Chromosome

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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[1].

Contents

Backround

The parallel between the properties of chromosomes and those of genes is evidence that genes are located on chromosomes[2]:

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 definition that genetic material is held on chromosomes after a series of experiments in the mid-1880s[3].

Structure of Chromosome:

  1. Sister chromatids.jpg
    Each chromosome compose of two identical sister chromatids.
  2. The sister chromatids are held 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. In metacentric chromosomes the centromere is positioned in the centre making the arms of equal length. When the centromere is positioned slightly off centre this is called submetacentric. When a chromosome is acrocentric the centromere is towards one end of the chromosome and in telocentric chromosomes is it positioned right at the end. This positioning is very important in determining the shape of daughter chromosomes in anaphase[4].
  4. Tp11 02.jpg
    The end of each chromosome contains a telomere, this is used to protect the chromosome from annealing with sister chromatids and distinguishes the real end of the chromosome from a damaged end of a chromosome. It is also used to solve the 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 contains an X chromosome and the rest contain a Y chromosome. The karotype of a human female is 46,XX while the karyotype 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[5].

Human Y chromosome

It was theorised that the Y chromosome originated from a mutated X chromosome during evolution. It contains far fewer genes 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 pieces 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[6]. Polytene chromsomes are giant chromosomes found in the larva of Drosophilia and other two-winged flies. These chromosomes contain around a thousand DNA molecules laterally aligned[7].

References

  1. Molecular Biology of the cell fifth edition, 2008
  2. Hartl D L., Ruvolo (2011), Genetics: Analysis od Genes and Genomes, 8th edition, London: Jones and 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. Hartl D, Jones E and Jones E, 2012. Genetics: Analysis of Genes and Genomes (Genetics Series) 8th Ed., Jones and Bartlett Publishers p 261
  5. 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
  6. Brown A., (2007), Genomes 3, 3rd edition, New York: Garland science
  7. Hartl D, Jones E and Jones E. 2012, Genetics: Analysis of Genes and Genomes (Genetics Series) 8th ed., Jones and Bartlett Publishers. p238
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