Morphology of Chromosomes

Shirin Shahana.N
BSc.Biotechnology
SIAS

1. M O R P H O L O G Y O F C H R O M O S O M E S S h i r i n S h a h a n a . N 3 - B i o t e c h n o l o g y S I A S

2. I N T R O D U C T I O N Chromosomes are rod shaped, dark stained bodies seen during metaphase. The term 'chromsome' was first used by Waldeyer in 1888. Each species has a definite chromosome number, represented by 2n. Matic Somatic cells contain two copies of each chromosomes, which are identical in morphology, gene content and gene order and they are known as homologous chromosomes.

3. M O R P H O L O G Y D u r i n g c e l l d i v i s i o n the following structural features can be seen under light microscope by staining. 1.Chromatid . 2.Centromere. 3. Telomere . 4.Secondary constriction 5 . S a t e l l i t e c h r o m o s o m e s .

4. CHR O M A T I D It is the structural and functional unit of chromosomes. At Metaphase, each chromosome appears to the longitudinally divided into two identical parts, each of which is known as a Chromotid . The chromatids of a chromosome appear to have joined the giller at a point called centromere . The two chromatid making up a chromosome are produced through replication of a single chromatid , they are referred as Sister chromatids .

5. In contrast , the chromat ids of homologous chromosomes are known as N o n - sister chromatids .

6. C E N T R O M E R E The region where 3 sister chromatids of a chromosome appeared to be held together is called cent r omere. Under light microscope, centomere generally appears as a constriction in the chromosome, here it is also termed as primary constriction . Centromeres are the first part moving towards the opposite poles during anaphase; the remaining regio n s lag behind and appear as if they were being pulled by the centomere. T herefore, chromosome movement is due to the centromeres of chromosomes hence they are also known as k i n e t o c h o r e s .

7. In most spells each chromosome has a single centromere in a fixed position which does not change except due to structural chromosome aberrations. Therefore, the position of centromere serves as an important landmark in the identification of different chromosomes of a species. Each chromosome is divided into two transverse parts by its centromere; t hese parts are called a r ms .

8. TY P E S O F C H R O M O S O M E S B a s e d o n position of centromere : Metacentric : center of the chromosome having equal arms and appeared as 'V' shaped during anaphas e . Submetacentric : Centromere is on one side . Actocentric : C entromere is located close to one end . Telo centric : C entromere appeared to be at one end of the chromosome .

9. TE L O M E R E T h e two ends of a chromosome are c a l l e d t e l o m e r e s . They are highly stable and do not fuse with other chromosomes. It is generally accepted that, the structural integrity and individuality of chromosomes is maintained due to the telomeres and that all stable chromosome ends are composed of telomeres.

10. S E C O N D A R Y CONSTRI C T I O N A N D S A T E L L I T E CHROMOSOMES I n s o m e chromosomes , a s e c o n d constriction in addition to primary constriction is also pres e n t . I t i s cal l e d s e c o n d a r y c o n s t r i c t i o n . It is present in short arm near one end or in many chromosomes , t hey are located in the long arm nearer to the centromere. The region between the secondary constriction and the nearest telomere i s c a l l e d satellite. C hromosomes having a secondary constitution are called s a t e l l i t e c h r o m o s o m e s ( S A T ) . The number of Sat Chromosomes in the genome varies from one species to the other.

11. Human somatic cells have 10 Sat Chromosomes. N ucleolus is always associated with the secondary constriction of S A T c h r o m o s o m e s . Therefore , secondary constrictions are also called as Nucleolus oraniser Region (NOR) S A T Chromosomes are often referred to as Nucelolus organism chromosome (NOC) . NOR contains several hundred copies of the gene coding for ribosomal RNA. (r RNA).