AS Bio notes Raida 12D
The mitotic cell cycle
5.1 Growth and Reproduction
Cells reproduce by dividing and passing on copies of their genes to ‘daughter’ cells
The process must be very precisely controlled so that no vital genetic information is lost
The nucleus always divides before a cell divides
Each of the two daughter cells therefore contains its own nucleus
This is important because the nucleus controls the cells activities, this is because it
contains the genetic material, DNA, which acts as a set of instructions/code for life
All the cells in the bodies of multicellular organisms are genetically identical, apart from
the reproductive cells known as gametes
This is because they all come from one cell, the zygote
This is the cell formed when one gamete from the mother and one gamete from the
father fuse
When the zygote starts the process of growth, it divides into two cells with identical
nuclei
This involves a type of nuclear division called mitosis
Nuclear division followed by cell division continues to be repeated in a cycle called the
mitotic cell cycle to produce all the cells of your body
5.2 Chromosomes
The structure of chromosomes
The chromosome at this stage is a double structure
Made of two identical structures called Chromatids, joined together
Two identical chromatids of one chromosome are known as sister chromatids
There are two chromatids because during the period between nuclear divisions,
interphase, each DNA molecule in a nucleus makes an identical copy of itself
Each chromatid contains one of these DNA copies
The sister chromatids are held together by Centromere to form a single chromosome
The centromere could be anywhere along the length of the chromosome
DNA is the molecule of inheritance and is made up of a series of genes
Each gene is one unit of inheritance
The two DNA molecules [one in each of the sister chromatids] are identical
, This means the gene on the chromatids are also identical
The fact that there are two identical chromatids is the key to precise nuclear division
When cells divide, one chromatid goes into one daughter cell and one goes into the
other daughter cell, making the daughter cells genetically identical
A precise scaffolding made of protein molecules prevents the DNA from getting tangled
up into knots
The DNA is wound around the outside of these protein molecules
The combination of DNA and proteins is called Chromatin
Most of the proteins are basic and are a type known as Histones
Because they are basic, they can interact easily with DNA, which is acidic
Chromosomes also possess two more features essential for successful nuclear division:
centromere and telomeres
Telomeres are visible if chromosomes are stained appropriately
5.3 The cell cycle
Mitosis is nuclear division that produces two genetically identical daughter nuclei, each
containing the same number of chromosomes as the parent nucleus
Mitosis is part of a precisely controlled process called the cell cycle
The cell cycle is the sequence of events that takes place between one cell division and
the next
It has 3 phases: interphase, nuclear division and cell division
1. During interphase, the cell grows to its normal size after cell division and carries out its
normal functions
At some point during interphase, a signal may be received that the cell should divide
again
If this happens, the DNA in the nucleus replicates so that each chromosome consists of
two identical chromatids
This phase of the cell cycle is called the S phase [S stands for synthesis of DNA]
This is a relatively short phase
The gap after cell division and before the S phase is called G1 phase (G for gap)
The gap after the S phase and before nuclear division is called the G2 phase
Interphase therefore consists of G1, S and G2
During G1, cells makes the RNA, enzymes and other proteins needed for growth
At the end of G1, the cell becomes committed to dividing or not dividing
During G2, the cell continues to grow and the new DNA that was made during the S
phase is checked
Any errors are usually repaired
Preparations are also made to begin the process of division
[For example, there is a sharp increase in production of the protein tubulin which is needed to
make microtubules for the mitotic spindle]
Nuclear division follows interphase
It is referred to as the M phase (M for mitosis)
Growth stops temporarily during mitosis
After the M phase, when the nucleus is divided into two, the whole cell divides to create
two genetically identical cells
In animals cells, cell division involves the constriction of the cytoplasm between the two
new nuclei, a process called Cytokinesis
[In plant cells it involves the formation of a new cell wall between the two new nuclei]
The mitotic cell cycle
5.1 Growth and Reproduction
Cells reproduce by dividing and passing on copies of their genes to ‘daughter’ cells
The process must be very precisely controlled so that no vital genetic information is lost
The nucleus always divides before a cell divides
Each of the two daughter cells therefore contains its own nucleus
This is important because the nucleus controls the cells activities, this is because it
contains the genetic material, DNA, which acts as a set of instructions/code for life
All the cells in the bodies of multicellular organisms are genetically identical, apart from
the reproductive cells known as gametes
This is because they all come from one cell, the zygote
This is the cell formed when one gamete from the mother and one gamete from the
father fuse
When the zygote starts the process of growth, it divides into two cells with identical
nuclei
This involves a type of nuclear division called mitosis
Nuclear division followed by cell division continues to be repeated in a cycle called the
mitotic cell cycle to produce all the cells of your body
5.2 Chromosomes
The structure of chromosomes
The chromosome at this stage is a double structure
Made of two identical structures called Chromatids, joined together
Two identical chromatids of one chromosome are known as sister chromatids
There are two chromatids because during the period between nuclear divisions,
interphase, each DNA molecule in a nucleus makes an identical copy of itself
Each chromatid contains one of these DNA copies
The sister chromatids are held together by Centromere to form a single chromosome
The centromere could be anywhere along the length of the chromosome
DNA is the molecule of inheritance and is made up of a series of genes
Each gene is one unit of inheritance
The two DNA molecules [one in each of the sister chromatids] are identical
, This means the gene on the chromatids are also identical
The fact that there are two identical chromatids is the key to precise nuclear division
When cells divide, one chromatid goes into one daughter cell and one goes into the
other daughter cell, making the daughter cells genetically identical
A precise scaffolding made of protein molecules prevents the DNA from getting tangled
up into knots
The DNA is wound around the outside of these protein molecules
The combination of DNA and proteins is called Chromatin
Most of the proteins are basic and are a type known as Histones
Because they are basic, they can interact easily with DNA, which is acidic
Chromosomes also possess two more features essential for successful nuclear division:
centromere and telomeres
Telomeres are visible if chromosomes are stained appropriately
5.3 The cell cycle
Mitosis is nuclear division that produces two genetically identical daughter nuclei, each
containing the same number of chromosomes as the parent nucleus
Mitosis is part of a precisely controlled process called the cell cycle
The cell cycle is the sequence of events that takes place between one cell division and
the next
It has 3 phases: interphase, nuclear division and cell division
1. During interphase, the cell grows to its normal size after cell division and carries out its
normal functions
At some point during interphase, a signal may be received that the cell should divide
again
If this happens, the DNA in the nucleus replicates so that each chromosome consists of
two identical chromatids
This phase of the cell cycle is called the S phase [S stands for synthesis of DNA]
This is a relatively short phase
The gap after cell division and before the S phase is called G1 phase (G for gap)
The gap after the S phase and before nuclear division is called the G2 phase
Interphase therefore consists of G1, S and G2
During G1, cells makes the RNA, enzymes and other proteins needed for growth
At the end of G1, the cell becomes committed to dividing or not dividing
During G2, the cell continues to grow and the new DNA that was made during the S
phase is checked
Any errors are usually repaired
Preparations are also made to begin the process of division
[For example, there is a sharp increase in production of the protein tubulin which is needed to
make microtubules for the mitotic spindle]
Nuclear division follows interphase
It is referred to as the M phase (M for mitosis)
Growth stops temporarily during mitosis
After the M phase, when the nucleus is divided into two, the whole cell divides to create
two genetically identical cells
In animals cells, cell division involves the constriction of the cytoplasm between the two
new nuclei, a process called Cytokinesis
[In plant cells it involves the formation of a new cell wall between the two new nuclei]
