AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

STRUCTURE OF CHROMOSOME

Chromosome Morphology

• The chromosome morphology changes during cell division and mitotic metaphase is the most suitable stage for studies on chromosome morphology.
• The DNA of eukaryotic cells is tightly bound to small basic proteins (histones) that package the DNA in an orderly way in the cell nucleus.
• The complexes between eukaryotic DNA and proteins are called chromatin, which typically contains about twice as much protein as DNA.
• The major proteins of chromatin are the histones - H1, H2A, H2B, H3, and H4 which are very similar among different species of eukaryotes.
• The shape of the eukaryotic chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division.
• Chromosomes are thin, coiled, elastic, thread-like structures during the interphase.
• As cells enter mitosis, their chromosomes become highly condensed so that they can be distributed to daughter cells.
• In mitotic metaphase chromosomes, the following structural features can be seen under the light microscope.

Chromatid

• Each metaphase chromosome appears to be longitudinally divided into two identical parts each of which is called chromatid. Both the chromatids of a chromosome appear to be joined together at a point known as centromere. The two chromatids of chromosome separate from each other during mitotic anaphase (and during anaphase II of meiosis) and move towards opposite poles.
• Since the two chromatids making up a chromosome are produced through replication of a single chromatid during synthesis (S) phase of interphase, they are referred to as sister chromatids. In contrast, the chromatids of homologous chromosomes are known as non-sister chromatids.

Centromere (Primary constriction)

• Each chromosome has a constriction point called the centromere (Synonym: Kinetochore), which divides the chromosome into two sections or arms.
• The short arm of the chromosome is labeled the "p" arm. The long arm of the chromosome is labeled the "q" arm.

Telomere

• The sequences at the ends of eukaryotic chromosomes, called telomeres, play critical roles in chromosome replication and maintenance.

Secondary constriction

• In addition to centromere / primary constriction, one or more constrictions in the chromosome are present termed secondary constrictions.

Satellite

• A small chromosomal segment separated from the main body of the chromosome by a secondary constriction is called Satellite.