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
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