Question

Define these 4 terms: alleles, gene expression independent assortment, and chromosomal segregation.  Tell how they are related to one another in terms of genetics and meiosis.

Answer 1

A diploid individual has two sets of genomes (two haploid sets of chromosomes) in each somatic and germinal cell in its body,contributed by the male parent through the sperem and female parent through the egg. For example in humans each diploid cell has 46 chromosomes - 23 contributed by the father (paternal chromosomes) and 23 contributed by the mother (maternal chromosomes). Corresponding chromosomes derived the paternal and maternal side are called homologous chromosomes. Homologous chromosomes are identical in their shape and contains almost identical DNAs in them. A chromosomal locus is a specific position on homologous chromosomes where a particular allele is located. Alleles are different variants of the same gene present on the same locus of a pair of homologous chromosomes. For example the human RB gene is located on band q14 of human chromosome 13. The chromosome contributed by the mother (maternal chromosome) has a copy of this gene at this locus. Similarly the paternal chromosome also has the RB gene at the same locus. These are alleles present at the same locus of homologous chromosomes.

Gene expression - means production of the product specified by that gene. Genes encode RNA/RNA and protein. A protein coding gene encodes a protein. For example beta-globin gene of man encodes beta-gloin protein. In the synthesis of the beta-globin protein the beta-globin gene is first transcribed into mRNA and this mRNA is translated into beta-globin protein. Expression of beta-globin gene means production of beta-globin protein. The product of tRNA gene is tRNA and rRNA gene is rRNA; these RNAS are not translated into proteins.

Before cell division i.e. in the -S- subphase of interphase the chromosomes duplicates. The duplicated chromosomes are held together at the centromere of chromosomes by a ring of cohesin. Infact, sister chromatids of a chromosome are daughter chromosomes held together at the centromere. In the anaphse of cell division the sister chromatids (daughter chromosomes) separate and they migrate into opposite poles of the cell. This process is called chromosome segregation. Chromosome segregation occurs both in mitosis and in meiosis.

In the frist meiotic prophase exchange of corresponding DNA segments occurs between homologous chromosomes by crossing over. When the prophase is over each chromosome has lost their original identity because each contains DNA segments from both chromosomes. During first meiotic anaphase the paired homologous (bivalent or tetrad) chromosomes segregate into opposites poles of the cell. In this phase what is moving to the pole is a chromosome as such with two sister chromatids and not single chromatids. Each chromosome has the probability to move into either pole of the cell. This means that the chromosomes assort independently. Due to independent assortment of chromosomes the daughter cells produced by different parent cells are likely to be different. with random combinations of chromosomes.