Chromosomes

by Ramneet Kaur
620 views

Chromosomes
Chromosome term was given by Waldeyer.

  • Chromosomes are seen during the Metaphase stage of cell division.
  • Chromatin reticulum seen during the interphase coils to form chromosomes during cell division.

Chromatin shows 2 types of coiling behavior:

  • Euchromatin: loosely coiled chromatin. It is transcriptionally active.
  • Heterochromatin: densely coiled chromatin. It is transcriptionally inactive and consists of repetitive DNA. It is present in centromere, telomere & satellite.

Packaging of DNA in chromatin:

Chromatin is made of DNA and histone proteins, in the ratio 1:1.

Histone proteins are basic proteins, rich in Lysine and Arginine.

Advertisements
  • They are of 5 types H1, H2A, H2B, H3 & H4.

Chromatin consists of repeating units called Nucleosomes.

  • Nucleosomes appear like ‘beads on a string’.
  • Kornberg proposed the Nucleosome model.
  • A nucleosome consists of the core particle and a linker DNA.
  • Core particle is an octamer of 4 histone proteins; H2A, H2B, H3 & H4 each present twice. DNA strand of 146 bp surrounds the octamer. It makes 1¾ coil around the octamer.
  • Linker DNA; is a DNA strand of about 8 to 114 bp that joins 2 core particles & completes the 2nd coil around the octamer. H1histone protein stabilizes the 2 coils around the octamer.

nucleosome

Diameter of the nucleosome is 11nm.

Nucleosomes are coiled in the form of a solenoid, having 6 nucleosomes per coil. They form the chromatin thread having a diameter of 30nm.

The chromatin thread further coils with the help of Non-histone chromosomal proteins.

Non-histone chromosomal proteins form a central scaffold around which the solenoid forms loops. It further coils to form chromosomes.

  • Non-histone chromosomal proteins are rich in Tryptophan & Tyrosine.

choromsome formation

Chromosome structure:

A chromosome consists of two chromatids attached to each other at the centromere or the primary constriction.

Lying on the either side of the centromere are disc-like structures, the kinetochore to which spindle fibers attach during cell division.

Specialized ends of chromosomes are Telomeres.

  • Telomeres help the chromosomes to maintain their identity.
  • Telomeres are tandem repeats of ‘TTAGGG‘.

chromosome structure

Satellite chromosomes / SAT-chromosomes:
  • In some chromosomes besides the primary constriction, a secondary constriction is also present.
  • Also referred as the Nucleolar organizer (NOR). They are associated with the nucleolus and help in the formation of nucleolus at the end of cell division.
  • In man chromosome 13, 14, 15, 21, 22 have the NOR.
  • Satellite is the region outer to the secondary constriction & is usually spherical.
  • SAT represents Sine Acid Thymidine i.e., this region is without Thymidilic Acid.
Chromosomes are divided into different types according to the position of centromere:
  • Metacentric: the centromere is present at the center, dividing the chromatid into 2 equal arms.
  • Sub-metacentric: the centromere is present slightly away from the center, dividing the chromatid into 2 unequal arms.
  • Acrocentric: the centromere is present slightly away from the tip, dividing the chromatid into 2 unequal arms.
  • Telocentric: the centromere is at the tip.

types of chromosomes

Karyotype:

Complete set of chromosomes in a species or in an individual organism.

  • Karyotypes describe the chromosome number in an organism and how these chromosomes look like under a light microscope.
  • Length of a chromosome, the position of the centromere, its banding pattern, differences between the sex chromosomes and autosomes and any other physical characteristics are considered.

karyotype

Idiogram is a diagrammatic representation of a karyotype of a species.

idiogram

Genome: is the complete set of chromosomes/genes in an organism.

Autosomes: Chromosomes other than sex chromosomes (X & Y) are autosomes.

  • Autosomes appear in pairs.

Homologous chromosomes are chromosomes having the same number, type, and arrangement of genes. But one is paternal and one is maternal.

Also, read DNA-structure-Mnemonic

You may also like

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Lets make Biology simple

This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More

Subscribe
SUBSCRIBE NOW

Join Our Newsletter Today!

Stay updated with all latest MCQs, Topic Summary, Mnemonics, Videos, tips & tricks for Biology.
close-link

Send this to a friend

Skip to toolbar