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Summary Cytogenetics GNE2603
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- University Of South Africa (Unisa)
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Cytogenetics GNE2603 Examination notesLEARNING UNIT 1: GENOME STRUCTURE AND CELL DIVISION
Genome of all living organisms represents their hereditary material and is formed of DNA.
The organisation and packaging of eukaryotic DNA is more complex than those of
prokaryotic DNA (e.g., the DNA of bacteria), because the eukaryotes have much larger
genomes. The DNA is associated with proteins called histones (positively charged) or non-
histones (less positively charged). There are 5 histone proteins that together form a
nucleosome structure. The nucleosomes condense to form a structure, initially called a
solenoid, with a diameter of 30 nm. Many solenoid structures fold to form looped domains,
which further condense to form chromatin fibres. These chromatin fibres condense to form
chromosomes during cell division.
Genome organisation
DNA → histones → nucleosomes → solenoid → looped domains → chromatin fibres
→ chromosomes
The diploid number (or 2n) is the total number of chromosomes found in the somatic (body)
cells of an organism. When only one chromosome from each pair is present in a cell, we say
the cell has a haploid chromosome number (n). Germ cells (gametes) carry only one
chromosome of each pair – the haploid number. Germ cells are formed during meiosis, and
somatic body cells are replaced and increased through mitosis.
Mitosis- Essential for growth and development, the repair and replacement of cells and for
asexual reproduction. Mitosis is carefully timed in the evets of the cell cycle (divided into
four stages, G1, S, G2 and M). After mitosis, the new cells formed may become non-dividing
,and enter the G0 phase. These cells include muscle cells that do not divide further once
they have been formed. The cells may continue into the G1 phase, where they become
committed to continue to the S phase, which is the DNA synthesis phase. If everything
occurs as it normally should, they will complete G2 phase and mitosis (or M phase).
Following mitosis, the two cells formed either enter G and the cell will stop dividing or
G and the cycle is repeated.
During DNA replication (the S phase), the DNA double helix duplicates itself by
separating the two strands and then synthesising new DNA molecules on the existing
strands. Each chromosome now consists of two chromatids.
The G1 and G2 phases are the growth phases of the cell where proteins are synthesized,
and cell organelles are produced. During the S phase, replication of the DNA occurs.
After replication, each chromosome is now composed of two sister chromatids that are held
together at their centromeres. In the G2 phase, the chromosome continues to condense
from the 30-nm fibre. As the chromosomes become more condensed, they become visible
with the light microscope. This marks the start of mitosis. Mitosis is the phase of the cell
cycle where the sister chromatids are separated by spindle fibres. During mitosis, the
chromosomes continue to condense until metaphase. In metaphase, the chromosomes
align at the metaphase plate in preparation for the separation of the sister chromatids
by the spindle fibres. The sister chromatids are finally separated in anaphase and the
chromosomes begin to decondense in telophase preparing the DNA for replication as
the cell cycle starts again.
Features of chromosomes
,(1) Chromatid, which is one of the two identical parts (sister chromatids) of the chromosome
after the S phase has been completed.
(2) Short (p) arm
(3) Centromere or the point where the two chromatids touch, where the microtubules attach
and where the DNA is more tightly coiled.
(4) Long (q) arm
Chromosomes based on the position of the centromere.
Karyokinesis is a division of the cell nucleus that takes place in four stages during mitosis,
prophase, metaphase, anaphase and telophase, followed by division of the cytoplasm-
cytokinesis (the physical process of cell division, which divides the cytoplasm of a
parental cell into two daughter cells, starts during the nuclear division phase called
anaphase and continues through telophase.). The organelles in the cytoplasm replicate
themselves, or develop from existing cell membrane structures, or are formed anew in each
cell.
Prophase
The cytoplasmic organelles (centrioles) found outside the nuclear envelope, divide and
move to opposite ends of the cell to establish two poles. They organise cytoplasmic
microtubules into spindle fibres that run between the poles. As the centrioles move to
opposite sides of the cell, the nuclear envelope breaks down and slowly disappears.
The chromosomes have already replicated during the S phase of the cell cycle now
consisting of two chromatids, called sister chromatids joined at a centromere. The sister
, chromatids undergo chromosome condensation, and as a result they become smaller and
more compact.
Metaphase
The chromosomes place themselves on the metaphase plate (equatorial plate), which is
the midline region of the cell. The kinetochore is a granule of the centromere that is
attached to the spindle fibre. Two kinetochores form on opposite sides of the
centromere. The centromeres lie on the metaphase plate in such a way that the
kinetochore of one chromatid attaches to the spindle fibres of one pole. The
kinetochore of the other chromatid attaches to the spindle fibres of the other pole- Auto-
orientation. Each centromere is aligned at the plate so that each chromatid can
separate from the pole it is joined to by the spindle fibres.
Anaphase
The sister chromatids of each chromosome separate from each other and move to
opposite ends of the cell. Each centromeric region splits in two and is pulled to the
opposite pole, with the centromeres leading the way. Each chromatid is now a daughter
chromosome. Equational division- one chromatid of each chromosome goes to one pole,
and the other chromatid goes to the other pole, ensuring that there is an identical set of
chromosomes at each pole due to auto-orientation at metaphase.
Telophase
There is now an identical set of chromosomes at each pole. The chromosomes
decondense (uncoil and become diffuse chromatin again). The nuclear membrane
reforms around each set of chromosomes. Cytokinesis (the division of the cytoplasm
and all it contains between two new cells) now takes place. Two genetically identical,
diploid cells that are identical to the original diploid (2n) cell result.
Meiosis I- a reductional division where the diploid chromosome number is halved, thus
giving the haploid number. DNA replication takes place before cells enter into prophase I.
Prophase I
o Leptotene: The replicated chromosomes start to condense and appear as long
thin threads. The nuclear envelope and nucleolus are still present in the cells.
o Zygotene: Homologous chromosomes match up perfectly (synapsis) and pair
together gene for gene to form bivalents or tetrads. Each bivalent or tetrad therefore
consists of two homologous chromosomes synapsed together.
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