Chromosome

A chromosome is the name given to the structure that holds an organisms DNA, either all or just part of it. Both plant and animal chromosomes become visible when mitosis or meiosis occur, they become much more condensed.

In most cases bacterial chromosomes are often circular and not linear, as found in humans .

 Backround

The parallel between the properties of chromosomes and those of genes is evidence that genes are located on chromosomes:

1. Genes come in  pairs; chromosomes come in pairs

2. Alleles of a gene segregate; homologous chromosomes segregate

3. Unlinked genes undergo independent assortment; nonhomologous chromosomes undergo independent assortment

Theodor Boveri gave the definitive difinition that genetic material is held on chromosomes after a series of experiments in the mid 1880s.

Structure of Chromosome:

1. Each chromosome compose of two identical sister chromatids.

    
   2. The sister chromatids are hold on the centromere. Centromere in the human chromosome doesn't have any distinct sequence, it is just a large array of repeated sequence.
        The position of centromere determine the type of chromosome.

3. At the end of each chromosome also contain telomere, that is used to protect the chromosome from annealing with sister chromatids.
Also is used to distinguish the real end of chromosome from the end of chromosome caused by damaged; in addition, it is also used to solve end replication problem.        

Sex Chromosomes/Determination in Humans

Sex chromosomes are not present in pairs of morphologically similar homologs. The chromosomal mechanism for determining sex at the time of fertilisation is the difference in chromosomal constitution between males and females. While all eggs contain an X chromosome, only half of the sperm cell contain an X chromosome and the rest contain a Y chromosome. The karotype of a human female is 46,XX while the karotype of a human male is 46, XY. The human Y chromosome pairs with the X chromosome during meiosis in males, usually only along part of its length because of a limited region of homology.

Human Y chromosome

The Y chromosome contains far fewer genees than the X and encodes at least 26 unique proteins that are important for male fertility. It is divided into 3 different regions; the pseudoautosomal regions, male specific region and the sex determining region Y. The pseudoautosomal regions share homology with the X chromosome, they synapse and recombine with X during meiosis. The male specific region of the Y does not synapse with X chromosome. The sex determining region Y produces a product called testis determining factor which triggers undifferentiated gonadal tissue of the embryo to for testes.

Unusual Chromosomes

Some organisms contain special chromosomes which are not generally found in other organisms. Minichromosomes are short pices of DNA which are very rich in genes. B chromosomes are chromosomes which are only found in some members of a population, they arise due to unusual events in nuclear division and do not always have any affect on the organism. Holocentric chromosome have multiple kinetochores instead of just one single centromere.

References

2. Hartl D L., Ruvolo (2011), Genetics: Analysis od Genes and Genomes, 8th edition, London: Jones & Bartlett Learning

3. DNA from the Beginning - An animated primer of 75 experiments that made modern genetics (2002-2010), DNA Learning Centre. Available at: http://www.dnaftb.org/8/bio.html

4. Brown A., (2007), Genomes 3, 3rd edition, New York: Garland science