Module 1

Section 1.7

Continuity of Cells

AS Biology

Specification

The cell cycle

The need to divide

All 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 stages

G1-phase

The first gap phase

Here 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-phase

The 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-phase

The 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-phase

Mitosis

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 other

Cytokinesis

Once cells have divided they are both stuck

together

To separate the cells they undergo a process

called CYTOKINESIS

Cytokinesis

In 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 growth

Mitosis is required for growth of an organism

More 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

stability

DNA must be accurately copied and present in all

cells

The “extra” stage

Cells 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

Histones The 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 chromosomes

Cells contain 23 PAIRS of chromosomes

22 autosomal pairs and 1 sex pair

This means we have 46 chromosomes total in all

of our cells

Having all 46 means that a cell is DIPLOID

KARYOTYPE

Micrograph showing chromosome

arrangements

The stages of mitosis

The stages in a bit more detail Prophase

The chromatin begins to condense into chromosomes

The nuclear membrane disintegrates

Metaphase

The chromosomes arrange along the middle of the cell

Anaphase

The chromosome pairs are pulled to either end of the cell

Telophase

Nuclear membranes reform around the separating chromosomes

Cytokinesis begins

Haploid cells

Sex cells (eggs and sperm) are HAPLOID

That 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)

Meiosis

Meiosis 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 HAPLOID

The initial round of division

The second round of division

The sources of genetic variation Prophase 1 When 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 assortment

When 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

diversity

The sources of genetic variation