Table of Contents
- Chromosome Organization and Structure
- What are Homologous Chromosomes
- Frequently Asked Questions
Chromosome Organization and Structure
The number and structure of chromosomes vary between species. It can even vary within eukaryotic organisms. For example, the mammalian gametes are specialised cells that contain half the number of chromosomes (haploid) of the somatic cells (diploid).
A chromosome in a man consists of two separate arms divided by a structure called the centromere. Eventually, it results in a long arm (q) and a short arm (p).
What are Homologous Chromosomes?
Homologous Chromosomes are made of chromosomal pairs of the same length and centromere positions for the genes corresponding to the same loci. Typically one of the chromosomes is inherited from the father and the other from the mother. Their alleles might possibly be different, and thus it results in different phenotypes for a single gene.
Humans have 22 pairs of autosomes that are homologous chromosomes. The homogeneity of the 23rd pair depends on the sex of the individual. Females have XX homologous chromosomes while males have XY (non-homologous).
Structure of Homologous Chromosomes
The basic parts of a homologous chromosome include –
- Kinetochore or Centromere
- Short arm p
- Long arm q
The length of the arm and the position of the centromere plays a vital role in determining the homogeneity of the chromosomes. Based on the position of the centromere, they can be metacentric, acrocentric, telocentric and submetacentric. Two chromosomes with similar structures come together to form homologous chromosomes.
Also, note that homologous chromosomes are not identical chromosomes like the sister chromatids.
Typically sister chromatids are identical copies having the same size and also have the same genes. The genes are at the same loci and have the same alleles. Whereas homologous chromosomes carry the same genes at the same loci but can have different alleles.
- During mitosis, the homologous chromosomes duplicate and fuse to form sister chromatids. These sister chromatids get segregated to opposite poles and get divided between the daughter cells.
- During meiosis, the homologous chromosomes exchange genetic information in the process of recombination. Here, they form four daughter cells, with each having half the number of parental chromosomes.
- The segregation of homologous chromosomes happens during meiosis Ⅰ, and the segregation of sister chromatids occurs in meiosis Ⅱ.
- The failure in the segregation of homologous chromosomes during meiosis can result in nondisjunction.
Thus homologous chromosomes play a vital role in nuclear division. They also play a role in fixing damaged DNA. Any double-stranded damage can often be fixed by matching up the broken ends to their homologous partners.
Also Read: Chromosomal Abnormalities
Frequently Asked Questions
What are the four shapes of chromosomes?
The four shapes of chromosomes are metacentric, submetacentric, acrocentric and telocentric. These four distinct shapes of chromosomes are based on the centromere position –
- Metacentric – Here, the centromere is central, and thus the two chromosomal arms are of equal length. These chromosomes have a characteristic V-shape.
- Submetacentric – The centromere is slightly off-centre, resulting in unequal arms. Thus results in J or L shaped chromosomes.
- Acrocentric – The centromere is almost at the top of the chromosome, resulting in a very short and long arm.
- Telocentric – The centromere is at the proximal end of the chromosome and gives a characteristic I-shape.
What is nondisjunction?
Nondisjunction results as the failure of the homologous chromosomes to segregate during cell division. It leads to chromosomal abnormalities, which eventually result in physical defects. The two main types are monosomy and trisomy.
What is homologous recombination?
The exchange of hereditary information between two identical molecules of nucleic acids is called homologous recombination. This process facilitates chromosomal crossover, where similar but non-identical regions get exchanged between the homologous chromosomes.
Also, Check: Linkage and Recombination
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