question 5 on page 336
Reflection
These are the questions you should answer each
lesson, preferably in writing
• What learning was new today?
• What learning was revision or built on what I
already know?
• What did I find most challenging and what
strategies will I put in place to help me?
• What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 2
EL: To introduce or revise mitotic cell
division in eukaryotic somatic cells
What do you remember?
• Before we begin the lesson, write down or
draw what you remember about eukaryotic
cell division
Cell Division in Eukaryotes
• A division of the
nucleus (mitosis)
followed by a
division of the
cytoplasm
(cytokinesis)
• To accomplish this
task, the cell passes
through a series of
discrete stages or
phases
http://www.youtube.com/watch?v=VlN7K1-9QB0
• Cells spend the majority of their
time (about 95%) in interphase.
• Cultured mammalian cells usually
divide once every 18-24 hours.
• The cell appears “to be at rest”.
Nothing could be further from the
truth!
• During interphase most cellular
contents are synthesised
increasing cell mass. It is a time of
cell growth, DNA replication and
metabolic activity.
• The genetic material in the
nucleus is in the form of
chromatin fibres. Discrete
chromosomes are not visible.
Interphase
• Interphase starts with G1 or
Growth 1; its the time for
the cell to grow and carry
out its biochemical
activities. The length of this
phase is highly variable
between cells, typically 8-10
hours.
• Some cells sit in G1 for
weeks, months, years. Cells
that are arrested in G1 are
said to be in a G0 state.
Most nerve cells never leave
G0.
• The decision to commit to
cell division is made when
the cell passes through the
first checkpoint at the end
of G1.
The G1 Phase
• Once committed to cell
division the cell enters the S
Phase – S stands for
synthesis.
• This is the time for DNA
replication. This typically
takes 6-8 hours.
• The S phase ends when the
DNA content of the cell has
doubled. The evidence for
this becomes obvious when
the chromosomes become
visible at the start of the M
Phase. Each chromosome is
now made up of two sister
chromatids.
The S Phase
• Once the genetic material
has doubled the cell now
enters G2 – Growth 2. This
phase is more fixed in its
timing usually 4-6 hours for
most cells.
• During this phase the cell
actively prepares for cell
division. It is a period of high
metabolic activity and
protein synthesis.
• The cell passes through
checkpoint at the end of G2
to ensure that all is ready
for the division of the
The G2 Phase
• The M phase encompasses a
division of the nucleus
(mitosis) and then a division
of the cytoplasm
(cytokinesis).
• This phase explains how the
two copies of the
chromosomal DNA formed
in S phase are separated and
partitioned into daughter
cells.
• The M phase lasts for less
than 1 hour. The M phase is
divided into various phases
that are characterised by
particular chromosome
behaviour.
The M Phase
The M phase summary
Mitosis can be divided into five
stages:
1.Interphase - cell performs all
its normal functions. Before
mitosis begins, DNA on
replicates
2.Prophase – Nuclear
membrane disappears
3. Metaphase - Spindle is
visible and helps chromatids
line up on equator
4. Anaphase - Chromatids get
pulled to opposite poles.
5. Telophase - Two nuclei
reform around the
chromatids. The cell then
divides (cytokinesis) into
two daughter cells.
Activity
• Use your jelly snakes and jelly beans to model
mitosis with a partner using the information
on the upcoming slides
•
Prophase beings when the individual
chromosomes have condensed to become
discrete objects under the light microscope.
•
In the cytoplasm, adjacent to the nucleus,
the centrosomes, (duplicated in S phase)
move to opposite ends of the cell. Spindle
microtubules will form between these two
centrosomes.
•
Towards the end of prophase, the nuclear
envelop breaks down
•
The centrosomes are now at opposite ends of
the cell and growing spindle microtubules
enter the nuclear area and make contact with
the chromosomes.
•
Contact between a chromosome and spindle
microtubules occurs at a protein – DNA
complex region known as the kinetochore.
2. Prophase
The relationship between the centromere,
kinetochore and spindle microtubules.
3. Metaphase
• Chromosomes are now
maximally condensed and
lined up along the metaphase
plate.
• Chromosomes can now be
used in karyotype analysis.
• Metaphase occupies half the
time required for mitosis.
• The chromosomes appear
stationary, but each chromatid
is being tugged towards the
opposite poles by equally
strong forces.
• In animal cells the centrosome
contains a pair of centrioles.
4. Anaphase
• The centromere holding the two
chromatids abruptly separates.
• Each chromatid (now a single
chromosome) begins moving to opposite
spindle poles as the microtubules get
shorter and shorter.
• Anaphase is the shortest phase in mitosis
typically lasting only a few minutes.
5. Telophase
• Daughter chromosomes arrive
at the poles and revert to
extended fibres of chromatin.
• The spindle microtubules
disassembles and the nuclear
membrane forms around the
two groups of daughter
chromosomes.
• During this period the cell
usually undergoes cytokinesis
– an independent process –
that results in the division of
the cytoplasm.
Cytokinesis
Plant Cell
• Due to rigid cell wall, cytokinesis cannot constrict the plasma membrane inwards.
A new cell wall and plasma membrane is assembled across the cell plate.
Animal Cell
• Inward constriction of the plasma membrane results in cleavage furrow
during cytokinesis.
• The result of mitosis and cytokinesis are two new
daughter cells produced from one parent cell.
• The daughter cells contain the same (or virtually the
same) genetic information and the same number of
chromosomes as the parent cell.
Stage 5
Stage 4
Stage 1
Stage 2
Stage 3
http://www.youtube.com/watch?v=VlN7K1-9QB0
Checkpoints regulate the cell cycle
• The cell cycle is highly regulated by intracellular signalling
molecules and extracellular signalling proteins
Defective Cell Cycle Control Mechanisms
• When control mechanisms fail, uncontrolled cell
proliferation can produce a mass of cells called
a tumour. Tumours can be benign or malignant
(cancer).
• Mutations in the genes that express regulatory
proteins accumulate. This leads to genetic
instability and the development of cancer.
Animations and web links
• http://www.biology.arizona.edu/CELL_BIO/tutorials
/cell_cycle/MitosisFlash.html
• http://www.johnkyrk.com/mitosis.html
Apoptosis
• Apoptosis is programmed cell death or “cellular suicide”. It is a key
event in many biological processes. Removal of the tadpoles tail.
• The process is a specific sequence of events that result in the
ordered dismantling of the internal contents of a cell.
• A key event is the activation of a series of enzymes called caspases.
• The pathway can be triggered by
– (1) death signals or
– (2) the withdrawal of survival factors.
• Mutations in genes that express proteins involved in apoptosis can
lead to various cancers.
• NoBiology2 p.34-5
http://wehi.edu.au/education/wehitv/apoptosis_and_signal_transduction/
Activity
• Create a cell cycle poster with all the stages or
mitosis mapped out
Reflection
These are the questions you should answer each
lesson, preferably in writing
• What learning was new today?
• What learning was revision or built on what I
already know?
• What did I find most challenging and what
strategies will I put in place to help me?
• What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 3
EL: To introduce or revise meiotic cell
division in eukaryotic gamete cells
Introduction to meiosis
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter
12/animations.html#
• The formation of gametes
(i.e.sex cells) - sperm and eggsoccurs by a special type of cell
division called meiosis.
• The nuclei of sex cells contain
only half as many chromosomes
as the nuclei of all other cells (i.e.
haploid) – called reduction
division.
• When the nuclei of the sperm
and egg join during fertilisation,
the new cell then contains the
full complement of
chromosomes.
Meiosis
Meiosis
• There are two divisions in meiosis; the first division is meiosis 1
and the second is meiosis 2.
• The phases have the same names as those of mitosis. A
number indicates the division number (1st or 2nd):
– meiosis 1: prophase 1, metaphase 1, anaphase 1, and
telophase 1
– meiosis 2: prophase 2, metaphase 2, anaphase 2, and
telophase 2
• In the first meiotic division, the number of cells is doubled but
the number of chromosomes is not. This results in 1/2 as many
chromosomes per cell.
• The second meiotic division is like mitosis; the number of
chromosomes does not get reduced.
http://www.cellsalive.com/meiosis.htm
Meiosis I
Meiosis 2
Activity
• Once again, use your jelly snakes and jelly
beans to model mitosis with a partner using
the information on the upcoming slides
Interphase
Interphase: Before meiosis begins, genetic
material is duplicated. There are two
homologous pairs of each chromosome (i.e.
cell is diploid).
• Duplicated chromatin
condenses. Each
chromosome consists of
two, closely associated
sister chromatids.
• Synapsis and crossingover occur during the
latter part of this stage:
two chromosomes of a
homologous pair may
exchange segments
producing genetic
variation.
Meiosis 1:
Prophase 1
Meiosis 1: Metaphase and Anaphase 1
• Metaphase 1:
Homologous
chromosomes align at
the equatorial plate.
• Anaphase 1:
Homologous pairs
separate with sister
chromatids remaining
together.
Meiosis 1: Telophase 1
• Telophase 1: Two daughter cells are formed
with each daughter containing only one
chromosome of the homologous pair
• After Meiosis 1, there is usually a brief
interphase
Meiosis 2
• Prophase 2: Spindle forms, DNA does not
replicate.
• Metaphase 2: Chromosomes align at the
equatorial plate.
Meiosis 2:
• Anaphase 2:
Centromeres divide
and sister
chromatids migrate
separately to each
pole.
• Telophase 2: Cell
division is complete.
Four haploid
daughter cells are
obtained.
Animations
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/ch
apter12/animations.html#
• http://www.cellsalive.com/meiosis.htm
Mitosis vs Meiosis
Activity
• Complete qu 1-11 of activity 9.2 on pages 9091 of your activity manual (yes, you get to play
with play doh!)
• Quick check questions 9-11 pg 306
• Make a poster of the stages of meiosis
mapped out
Reflection
These are the questions you should answer each
lesson, preferably in writing
• What learning was new today?
• What learning was revision or built on what I
already know?
• What did I find most challenging and what
strategies will I put in place to help me?
• What percentage of the class did I spend on
task and how can I improve this if needed?
Test revision
SAMPLE EXAM QUESTIONS
ANSWER = B
At the end of meiosis I females have two daughter cells and
meiosis II only occurs if and when fertilization occurs by a sperm
cell.
At that time both daughter cells divide to form 4 cells and of the 4
cells formed, 3 are discarded as polar bodies and the 4th cell
having an enhanced cytoplasmic component combines its nuclear
component with the sperm cell's nuclear component and crossing
over occurs to form the embryo which then begins to divide via
mitosis to become two cells, then four and so on.
An egg cell that is not fertilized is ovulated as a pair of daughter
cells and there is no formation of polar bodies, hence, the eggs
that are ultimately discarded at menstruation are not "finished"
eggs. They have not undergone meiosis II.
ANSWER = C
ANSWER = C
ANSWER = A