Notes on Chromosomes | Grade 10 > Science > Chromosome and Sex Determination | KULLABS.COM

Chromosomes

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The thread-like structure which is located inside the nucleus of animal and plants cells are known as chromosomes. The word chromosome is derived from the Greek word 'chromo' which means color and 'soma' which means body. The size of the chromosome varies from one to another species which is visible during the cell division. All the plants and animals have a fixed number of chromosomes in their cells which is always present in pairs. 

Structural Features of Chromosomes

  1. Chromosomes comprise of a fine greatly coiled thread or filament which is known as chromonema. It is a coiled chromatin that has a single chemical molecule of double-stranded deoxyribonucleic acid (DNA), which is coiled tightly around proteins called histones. The proteins help the long DNA molecule to fit inside the nucleus of cells.

  2. Centromeres, secondary constrictions, nucleolar organizers, telomeres, and satellite are the other structures of a chromosome.

  3. The differential coiling between DNA, chromatin, and chromosome defines an association between them.

  4. It requires large number of euchromatin and less number of heterochromatin to form a chromosome. 

  5. The telomere or chromosomal end, primary constriction or centromere along with chromatin near the centromere and satellite region of a chromosome contain heterochromatin.

  6. The nuclear organizer, secondary constriction II and the rest of the chromosome contain euchromatin. The nuclear organizer contains genes for the synthesis of ribosomal ribonucleic acid (rRNA) while the rest part of a chromosome is capable of synthesizing messenger ribonucleic acid (mRNA) for protein synthesis.

  7. An exact copy of the DNA molecule in a chromosome is made so that at nuclear division the chromosome is a double structure, containing two identical DNA molecules. These two-part structures of chromosomes are called chromatids with each chromatid of the pair containing one of the two identical DNA molecules. The chromatids are held together at a point called the centromere. The centromere may occur anywhere along the length of the chromosome. Centromere is the only active part of the chromosome during cell division.

  8. Chromosomes are of five types, on the basis of presence or absence and position of centromere:

    Acentric: If centromere is absent, the chromosome is called acentric chromosome. This chromosome is genetically inactive and does not take part in cell division.

    Telocentric: If centromere is located at one end of the chromosome or in the telomeric region, it is called a telocentric chromosome.

    Acrocentric: If centromere is present in the subterminal region and the chromosome is divided into a very short arm at one region and other arm is very long, this chromosome is called acrocentric.

    Submetacentric: If centromere is present in the submedian region and the chromosome is divided into two equal arms, one of which is slightly longer than other forming L or J shaped chromosome. which is called sub-metacentric chromosome.

    Meta Centric: If centromere is present in the median position and the chromosome is divided into two equal arms forming a V-shaped chromosome, it is called metacentric chromosome.

  9. The region of the centromere is relatively narrow which is said to be primary constriction. At this region, a plate-like kinetochore is located in each chromatid of the chromosome.

  10. Besides primary constriction, the chromosomal arms may contain secondary constriction. It may be secondary constriction I or nuclear organizer and secondary constriction II.

  11. Each diploid set of chromosome contains two chromosomes with the secondary constriction I or nuclear organizer. They are necessary for the formation of the nucleolus.

  12. The part of the chromosome beyond the nuclear organizer is very short and like a sphere called satellite. The chromosome bearing a satellite is called SAT chromosome.

  13. Secondary constriction II is located in a fixed position in the chromosome. This is used for identification of chromosome.

  14. The tips of chromosomes are called telomeres. It facilitates attachment of chromosomes with the nuclear membrane.

Homologous Chromosomes

The one-half of the chromosome is derived from the mother and the other half is derived from father. The part of the chromosome derived from mother is maternal chromosome and the part of the chromosome derived from father is called parental chromosome. Thus, the homologous chromosome is the chromosome which is formed combinedly from parental and maternal chromosome. Similarly, we can say that a homologous chromosome is those chromosome pairs which is obtained from each parent that have a similar structure, height, gene position, and centromere location. The replicas of a single chromosome held together by a common centromere after DNA replication are known as sister chromatids.

There are 23 pairs of chromosomes in a human body. The one set of a chromosome is obtained from the female parent through egg and the other set of a chromosome is obtained from male parent through sperm. During the process of fertilization, the cell obtained from the fusion of sperm cell and egg cell is known as the zygote, which contains two sets of homologous chromosomes.

Function of Chromosomes

The major role of a chromosome is during the cell division and inheritance of hereditary characters from parents to their offsprings. They help in the growth and metabolic activities.

Diploid and Haploid Cells

Diploid and haploid are two types of cells found in the body of plants and animals. The diploid cells contain two complete sets of a chromosome which is represented by 2n. For example: somatic cells like skin cell, muscle cell , plant cells such as parenchyma cell, reproductive cell, etc. 

The haploid cells contain only one complete set of chromosomes which is half the number of chromosomes as comared to diploid cell that is represented by n. For example: spores, male gamete, egg, etc. 

The number of chromosomes varies in the cells of diffeent living organisms. Some of them are listed as follows:

Organisms Number of Chromosomes
Total Number Number in pairs
Human 46 23 pairs
Frog 26 13 pairs
Mice/ Rat 40 20 pairs
Monkey 54 27 pairs
Onion (Allium ceipa) 16 8 pairs
Pea (Pisum sativum) 14 7 pairs
Rice 24 12 pairs
Solanum nigrum 16 8 pairs
Potato 28 14 pairs
Sugarcane 80 40 pairs
Dog 78 39 pairs
Cow 60 30 pairs
Pine 24 12 pairs
Housefly 12 6 pairs
Gorilla 48 24 pairs
Mucor 2 1 pairs 



  1. Chromosomes are thread-like structures located inside the nucleus of an animal or plant cell which is found in pairs.
  2. There are 23 pairs of chromosomes in a human body.
  3. The chromosome derived from mother is maternal chromosome and the chromosome derived from father is parental chromosome.
  4. Chromosomes play a key role in cell division and inheritance of hereditary characters from parents to their offsprings.
  5. There are two types of cells: diploid cells and haploid cells.
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Very Short Questions

Chromosomes are known as hereditary materials because they transfer the genetic characters from parents to the offsprings from a generation to other generations.

Chromosome: Chromosomes are the genetic substances that help in the inheritance of characters from parents to their offsprings.

Chromonema: The fine greatly coiled thread or filament contained by chromosome is said to be chromonema.

Chromosomes are of five types, on the basis of presence or absence and position of centromere:

  1. Acentric: If centromere is absent, the chromosome is called acentric chromosome. This chromosome is genetically inactive and does not take part in cell division.

  2. Telocentric: If centromere is located at one end of the chromosome or in the telomeric region, it is called a telocentric chromosome.

  3. Acrocentric: If centromere is present in the subterminal region and the chromosome is divided into a very short arm at one region and other arm is very long, this chromosome is called acrocentric.

  4. Submetacentric: If centromere is present in the submedian region and the chromosome is divided into two equal arms, one of which is slightly longer than other forming L or J shaped chromosome. which is called sub-metacentric chromosome.

  5. Meta Centric: If centromere is present in the median position and the chromosome is divided into two equal arms forming a V-shaped chromosome, it is called metacentric chromosome.


Homologous chromosomes are the pair of chromosomes, one from each parent, which are similar in length, gene position and centromere location.

The major role of chromosomes during the cell division is to inheritance of hereditary characters from parents to their offsprings. They also help in growth and metabolism.

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  • How many pairs of chromosomes are there in pea?

    14
    9
    7
    1
    80
    78
  • How many number of chromosome does a cow have?

    29
    2
    108
    60
    48
    30
  • There are ______ pairs of chromosomes in human.

    16
    28
    43
    46
    2
    23
  • ______ is the example of diploid cells.

    Egg
    Zygote
    Ova
    Sperm
    Spores
    Oocytes
  • Chromosome consists of a fine greatly coiled thread or filament called ______.

    secondary constrictions
    histones
    chromonema
    euchromatin
    heterochromatin
    centromeres
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