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CHROMOSOMES

INTRODUCTION

Chromosomes are compact rod shaped structures formed during the cell division. Each chromosome is made up of a long and single DNA molecule wrapped around proteins.


In a non-dividing cell, DNA is already present in a coiled and compact form as Chromatin. Chromatin is formed when negatively charged DNA is wrapped around positively charged histone proteins. Due to this special packing, it is possible for very long DNA molecule to fit inside the nucleus. The DNA consist of genetic information in the form of sequence of nucleotides called Genes. So, it is very important that at the time of cell division, the DNA remains intact and gets evenly distributed among the daughter cells. To ensure this, the chromatin are further condensed into long, compact and rod-shaped structures called Chromosomes.


structure OF CHROMOSOME


Chromatid

Each chromosome consist of either a single or a pair of Chromatids (depending of the phase of the cell cycle). When a pair of chromatids is present in a chromosome, these are called sister chromatids as the DNA content is exactly the same.


CENTROMERE

Centromere is the narrow and constricted part of the chromosome. It consists of proteinaceous disc like structures called Kinetochores which serves as the site of attachment of spindle fibres during cell division. Centromere is also called Primary constriction.


ARMS

The part of chromatid present on either side of the centromere are called Arms of the chromosome. A chromosome can have 1-4 arms in number.


TELOMERES

Telomeres are the repetitive part of DNA (Guanine rich) located at the terminal part of chromosomes. In many cells, some part of the telomere is lost every time the DNA is replicated.

A chromosome can have 1-4 telomeres.

CONSTRICTION

Secondary constrictions are the constricted or the narrow region found at any point of the chromosome other than that of centromere.

The secondary constriction marks the site of a Nucleolar Organizer Region (NOR), a region containing multiple copies of the 18S and 28S ribosomal genes that synthesize ribosomal RNA required by ribosomes. The part of the chromosome present beyond the secondary constriction is called Satellite DNA.













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