CHROMOSOME: THE BASIS OF HEREDITARY
LOCATION AND STRUCTURE OF CHROMOSOMES
Chromatin granules (thread – like structures) found in the nucleus of eucaryotic cells are the precursors or raw materials of chromosomes.
Chromosomes occur in pairs known as homologous chromosomes. Each chromosome is made up of two threads called chromatids joined at a point called centromere. Each human somatic cell has 46 chromosomes. These are present in 23 pairs of homologous chromosomes. The number of chromosomes in each somatic cell of an organism is called diploid number (2n).
Each chromosome is made up of protein units in a strand of deoxyribonucleic acid, DNA (in double helix). Along its length are genes arranged which are actually DNA segments. The DNA is a very large molecule made up of repeating units called nucleotides. Each nucleotide is made up of deoxyribose (a sugar molecule), phosphate and an organic nitrogenous base which may be adenine, guanine, thymine or cytosine. Guanine always pairs with cytosine and adenine with thymine. The two helical chains are referred to as complementary strands of DNA since one is the exact opposite of the other.
Sex chromosomes and autosomes
There are forty-four autosomes which are similar in shape and size in both male and female. The last pair is called sex chromosome which are of genotype XX in female and XY in male. Exception to this is in birds, moths and butterfly where the female has genotype XY and the male XX. Also, in certain grasshoppers, the Y chromosome is absent so that the male has the genotype XO.
Just before cell division, the protein bundles come together and the DNA strands coil tightly around them, causing the chains to shorten and become visible under the light microscope. This process is called condensation
Each DNA molecule is made up of thousands of genes. The DNA molecules coil around the 23 pairs of chromosomes. In human body cells are about 50,000 genes. Each DNA molecule can make an exact copy of itself in a process called replication. This forms the basis for the transmission of hereditary materials from parents to the offspring.
ROLE OF CHROMOSOME IN TRANSMISSION OF HEREDITARY CHARACTERS
Genes are the expression of hereditary characters in organisms and are located on the chromosomes of body cells. Therefore chromosomes are responsible for the transmission of characters from parents to offspring. Chromosomes are arranged in pairs known as homologous chromosomes (exactly alike in shape and size and carry genes responsible for the transmission of the same characteristics). The genes relating to the same character e. g. tallness and shortness occupy identical loci on the homologous pair. The genes on homologous pair of chromosomes determine whether the individual will be homozygous or heterozygous for certain characters.
PROCESSES OF TRANSMISSION OF HEREDITARY CHARACTERS BY CHROMOSOMES
- The chromosomes pass the genes into the gamete during meiosis.
- Homologous chromosome separate into two daughter cells during the first stage of meiosis.
- The two chromatids of each chromosome separate during the second stage of meiosis. Each gamete therefore has one set of chromosomes hence one copy of genes.
- During fertilization, the gametes fuse together to form a zygote. The zygote receives two genes for the same character (one from one chromosome in the egg and the other from one chromosome in the sperm).
- When the two genes are the same, the offspring is a homozygous but when they are different, the offspring is a heterozygous (hybrid).