Unit 1Cell and Molecular

Biology

Section 2

Cell cycle, cell growth and differentiation

The cell cycle

The cell cycle, or mitosis, is the process by whichnew cells are produced for growth. All cellsresulting from this process are identical to eachother 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 andsynthesis of organelles

Cell growth andsynthesis of organelles

DNA replication

The stages G1, S and G2 are collectivelyknown as interphase.

The M phase or cell division phase is known asthe 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 actinfibres which split the cytoplasm intwo - stage 6

Stages of MitosisStage 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 andDNA 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 between the two cells

Activity – Read Dart Pg 9-14

- 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 S-phase 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 dominantOnly 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 recessiveBoth alleles of the gene are required to be

mutated for mutation to take affectMutations 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

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

3. 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/anisamples/nonmajorsbiology/stemcells.html

•http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/lacoperon.html

•http://science.howstuffworks.com/stem-cell.htm