Genetics 3
HC 9
3.1
Chromosomes are the structures in living cells that contain the genetic information, since
the genes are physically located on them. In fact, it contains long DNA sequences combined
with proteins, together named the chromatin.
Prokaryotes, like bacteria, do not have their chromosomes contained inside the membrane
of a nucleus, but have their circular chromosomes in a nucleoid, which is simply a region in
the cytoplasm. Eukaryotes cells are more complex, with internal membranes forming
enclosed specialised compartments, organelles, each with a distinct function. One such
organelle, the nucleus, encloses the linear chromosomes. N. B. the mitochondria have their
own set of DNA.
Cytogenetics is the field of microscopic examination of the chromosomes, thus a
cytogeneticist examines the chromosomal composition of a particular cell, which is mostly
done in cells ready to divide, because the chromosomes are more visible.
Most of the cells in our body are somatic cells (diploid), the other sort is gametes or germ
cells (haploid). All of their chromosomes can be isolated, identified and organised, giving
form to a karyotype.
Diploid indicates each type of chromosome is member of a pair, called homologs, so a
homologous pair. In each pair, one chromosome came from the mother, the other from the
father. They are nearly identical; in size, banding pattern and similar genetic material.
However, they may have different versions of a gene, so different alleles. Alleles can be
dominant or recessive. Only the sex chromosomes differ completely in every aspect, which
is why they are not considered a homologous pair. Homozygous means two of the same
alleles, meaning both dominant or both recessive. Heterozygous means one dominant, one
recessive allele. The physical location of the gene is a locus.
3.2
Why do cells divide?
- Asexual reproduction: a pre-existing cell divides into two new cells; a mother gives
two daughter cells.
- Multicellularity: from one cell, many can spawn thanks to cell division. For this
though, it is critical the chromosomes are properly transmitted.
Bacteria (unicellular, one circular chromosome in the cytoplasm) proliferate via binary
fission:
1. Copy chromosomal DNA
2. Each copy moves to one side of the cell
3. FtsZ protein moves to division site and recruits other proteins to form cell wall and
physical division between cells.
4. Then binary fission, two daughter cells become separated by septum.
This is a form of asexual reproduction, because it does not involve the interaction of two
different gametes.
Eukaryotic cells have a specific cell cycle, divided into several phases. G-phases are gap
phases between S (synthesis) and M (mitosis) phase, they nevertheless are critical periods
with many molecular changes. The G 1 S and G2 phases together, form the interphase. There
HC 9
3.1
Chromosomes are the structures in living cells that contain the genetic information, since
the genes are physically located on them. In fact, it contains long DNA sequences combined
with proteins, together named the chromatin.
Prokaryotes, like bacteria, do not have their chromosomes contained inside the membrane
of a nucleus, but have their circular chromosomes in a nucleoid, which is simply a region in
the cytoplasm. Eukaryotes cells are more complex, with internal membranes forming
enclosed specialised compartments, organelles, each with a distinct function. One such
organelle, the nucleus, encloses the linear chromosomes. N. B. the mitochondria have their
own set of DNA.
Cytogenetics is the field of microscopic examination of the chromosomes, thus a
cytogeneticist examines the chromosomal composition of a particular cell, which is mostly
done in cells ready to divide, because the chromosomes are more visible.
Most of the cells in our body are somatic cells (diploid), the other sort is gametes or germ
cells (haploid). All of their chromosomes can be isolated, identified and organised, giving
form to a karyotype.
Diploid indicates each type of chromosome is member of a pair, called homologs, so a
homologous pair. In each pair, one chromosome came from the mother, the other from the
father. They are nearly identical; in size, banding pattern and similar genetic material.
However, they may have different versions of a gene, so different alleles. Alleles can be
dominant or recessive. Only the sex chromosomes differ completely in every aspect, which
is why they are not considered a homologous pair. Homozygous means two of the same
alleles, meaning both dominant or both recessive. Heterozygous means one dominant, one
recessive allele. The physical location of the gene is a locus.
3.2
Why do cells divide?
- Asexual reproduction: a pre-existing cell divides into two new cells; a mother gives
two daughter cells.
- Multicellularity: from one cell, many can spawn thanks to cell division. For this
though, it is critical the chromosomes are properly transmitted.
Bacteria (unicellular, one circular chromosome in the cytoplasm) proliferate via binary
fission:
1. Copy chromosomal DNA
2. Each copy moves to one side of the cell
3. FtsZ protein moves to division site and recruits other proteins to form cell wall and
physical division between cells.
4. Then binary fission, two daughter cells become separated by septum.
This is a form of asexual reproduction, because it does not involve the interaction of two
different gametes.
Eukaryotic cells have a specific cell cycle, divided into several phases. G-phases are gap
phases between S (synthesis) and M (mitosis) phase, they nevertheless are critical periods
with many molecular changes. The G 1 S and G2 phases together, form the interphase. There
