Download Cell growth, cell cycle and differentiation

Unit 1
Cell and Molecular
Biology
Section 2
Cell cycle, cell growth and
differentiation
The cell cycle
The cell cycle, or mitosis, is the process by which
new cells are produced for growth. All cells
resulting from this process are identical to each
other and the parent cell
The cycle has 4 main stages as follows : G1 – Cell growth and synthesis of organelles
 S – DNA replication
 G2 - Cell growth and synthesis of organelles
 M – Cell division
Four Phases of Cell Division
Cell growth and
synthesis of
organelles
Cell growth and
synthesis of
organelles
DNA replication
The stages G1, S and G2 are collectively
known as interphase.
The M phase or cell division phase is known as
the mitotic phase.
Mitosis has a number of stages as shown
below:
Cell Division and the Cell Cycle
Nuclear division is controlled
by microtubules which form the
spindle fibres and move
chromosomes - stages 1-5
Cytokinesis is controlled by actin
fibres which split the cytoplasm in
two - stage 6
Stages of Mitosis
Stage
Description
Prophase
No distinct chromosome. Nuclear envelope intact
Prometaphase
Chromosome become visible. Nucleus breaks down
Metaphase
Chromosomes line up across the centre of cell (
equator)
Anaphase
Chromosomes divide into chromatids which are pulled
to opposite poles by spindle fibres. These are made of
microtubules and radiate from the centrosome.
Telophase
Daughter chromosomes ( chromatids) reach opposite
poles and begin to de-condense
Cytokinesis
Cell divides into two by contraction of actin fibres
Interphase
Follows the M phase and involves cell growth and
DNA replication. Made up of G1, S and G2
Mitosis - Prophase
The replicated chromosomes each consisting of two closely
associated sister chromatids condense
Outside the nucleus the mitotic spindle assembles between the two
centrosomes which have replicated and moved apart.
Mitosis - Prometaphase
The nuclear envelope suddenly breaks down
Chromosomes attach to the spindle microtubules via structures
known as kinetochores
Chromosomes start to actively move
Mitosis – Metaphase
The chromosomes are moved to the equator by the spindle fibres
The kinetochores of all chromosomes align on the equator, midway
between the poles at a structure known as the metaphase plate
The paired microtubules attached to each chromosome attach to
opposite poles of the spindle
Mitosis - Anaphase
The paired chromatids from each chromosome separate to
form two sister chromatids.
Daughter chromosomes are pulled to opposite poles by
the simultaneous shortening and lengthening of microtubules
Mitosis - Telophase
The two sets of daughter chromosomes arrive at the poles
A new nuclear envelope reassembles around each set forming to
separate daughter nuclei and marking the end of Mitosis
Cytokinesis
In animal cells the cytoplasm is divided into two by a contractile
ring of actin and myosin which pinches in the cell to create two
daughter cells.
Cytokinesis cont…

In plants



Membrane vesicles
spread across the
equator of the cell
They merge to form
plasma membrane
The new
membranes lay
down the cell wall
Activity – Read Dart Pg 9-14
between the two
cells
- Look at web animation
(www.biozone.co.uk/links.html)
Cell Cycle

There are 3 checkpoints in the cell cycle
CK
Control of cell cycle
G1 Checkpoint
 End of the G1 phase – the cell size is assessed.
If large enough the cell enters S-phase.
 The cell is usually pushed past this point by
signals (growth factors) from outside the cell.

If conditions are met DNA replication enzymes
called polymerase are transcribed to allow Sphase to begin

If conditions are not met
 Cells don’t divide and remin in G0
 Many mature cells e.g. nerve cells, skeletal muscle
cells, RBC’s don’t divide.
G2 Checkpoint
 DNA replication success is monitored
 If replication is successful
 DNA polymerase enzymes are deactivated
 Metaphase enzymes are activated
 If replication is unsuccessful
 Any cell with unreplicated or damaged DNA that cant
be repaired is destroyed (apoptosis = cell suicide)
Control of cell cycle - MPF

Mitosis (maturation) Promoting Factor (MPF)
 Promotes transition of G2 to M phase
 Acts as a catalyst for the conversion of
metaphase enzymes from an inactive to an
active state (by phosphorylation)
M Checkpoint
 Occurs during metaphase
 Checks the spindle has assembled
properly
 All chromosomes are attached properly
(by the kinetochores

If conditions are met
 Metaphase enzymes are deactivated
 Anaphase enzymes are activated
Abnormal cell division: Cancer
Cancer cells by-pass normal cell control
mechanisms. As a result they divide
uncontrollably to form lumps of tissue
(tumours) that no longer carry out their
function.
Mutation to Proliferation
Genes

Normal proliferation genes are called Proto-oncogenes
 During normal cell division proto-oncogenes code for
proteins (e.g. growth factors) that promote cell division

Mutated Proliferation genes are called oncogenes
 Oncogenes act to produce cells that are not required.
E.g.
 Produce a protein which triggers a response in the
cell as if growth factors are present
 Over production of growth factors
Oncogenes are dominant


Only 1 gene in the pair of alleles needs to
mutate for it to have an effect.
Mutations in several different genes are
usually required for cancer to develop.
Mutation to Anti-proliferation
genes



(AKA Tumour Suppressor Genes)
Normal Anti-proliferation Genes
 Switch off cell division when something goes wrong
 If the cell is damaged beyond repair apoptosis occurs
Mutations to Anti-proliferation Genes
 Cause the cell to continue dividing when faulty
 E.g. p53 is a protein produced by a anti-proliferation
gene. It binds to damaged DNA stopping cell
division until it is repaired. A mutation to this gene
results in a faulty protein and cell division with faulty
DNA

Mutations to anti-proliferation genes are
recessive


Both alleles of the gene are required to be
mutated for mutation to take affect
Mutations in several different genes are
usually required for cancer to develop
Activity – Read Dart Pg 14-17
Development


An organism starts life as a zygote (single
fertilised cell).
It undergoes three main stages to develop into
an individual
1. Mitotic division to form a group of cells called
the blastula.
2. Gastrulation
 Infolding of the cells to form a cup shape
called a gastrula

3.
The gastrula has three germ layers
 Endoderm
 Develops into the alimentary canal
 Ectoderm
 Develops into skin and nervous system
 Mesoderm
 Develops into the muscles, skeleton,
circulatory system, excretory system
Cell division and differentiation
(specialisation) results in tissue and organ
formation.
Differentiation


Nearly all cells in an organism have the same
DNA
Differentiation depends on gene expression (the
transcription of a gene into mRNA)
i.e. which genes are ‘switched on’ and which
genes are ‘switched off’.
During development the control of gene
expression may be:
 Temporal (different genes expressed at
different times in development)
 Spatial (cells in different places in the
embryo expressing different genes)
Example of differentiation to form an
organism:
Drosophila melanogaster
Control of gene expression in
bacteria

Lac operon (aka Jacob-Monod hypothesis)
Stem Cells



A stem cell is an undifferentiated cell which can
undergo unlimited division to form other cells
Source of stem cells
 Adult e.g. bone marrow
 Embryonic (from blastula stage ~ 150 cell
stage)
 Cancer cells
 Umbilical Cord Blood
Stem cells have the ability to differentiate, unlike
specialised cells
Activity –
•Read Dart Pg 18-20
•Web animations from Biozone website
•http://www.sumanasinc.com/webcontent/anisam
ples/nonmajorsbiology/stemcells.html
•http://www.sumanasinc.com/webcontent/anisam
ples/majorsbiology/lacoperon.html
•http://science.howstuffworks.com/stem-cell.htm