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What is a Chromosome?

Biology | 7-14 yrs | Interactive

Definition of Chromosome

The word chromosomes is derived from two words, Chroma, meaning colour and Soma, meaning body. This is because when they were first discovered, they had used very strong colour dyes to detect them.

What are chromosomes made of?

They are thread like structures, located inside the nucleus of cells of both plants and animals. Each chromosome is made of protein and single molecule of DNA (Deoxyribonucleic Acid). DNAs are responsible for carrying forward the genetic make up of an individual plant or animals and so the species.

In humans, animals, and plants, most chromosomes are arranged in pairs within the nucleus of a cell. Humans have 22 of these chromosome pairs, called autosomes.

What does a chromosome structure look like?

  • Chromosomes tend to show up only when the cell is undergoing cell division.
  • Chromosomes are made of DNA that are tightly coiled around positively charged proteins called histones. This is to condense the DNA material.
  • On each chromosome, there is a constriction point known as the centromere, that separates into four arms. The centromere plays an important role in the overall shape of the chromosome and can be used to locate a specific gene. Short arms are called p arms and long arms are called q arms. This gives the chromosome an X like appearance.
  • Studies have shown that chromosomes without centromeres, segregate randomly, are eventually lost from cells. On the other hand that have multiple centromeres tend to fragment, causing genetic mutations and disorders.

4 Types of chromosomes

1. Metacentric Chromosome

Metacentric chromosomes have the centromere in the center, such that both sections are of equal length. Human chromosome 1 and 3 are metacentric.

2. Submetacentric Chromosome

Submetacentric chromosomes have the centromere slightly offset from the center leading to a slight asymmetry in the length of the two sections. Human chromosomes 4 through 12 are submetacentric.

3. Acrocentric Chromosome

Acrocentric chromosomes have a centromere which is severely offset from the center leading to one very long and one very short section. Human chromosomes 13,15, 21, and 22 are acrocentric.

4. Telocentric Chromosome

Telocentric chromosomes have the centromere at the very end of the chromosome. Humans do not possess telocentric chromosomes but they are found in other species such as mice.

How many chromosomes does a human cell have?

  • Human cells have 23 pairs of chromosomes, making a total of 46 chromosomes. Of these, 22 pairs are called autosomes and look the same in both male and female humans.
  • In females, the 23rd pair has two X shaped chromosomes.
  • In males, the 23rd pair has one X and one Y chromosome.
  • The sex or the reproductive cells, the gametes, have 23 chromosomes out of which the sperm cell will carry an X or a Y chromosome. The egg will carry 23 chromosomes, out of which the 23rd will be an X. This is a haploid cell.
  • On fertilization of the egg, depending on the 23rd chromosome, the gender of the zygote is decided. The zygote will carry 46 chromosomes. This is known as a diploid cell.

Why are chromosomes so important?

Chromosomes are important to ensure that when cell duplicate or undergo cell division, to produce new cells, they carry the genetic information accurately to the next generation of cells, and hence the organism. This keeps the genetic information intact and is important to keep the species alive. It is also important for cells of organs to regenerate.

What is a chromatid?

A chromatid is one of the replicated copies of a chromosome. Identical sister chromatids are produced as a result of DNA replication.

In contrast, homologous chromosomes are derived from either the mother or the father of the organism. Although they contain the same set of genes, they usually have genetic differences.

Do bacteria and viruses have chromosomes?

  • Bacteria and virus have simplified chromosome structures.
  • In bacteria or prokaryotes, the chromosome is dispersed within the cell and is not enclosed by a separate membrane. It is called a nucleoid. It is also common for bacterial species to have to possess extrachromosomal genetic elements called plasmids. Plasmids play a major role in drug/antibiotic resistance and hence spread of bacteria borne diseases.
  • Virus on the other hand have very basic, fragments of genetic material inside the protein coat or shell. The shell is called capsid.
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