Class 12 Genes and Chromosomes NEET AIIMS
Genes and Chromosomes
Chromosomal basis of inheritance
What are Genes?
Genes are discrete particles which act as the carriers of inheritance.
The Big Question is ‘How do genes travel from one generation to another’?
The answer was proposed by Sutton and Boveri in 1902 as ‘The Chromosomal Theory of Inheritance’. This theory was later improved upon by Morgan.
The concepts proposed by Chromosomal Theory of Inheritance are –
1. Genes are carried by chromosomes.
2. The specific location of a gene on the chromosome is called locus.
3. A somatic (body) cell contains Diploid (2n) = a double number of chromosomes. It automatically means it has 2 sets of genes or factors.
4. The Gametic Cells – Sperm (from males) and Ovum (from females) have haploid – Single set of chromosomes. It automatically means it has one set of genes.
5. Chromosomes are usually present in the nucleus. Hence the nucleus plays a role in heredity. (proved by Hammerling while experimenting an Acetabularia – a green alga).
6. During gamete formation, the number of chromosomes is halved.
7. The number of chromosomes is restored at the time of fertilization due to the fusion of gametes.
8. Chromosomes (like genes) follow the Law of Segregation and the Law of Independent Assortment.
Fact Foundation
1. A human cell consists of 2.2 m of DNA distributed among 46 Chromosomes.
2. Each chromosome contains 4.8 cm of DNA.
3. Human chromosomes are as an average 6µ long.
4. The Packing Ratio is 8000; 1, i.e., the DNA is compressed 8000 times to be enclosed in the chromosome.
Chromosomes
· The term chromosomes means (chrome + soma = coloured body). They were first seen by Hofmeister in pollen mother cells of Tradescantia plant (Rhoeo discolour)
· Term chromosomes were first given to Waldeyer.
General Features of Eukaryotic Chromosome :
SHAPE: They may be Rodlike, Bent, Horseshoe shaped or Dot like structures.
SIZE = A normal chromosome measures 0.5 – 30 µm in length and 0.2 – 3.0 µm in breadth.
A general rule is that organisms having a lesser number of chromosomes have larger chromosomes.
Structural Details of Eukaryotic Chromosome
Follow the sequence –
· DNA + Proteins Þ Chromonema Þ Chromonema joined Together Þ Chromatin Threads (seen during Interphase)
· Chromatin Threads condense to form Chromosomes (after Interphase).
· Chromatin Threads have a ‘string of bead’ appearance when seen under a microscope.
· The string of beads is made up of a complex of proteins and DNA entwined with each other.
· The complex is called Nucleosome of Nu Body.
As in the diagram –
· nucleosome is made up of a core protein molecule.
· This core is made up of 8 Histone Protein Units. (Octamer).
What are Histone Proteins?
1. Histone Proteins are Low Molecular Weight proteins.
2. They are rich in Basic Amino Acids – Lysine and Arginine. 3. 5 types of Histone Proteins are known to occur H 1 – Rich in lysine
4. Histone Proteins are found only in Eukaryotic chromosomes.
5. Histone Proteins are responsible to prevent RNA Transcription.
The core molecule is made up of 2 units each of H2A, H2B, H3, H4. There is a coiled DNA taking 1¾ turn made up of 146 base pairs.
· A linker DNA made up 54 – 60 base pairs hangs out of the core molecule and attaches to 1 unit of H 1 Protein.
· Thus a total of 200 – 206 base pairs of DNA, along with the core protein constitutes the nucleosome (Nu Body).
· The structure of Nucleosome was described by Oudet.
Remember :
Another type of protein called Non-Histone Protein (NHP) is also present in the chromosome.
1. These are High Molecular Weight proteins.
2. They are rich in amino acids like Tyrosine and Tryptophan.
3. They are present bothy in Eukaryotic and Prokaryotic Chromosomes.
4. They help in stimulating RNA Transcription.
5. They may either form Structural or Scaffold Protein; Enzymatic Proteins (DNA or RNA polymerase) or Regulatory proteins like HMG (High Mobility Group).
Sometimes a secondary constriction is also seen in the chromosome. Secondary constriction has 2 main parts – Joints and NOR (Nucleolar Organizer Region).
In Humans-Chromosome 13, 14, 15, 20, 21 has NOR, while Chromosomes 13, 14, 15, 21, has NOR, while Chromosomes 13, 14, 15, 21, 22 are SAT chromosome.
SAT (Sine Acid Thymonucleico) – which means (Less of Nucleic Acid containing Thymine) i.e., Less DNA. Hence it is stained poorly.
The part of chromosome beyond the secondary constriction is called Satellite. The part of satellite beyond NOR is called Trabant.
The centromere is almost entirely without DNA and appears as an unstained portion of the chromosome.