Describe the ultrastructure of chromosomes.

Ans.  : A)   Chromosomes appear as a thread like structure by the end of prophase and in metaphase and anaphase show distinct shape and size.

B) The structure can be discussed as follows -

a) Size:

1) Different species of organisms vary in their size of chromosomes.

2)  Metaphase chromosomes are 0.1 to 33 mm in length and 0.2 to 2 mm        in thickness.

3)  Chromosomes in plants are larger than animals.

b)  Shape:

1)   The shape of the chromosome is decided by the stages of cell division.

2)  During interphase the chromatin network appears representing        chromosome.

3)  In metaphase, due to maximum condensation takes place and chromosomes become thick & short.

c)  Structure:

1)   The metaphase chromosome shows two identical parts of chromatids adjacent to each other known as sister chromatids.

2)  These two parts are attached at a point called on primary constriction on centromere.

3)  Each chromatid shows presence of subchromatids known as      chromonemata.

4)  The centromere bears plate like kinetochore.

5)  Attachment of spindle fibres at the time of cell division takes place at centromere.

6)  Along with primary constrictions, secondary constrictions are also observed.  These few are called as nucleolar constrictions as the play major role in formation of nucleolus.

7)  The part of chromosome beyond the nucleolar organizer is short,     spherical called as satellite

8)  Telomere represents the tips of chromosomes.

9)  Telomere helps in attachment of chromosome with the nuclear envelope but prevents sticking of chromosomes together.

10) The surface of chromosomes shows number of swellings called as chromosomes.

        (REFER DIAGRAM FROM TEXTBOOK)

Explain different types of chromosomes in details?

Ans.  : On the basis of position of centromere the different types of chromosomes are as follows -

a)  Metacentric:

In this case the centromere occupies exactly middle position.

Due to which the two halves become equal and chromosome appears V - shaped.

b)  Sub Metacentric:

In such chromosomes, centromere occupies position slightly away from the mid-point and due to this the chromosomes appears ‘L’ or ‘J’ shaped.

c)  Acrocentric:

These are rod shaped chromosomes where the centromere is very near the proximal end and thus has a small arm on this end.

d)  Telocentric:

Centromere occupies position at the proximal end on a rod like chromosomes.

 (REFER DIAGRAM FROM TEXTBOOK)

What are the various function of chromosomes?

Ans.:1) Chromosomes are responsible for inheritance of characters.

         2) Regulation of protein synthesis.

         3) Helps in cell division and cell growth.

         4) Control cell metabolism through regulation of enzyme synthesis.

         5) Determination of sex is controlled by sex chromosomes.

         6)  Replication is important property as it produces exact copies which are passed onto daughter cells.

    Describe the structure of sex chromosomes or X & Y chromosomes.

     Ans. : The two sex chromosomes are X-chromosome and Y- chromosome. The structure can be explained as follows:

     A) X - Chromosome:-

     1) The X - chromosomes is sub metacentric with the centromere located slightly away from the middle.

     2) It shows two types of segments namely, homologous and non-homologous chromosomes.

     3) The homologous part shows identical gene loci and cross over during meiosis.

     4) The non- homologous region carry X - linked genes which show

     X - linked inheritance.

     5) The X - chromosomes shows large amount of Euchromatin and small amount of heterochromatin.

     6) X - chromosome is present in both males and females.

     7) X - chromosomes is present in both males and females.

     8) It decides female sex of zygote.

     9) The linkage shown by X - chromosome is known as X - linkage.

     10)  X - chromosomes shows crisscross inheritance.

     B) Y - Chromosomes:-

     1) Y - chromosomes is present in males and absent in females.

     2) It shorter than X - chromosomes and curved at end except humans.

     3) It shows centromere at one end hence it is acrocentric in nature.

     4) If shows homologous segment which shows crossing over during meiosis.

     5) The non-homologous part do not carry same gene loci and do not show crossing over.

     6) The genes present on non-homologous part are called as Y - linked gene and    show Y - linked inheritance

     7) Y - Chromosomes contain small amount of Euchromatin and large amount    of heterochromatin.

     8) The linkage shown by Y - Chromosomes is known as Y - linkage.

     9) Y - Chromosomes shows straight inheritance.

     10)  It determines male sex.

             (REFER DIAGRAM FROM TEXTBOOK)

    Write a note on importance of heterosomes.

     Ans. : 1)  Heterosomes play an important role in determination of sex of an individual.

     2) The maleness is determined by Y - chromosome while femaleness is decided by X - chromosomes

     3) Heterosomes carry several sex linked genes.

     4) The X - linked genes are responsible for transmission of number of sex linked   diseases such as colour blindness, hemophilia etc.

     5) The Y - linked genes are called as holandric genes and are responsible for hypertrichoisis.