Chapter 9 – Chromosomes, the Cell Cycle, and Cell Division
Cell Cycle
The normal cycle of a cell consists of 4 stages:
1. Gap 1 – during this phase the cell decides whether it will or will not divide. Cells
that are not going to divide stay in this stage.
2. S phase – DNA replication occurs during this stage.
3. Gap 2 – during this phase the cell prepares to divide by manufacturing more
cytoplasmic elements such as cytoskeletal elements and organelles.
4. Mitosis- division of the nucleus and the cell into two identical cells.
(Gap 1, S phase, and Gap 2 make up interphase.)
During G1, certain proteins are made that determine whether the cell will enter S or will
stay in G1. Cyclin D is manufactured during G1 and it binds to CDK 4 to form a
CDK/cyclin complex. This complex becomes active and phosphorylates enzymes that
initiate the progression of the cell into the S phase. At the end of G1, cyclin D is broken
down, which shuts off the action of the complex.
The G2 phase is similarly controlled by a CDK/cyclin complex, but during this phase
CDK 2 and cyclin b are present. The activity of this complex initiates chromosome
condensation.
Q.
If you add an inhibitor of CDK 4 to a culture of human cells, how many
chromosomes would you expect to find in each cell?
Chromosome structure
Definitions:
Chromosome – a single linear strand of DNA. Also, two identical strands of DNA that
are joined at the centromere.
Chromatid – one of the identical strands of a chromosome.
Chromatin – complex of DNA and proteins in a chromosome.
Sister chromatids – each identical strand of DNA in a chromosome is the “sister” of the
other strand. Therefore, a chromosome is made up of two sister chromatids.
Homologous chromosomes – you have a pair of each chromosome, one from your father
and one from your mother. They contain the same genes, although the alleles may not be
identical. So, chromosome 16(paternal)’s homolog is chromosome 16(maternal).
Centromere- the region of a chromosome where the sister chromatids are joined.
Kinetochore – structure on the chromosome, near centromere, that microtubules of the
spindle attach to.
Histone – proteins around which the DNA wraps while condensing. There are 8 core
proteins and 1 that “clamps” the DNA to the core.
Nucleosome – unit consisting of a set of histone proteins and the DNA wrapped around
them. They become super-coiled in condensed DNA.
Mitosis
The goal of mitosis is to make two daughter cells that are identical (at least genetically) to
the parent cell.
prophase
prometaphase
telophase
metaphase
anaphase
Stages:
1. Interphase – this is when the cell decides whether to divide or not. If it decides to
divide then the DNA is duplicated as well as the centrosomes (contains 2 centrioles,
serves as the microtubule organizing center). See description above.
2. Prophase – the nucleus prepares itself to divide and the centrosomes migrate to
opposite poles. The nucleolus disappears while the chromosomes begin to condense.
The spindle begins to form at each centrosome.
Cell in prophase
3. Prometaphase – the nuclear envelope disappears and the chromosomes finish
condensing. The breakdown of the nuclear membrane allows the spindle
microtubules to attach to the kinetochores of the chromosomes. Once attached, they
are called kinetochore microtubules. The chromosomes can now be pulled by the
spindle.
4. Metaphase – all of the kinetochores line up along the equatorial plate. At the end of
metaphase the centromeres split.
Cell in metaphase
5. Anaphase – once the centomeres have split, anaphase begins. During anaphase the
chromosomes are pulled by the microtubules toward the poles, with sister chromatids
going to opposite poles. This movement ensures that each daughter cell receives 1
copy of each chromosome.
Cell in anaphase
6. Telophase – the chromosomes cluster at each pole while the spindle breaks down and
the nuclear envelope reforms. The chromosomes begin to uncoil and the nucleolus
reforms. Then another interphase begins.
7. Cytokinesis – although not an official stage, cytokinesis is necessary for mitosis to be
complete in most cells. Cytokinesis is the division of the cytoplasm and it is
accomplished by a ring of actin and myosin pulling the plasma membrane in between
the two new nuclei and pinching the cell in two.
Q.
Which cells of the body would you expect to be constantly undergoing mitosis?
Meiosis
The goal of meiosis is to produce 4 haploid cells, none of which have identical genetic
information.
Meoisis I
Meiosis II
Stages:
Meiosis I:
(the goal of meiosis I is to separate the homologous pairs)
1. Prophase I – the chromosomes condense and the centrosomes migrate. A unique
characteristic of this phase is the presence of chiasmata. Chiasmata are the sites at
which homologous chromosomes cross-over and exchange a section of their DNA.
This involves the breaking and reforming of the phosphodiester linkages between
nucleotides. Because genes occur in the same location on homologous chromosomes,
this process swaps maternal genes with paternal genes and generates unique
chromosomes.
2. Prometaphase I – nucleolus disappears, nuclear membrane breaks down, spindle
forms, microtubules attach to kinetochores. In this division there is only one
kinetochore per chromosome so that the homologous pairs will split and not the sister
chromatids.
3. Metaphase I – chromosomes line up along the equatorial plate with the homologous
pairs next to each other.
4. Anaphase I – homologous pairs are pulled to opposite poles. The decision as to
which pole the maternal and paternal chromosomes go to is arbitrary. This random
assortment produces varied combinations of maternal and paternal DNA in each of
the daughter cells.
5. Telophase I – (only incertain species) the nuclear membrane reforms and the
chromosomes begin to elongate, etc. Will be followed by interkinesis, which is
similar to interphase but without the duplication of DNA.
Meoisis II
(the goal of this division is to separate the sister chromatids and for four cells)
This division is basically the same as mitosis, except for the end products. Following
cytokinesis, 4 haploid cells have been formed. Each cell contains DNA that is different
not only from the DNA of the gametes of the organisms parents, but also different from
the cells of the organism’s own body.
Q.
During which phase(s) of meiosis may Down’s syndrome develop?