Significance of Chromosomes
The entire human body develops from the fertilized ovum.
It is, therefore, obvious that the fertilized ovum contains all the information necessary
for formation of the numerous tissues and organs of the body, and for their orderly
assembly and function.
Each cell of the body inherits from the fertilized ovum, all the directions that are
necessary for it to carry out its functions throughout life.
This tremendous volume of information is stored within the chromosomes of each
cell.
Each chromosome bears on itself a very large number of structures called genes.
Traits (characters) of an individual are determined by genes carried on his (or her)
chromosomes. As we have seen half of these are inherited from the father and half
from mother.
We have seen above that chromosomes are made up predominantly of a nucleic acid
called DNA, and all information is stored in molecules of this substance.
When the need arises, this information is used to direct the activities of the cell by
synthesizing appropriate proteins.
To understand how this becomes possible, we must consider the structure of DNA in
some detail.
Duplication of Chromosomes
One of the most remarkable properties of chromosomes is that they are able to duplicate
themselves.
Duplication of chromosomes involves the duplication (or replication) of DNA.
This takes place as follows:
• The two strands of the DNA molecule to be duplicated unwind and separate from each
other so that their bases are “free”.
• A new strand is now synthesized opposite each original strand of DNA in such a way that
adenine is formed opposite thymine; guanine is formed opposite cytosine, and vice versa.
, • This new strand becomes linked to the original strand of DNA to form a new molecule.
• As the same process has taken place in relation to each of the two original strands, we
now have two complete molecules of DNA.
• It will be noted that each molecule has one strand that belonged to the original molecule
and one strand that is new.
• It will also be noted that the two molecules formed are identical to the original molecule.
Structure of Fully Formed
Chromosomes
• Each chromosome consists of two parallel rod-like elements that are called chromatids.
• The two chromatids are joined to each other at a narrow area that is light staining and is
called the centromere (or kinetochore).
• In this region, the chromatin of each chromatid is most highly coiled and, therefore,
appears to be thinnest.
• The chromatids appear to the “constricted” here and this region is called the primary
constriction.
• Typically, the centromere is not midway between the two ends of the chromatids, but
somewhat toward one end.
• As a result, each chromatid can be said to have a long arm (denoted by letter q) and a
short arm (denoted by letter p). Based on the position of centromere the chromosomes are
classified as:
1. Metacentric: Centromere is centrally placed and the two arms are of equal length.
2. Submetacentric: Centromere is slightly away from the center and the two arms are
only slightly different in length.
3. Acrocentric: Centromere is nearer to one end and the difference in length of arms is
marked.
4. Telocentric: Centromere lies at one end.
Differences in the total length of chromosomes and in the position of the centromere are
important factors in distinguishing individual chromosomes from each other.
Additional help in identification is obtained by the presence in some chromosomes of
secondary constrictions. Such constrictions lie near one end of the chromatid.
The entire human body develops from the fertilized ovum.
It is, therefore, obvious that the fertilized ovum contains all the information necessary
for formation of the numerous tissues and organs of the body, and for their orderly
assembly and function.
Each cell of the body inherits from the fertilized ovum, all the directions that are
necessary for it to carry out its functions throughout life.
This tremendous volume of information is stored within the chromosomes of each
cell.
Each chromosome bears on itself a very large number of structures called genes.
Traits (characters) of an individual are determined by genes carried on his (or her)
chromosomes. As we have seen half of these are inherited from the father and half
from mother.
We have seen above that chromosomes are made up predominantly of a nucleic acid
called DNA, and all information is stored in molecules of this substance.
When the need arises, this information is used to direct the activities of the cell by
synthesizing appropriate proteins.
To understand how this becomes possible, we must consider the structure of DNA in
some detail.
Duplication of Chromosomes
One of the most remarkable properties of chromosomes is that they are able to duplicate
themselves.
Duplication of chromosomes involves the duplication (or replication) of DNA.
This takes place as follows:
• The two strands of the DNA molecule to be duplicated unwind and separate from each
other so that their bases are “free”.
• A new strand is now synthesized opposite each original strand of DNA in such a way that
adenine is formed opposite thymine; guanine is formed opposite cytosine, and vice versa.
, • This new strand becomes linked to the original strand of DNA to form a new molecule.
• As the same process has taken place in relation to each of the two original strands, we
now have two complete molecules of DNA.
• It will be noted that each molecule has one strand that belonged to the original molecule
and one strand that is new.
• It will also be noted that the two molecules formed are identical to the original molecule.
Structure of Fully Formed
Chromosomes
• Each chromosome consists of two parallel rod-like elements that are called chromatids.
• The two chromatids are joined to each other at a narrow area that is light staining and is
called the centromere (or kinetochore).
• In this region, the chromatin of each chromatid is most highly coiled and, therefore,
appears to be thinnest.
• The chromatids appear to the “constricted” here and this region is called the primary
constriction.
• Typically, the centromere is not midway between the two ends of the chromatids, but
somewhat toward one end.
• As a result, each chromatid can be said to have a long arm (denoted by letter q) and a
short arm (denoted by letter p). Based on the position of centromere the chromosomes are
classified as:
1. Metacentric: Centromere is centrally placed and the two arms are of equal length.
2. Submetacentric: Centromere is slightly away from the center and the two arms are
only slightly different in length.
3. Acrocentric: Centromere is nearer to one end and the difference in length of arms is
marked.
4. Telocentric: Centromere lies at one end.
Differences in the total length of chromosomes and in the position of the centromere are
important factors in distinguishing individual chromosomes from each other.
Additional help in identification is obtained by the presence in some chromosomes of
secondary constrictions. Such constrictions lie near one end of the chromatid.
