Chromosomes can be seen with an optical microscope during cell division metaphase. During cell division, each metaphase chromosome consists of two parts, called sister chromatids. They are a result of duplication of DNA through the cell division process and thus are completely identical. Diploid cells have two copies of each chromosome, known as homologous chromosomes. The two homologous chromosomes are similar in size, shape, and centromere location.
The type of genetic information they carry is also similar. In humans, each somatic cell has 23 pairs of homologous chromosomes, giving a total of 46 chromosomes. Two of them are the sex chromosomes that determine the sex of a person. In preparation for cell division, the DNA molecule doubles, and forms two sister chromatids. They form at a point in the cycle and the development of the cell — the S subphase of the interphase.
For the rest of the time, the chromosome is a single strand with a specific pinch narrower part , called a centromere. The centromere splits the chromosome into two arms. Peripherally, a structure called a kinetochore is built around the centromere.
It serves to attach microtubules to the division of chromatids in cell division. Chromosomes with equal arms are called metacentric. If one of the arms is visibly longer than the other, the chromosome is called submetacentric. There are also the so-called acrocentric chromosomes, in which there is a secondary pinching, which separates a small section called a satellite. Such chromosomes in human cells are 13, 14, 15, 21 and Sister chromatids are formed in the prophase I or Meiosis I if you consider the larger picture.
These sister chromatids are identical. Homologous chromosomes are also relevant for mitosis. In diploid cells that have to duplicate or undergo cellular division through the asexual process of mitosis, the homologous chromosomes are existent in the original cell undergoing duplication.
Homologous chromosomes are of the same shape and size, they may or may not contain the same genes and they are certainly not a duplicate version of one another. Sister chromatids are essentially a replicate or a copy of the original chromatid.
Hence, the two are identical or exactly similar. Thus, an individual may be homozygous or heterozygous for a particular character.
Humans have 22 homologous pairs of autosomes and 2 sex chromosomes. Female sex chromosomes, X and X are homologous while male sex chromosomes, X and Y are not truly homologous. X and Y differ from their size and genetic composition.
The pairing of homologous chromosomes allows random segregation of genetic material. Homologous chromosomes exchange their DNA by recombination. Recombination at sexual reproduction leads to genetic variations in the offspring which consequently facilitates the evolution. Homologous chromosomes separate during anaphase I of meiosis I. Four haploid daughter cells are produced at the end of the meiosis. Chromosomal abnormalities such as trisomy and monosomy can occur due to the nondisjunction of homologous chromosomes.
Figure 1: Homologous Chromosomes. Sister chromatids are two identical chromatids resulting in DNA replication during the S phase of interphase. They are joined together by the centromere. A sister chromatid is a one-half of a replicated chromosome. Hence, each replicated chromosome is composed of two sister chromatids. Answered by Phoebe L. Answered by Xi Z. Answered by. Answered by Stasa T.
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