AS Biology

Module 1Section 1.7Continuity of CellsAS BiologySpecification

The cell cycle

The need to divideAll cells have to divide as a means to pass on their genetic material before they die

Bacterial cells can divide every 30 minutes if conditions are right!

Human cells can divide every 2 hours (the liver), months or years (nerve cells) or less than 1 hour (stomach cells)

The cycle that leads to cell division has FOUR main stagesG1-phaseThe first gap phaseHere the cell is just growing and living as normal

During this stage protein synthesis occurs and the cell grows to almost double its size

More organelles are produced, cytoplasmic volume is increased

You need more cytoplasm and organelles if you are to make an identical new cell afterall! S-phaseThe synthesis phase

Here the cell replicates its DNA (by semi-conservative replication)

One set of the DNA will remain in the parent cell and the other set will end up in the newly synthesized daughter cell

G2-phaseThe second gap phase

Here the cell resumes growing and getting ready to divide

These previous three stages (G1, S and G2) are all collectively called INTERPHASE

Of a total 24 hour cell cycle, interphase lasts for approximately 20 hours!

This means that it only takes 4 hours for M-phase to occur.M-phaseMitosis

Has itself FOUR steps;

Prophase (Pre)Metaphase (Molars)Anaphase (Are)Telophase (Teeth)

Results in formation of TWO daughter cells that are genetically identical to each otherCytokinesisOnce cells have divided they are both stuck together

To separate the cells they undergo a process called CYTOKINESIS

CytokinesisIn animals the cells are divided by a sealing of the plasma membrane

In plants vesicles form along the middle of the cell and seal to split it into two cells

Mitosis as a means of growthMitosis is required for growth of an organismMore cells means you have got bigger!

Some organisms that reproduce ASEXUALLY carry out mitosis only as a means to procreate

Mitosis is vitally important in maintaining genetic stabilityDNA must be accurately copied and present in all cells

The extra stageCells that do not divide very often, or at all, enter a stage called the G0 phase which can be thought of as permanent G1 phase

This means that the cell is essentially out of the cell cycle!DNA and chromosome structure

HistonesThe DNA double helix is massive in length!

To fit it in the cell it is tightly wrapped around proteins called HISTONES

These allow the DNA to be compressed and folded up

23 chromosomesCells contain 23 PAIRS of chromosomes22 autosomal pairs and 1 sex pair

This means we have 46 chromosomes total in all of our cellsHaving all 46 means that a cell is DIPLOID

KARYOTYPEMicrograph showing chromosome arrangements

The stages of mitosis

The stages in a bit more detailProphaseThe chromatin begins to condense into chromosomesThe nuclear membrane disintegrates

MetaphaseThe chromosomes arrange along the middle of the cell

AnaphaseThe chromosome pairs are pulled to either end of the cell

TelophaseNuclear membranes reform around the separating chromosomesCytokinesis begins

Haploid cellsSex cells (eggs and sperm) are HAPLOIDThat is, they only have 23 chromosomes

So when a sperm fertilizes an egg and they both combine, the zygote has 46 and is a viable diploid cell

How haploid cells are formed is through a process called MEIOSIS

This happens only in sperm and egg cells

A)

B)

C)

D)

E)

MeiosisMeiosis is a process that results in formation of FOUR daughter cells from a single cell

It is a major source of genetic variation due to two steps throughout it that lead to chromosome rearrangements

This is how eggs and sperm cells are produced and explains why they are HAPLOIDThe initial round of division

The second round of division

The sources of genetic variationProphase 1When the chromosomes have replicated and there are two pairs, the pieces of DNA can swap regions of DNA

This process is called CHIASMA formation or CROSSING-OVER

The chromosomes swap regions of DNA at random, leading to variable DNA sequences

Independent chromosome assortmentWhen the chromosomes align along the centre spindle plate during metaphase, the chromosome that is pulled into each cell is random

This means that the chromosome will have a 50% chance of being in either cell

During prophase II this means that the chromosomes will cross-over and generate more diversityThe sources of genetic variation