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Chromosomes Function and Types

Introduction:

  • The DNA of a eukaryotic cell is packaged into structures called chromosomes.
  • Each chromosome is a long molecule of DNA, which carries hundreds to thousands of genes, with associated proteins that maintain chromosome structure and help control gene activity.
  • This DNA-protein complex is called chromatin.
  • The chromatin of each chromosome is long and thin when the cell is not dividing.
  • Prior to cell division, each chromosome is duplicated, and the resulting sister chromatids are attached to each other by proteins at the centromeres and, for many species, all along their lengths (a phenomenon called sister chromatid cohesion).
  • Chromosomes are present inside the nucleus of the eukaryotic cell and in the nucleoid region of the prokaryotic cell.
  • Chromosome structure was observed by Nageli and the term ‘chromosome’ was coined by Waldeyer.
  • They are meant for equitable distribution of the genetic material.
  • A single human cell has approximately two meter long thread of DNA distributed among its forty six (twenty three pairs) chromosomes.
  • The best stage to study the morphology of chromosomes is the metaphase of cell division as the chromosomes are most condensed at this stage.

Detailed Explanation:

  • The Metaphase stage is the best stage to observe the size, shape, morphology etc. of chromosomes.
  • The metaphasic chromosome (visible only in dividing cells) is a duplicated chromosome which consists of two sister chromatids which are attached at the centromere or primary constriction.
  • On either side of the centromere or with each chromatid is a disc-shaped structure called a kinetochore.
  • Kinetochore is attached to the pole of its side through spindle fibers so it helps in segregation of the duplicated chromosomes towards opposite poles.


    chromosome

     
  • On the basis of position of the centromere, chromosomes are of following types:
    (i) Metacentric : Centromere is found in the middle and at anaphase the chromosome is V-shaped. The two arms are of equal length (Isobrachial).
    (ii) Sub-metacentric : The position of the centromere is subcentral. Anaphasic stage L-shaped. The two arms are unequal (heterobrachial). The short arm is the p arm while the long arm is the q arm.
    (iii) Acrocentric : The centromere is situated close to its end forming one extremely short and one very long arm. Anaphasic stage J-shaped.
    (iv) Telocentric : Centromere terminal, the anaphasic stage is I- shaped.

Ultrastructure of Chromosome:
 

Eukaryotic chromosome contains the following parts:

(i) Chromatid:

  • A metaphasic chromosome consists of two cylindrical structures called chromatids. Both the sister chromatids are joined together at centromere or primary constriction.

(ii) Chromonema :

  • During mitotic prophase, the chromosomal material becomes visible into very thin filaments, called chromonemata.
  • A Chromonema represents a chromatid in the early stages of condensation.
  • Therefore, 'chromatid' and chromonema is the name of the same structure which is a single linear DNA molecule with the associated proteins.
  • The chromonemata form the gene-bearing portion of the chromosomes.
  • According to old observation, a chromosome may have more than one chromonemata.
  • This chromonemata is embedded in the achromatic and amorphous substance, called matrix.
  • The matrix is enclosed inside the sheath or pellicle.
  • Matrix and sheath are non-genetic material which appears in the metaphasic state when the nucleolus disappears.
  • It is also believed that the nucleolar material and matrix are interchangeable.

(iii) Chromomeres:

  • The chromomeres are bead-like accumulations of chromatin material that are sometimes visible along the interphase chromosomes.
  • The chromomere-bearing chromatin has an appearance of a necklace in which several beads occur on a string.
  • Chromomeres become especially clear in polytene chromosomes where they become aligned side by side, constituting the chromosome beads.
  • At metaphase the chromosomes are tightly coiled and the chromomeres are no longer visible.
  • Chromomeres are regions of tightly folded DNA and are believed to correspond to the units of genetic function in the chromosomes

(iii) Centromere (Primary constriction):

  • Narrow non stainable area where two chromatids are joined.The surface has disc or kinetochore on either side for attachment of microtubules belonging to chromosomal fiber.

(iii) Secondary constriction:

  • They are narrow areas of two types. NOR and joints. The NOR or nucleolar organizer region is secondary constriction-I capable of forming nucleolus in telophase. It is found on chromosome number 13, 14, 15, 21, 22 in human beings. Joints or secondary constriction-II are areas involved in breaking and fusion of chromosome segments.

(iv) Satellite:

  • Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.
  • It is knob like part distal to NOR. The chromosome that bears satellite is called SAT (Sine acid Thymonucleinico) chromosome.

(v) Telomeres:

  • These are non-sticky terminal ends of chromosomes or seal ends of chromosomes. They prevent the sticking of one chromosome with another. They are rich in guanine base. Telomerase enzymes are required for replication of this part of the chromosome.


    chromosome

     

The function of chromosomes:

  • The structure of chromosomes contains genes that contain the hereditary information which gets transferred from one generation to another.
  • This controls the synthesis of structural proteins in the cells that supports very vital processes of the living organism such as cell growth and cell division.
  • They control cellular differentiation.
  • This structure has the ability to replicate and produce exact copies of itself which is passed on to daughter cells and the next generation.
  • The sex chromosomes are a type of chromosomes that is responsible for determining the sex of an individual.
  • The process of crossing over in the chromosomes can induce variation.
  • Mutation in chromosomes is defined as the sudden changes or alternation in the genetic sequences of an organism which can lead to changes in the whole population's genetics.
  • The haploid organism contains one set of chromosomes whereas the diploid organism cell contains two sets of chromosomes.
  • SAT chromosomes produce nucleoli for synthesis of ribosomes.

Frequently Asked Questions (FAQs)
 

Question 1. Name and describe the four different types of chromosomes on the basis of centromere ?
Answer:

  • On the basis of position of the centromere, chromosomes are of following types:
    (i) Metacentric : Centromere is found in the middle and at anaphase the chromosome is V-shaped. The two arms are of equal length (Isobrachial).
    (ii) Sub-metacentric : The position of the centromere is subcentral. Anaphasic stage L-shaped. The two arms are unequal (heterobrachial). The short arm is the p arm while the long arm is the q arm.
    (iii) Acrocentric : The centromere is situated close to its end forming one extremely short and one very long arm. Anaphasic stage J-shaped.
    (iv) Telocentric : Centromere terminal, the anaphasic stage is I- shaped.

Question 2. What are the SAT chromosomes?
Answer:

  • Sometimes a few chromosomes have non-staining secondary constrictions at a constant location.
  • This gives the appearance of a small fragment called the satellite. It is knob like part distal to NOR.
  • The chromosome that bears satellite is called SAT (Sine acid Thymonucleinico) chromosome.

Question 3. State the function of chromosomes?
Answer: The function of chromosomes:

  • The structure of chromosomes contains genes that contain the hereditary information which gets transferred from one generation to another.
  • This controls the synthesis of structural proteins in the cells that supports very vital processes of the living organism such as cell growth and cell division.
  • They control cellular differentiation.
  • This structure has the ability to replicate and produce exact copies of itself which is passed on to daughter cells and the next generation.
  • The sex chromosomes are a type of chromosomes that is responsible for determining the sex of an individual.
  • The process of crossing over in the chromosomes can induce variation.
  • Mutation in chromosomes is defined as the sudden changes or alternation in the genetic sequences of an organism which can lead to changes in the whole population's genetics.
  • The haploid organism contains one set of chromosomes whereas the diploid organism cell contains two sets of chromosomes.
  • SAT chromosomes produce nucleoli for synthesis of ribosomes.

Question 4. How many pair of chromosomes are present in a human cell?
Answer:

  • The human cell is composed of 23 pairs of chromosomes, for a total of 46 chromosomes.
  • The 22 pairs of chromosomes are considered to be autosomes.
  • The 23rd pair of chromosomes is said to be sex chromosomes that differ between males and females.

Question 5. Explain the structure of the metaphasic chromosome.
Answer:

  • The Metaphase stage is the best stage to observe the size, shape, morphology etc. of chromosomes.
  • The metaphasic chromosome (visible only in dividing cells) is a duplicated chromosome which consists of two sister chromatids which are attached at the centromere or primary constriction.
  • On either side of the centromere or with each chromatid is a disc-shaped structure called a kinetochore.
  • Kinetochore is attached to the pole of its side through spindle fibers so it helps in segregation of the duplicated chromosomes towards opposite poles.
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