Tnpsc: Genetics, DNA, Genes and Chromosomes

What is Genetics?

Genetics is the branch of biology that deals with the genes, genetic variation, and heredity of living organisms.

What is heredity?

It is the character transmission from one generation to the next generation.

What is Variation?

It is the differences exhibited by the individuals of the same species and also by the offspring of the same parent.

All the above three happen only because of chromosomes and their composition with DNA which forms the genetic material.

Father of Genetics

Gregor Johann Mendel (1822-1884), an Austrian Monk, found the basics of heredity by his experiments which are considered the foundation of modern genetics. Mendel did his experiments on the garden Pea plant.

He conducted an experiment on nearly 10000 pea plants of 34 different varieties. Mendel noted that each pea plant differs from one another. He chose 7 pairs of distinct characters from these plants they are

Characteristics of Pea plant by Mendel

• Pea/Seed shape is round or wrinkled
• Pea/Seed colour is yellow or green
• Pea/Seed coat colour is coloured or white

• Pod shape is constricted or inflated
• Pod colour is green or yellow
• stem size is tall or short
• Position of flowers is axial or terminal

The basis for success for Mendels Experiments

Choosing the pea had many advantages for experiments such as:

1. It is very easy to grow pure breeding Peas, as it is naturally self-pollinating.
2. It has a short life span as it is an annual crop and so it was feasible to go along with several generations.
3. Easy cross-pollination.
4. Contrasting character is deeply defined.
5. It has bisexual followers.

Some outcomes from Mendel Genetics Experiments

Factors that are passed from one generation to another are called Genes. Factors that occur in pairs and maybe alike as in pure breeding tall plants (TT) and dwarf plants (tt), it is called Homozygous.

If they are unlike (Tt) they are called Heterozygous. Two factors making up a pair of contrasting characters are called Alleles or allelomorphs, one of each pair is contributed by one parent.

The character which expresses itself is called the Dominant condition and that which is masked is called the recessive condition.

Mendel’s Laws of Genetics

Based on experiments with Monohybrid and Dihybrid cross, Mendel devised three laws called the Law of Heredity.

Law of Dominance

” When two homozygous individuals with one or more sets of contrasting characters are crossed, the characters that appear in the F1 hybrid are dominant and those that do not appear in F1 are recessive characters”.

Law of Independent assortment

“In case of inheritance of two or more pairs of characters simultaneously, the factors or genes of one pair assort out independently of the other pair.”

Law of Segregation or Law of purity of gametes

“When a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote or hybrid, the two members of the allelic pair remain together without mixing and when gametes are formed, the two separate out, so that only one enters each gamete.”

Chromosomes, DNA and Genes

Every living organism is made up of cells and the nucleus of each cell contains thin thread-like structures known as Chromosomes. In 1888, the word Chromosome was coined by Waldeyer.

The chromosomes are the transporter of hereditary material which contains the heredity data. The chromosome is packed with DNA (Deoxyribonucleic acid) which forms genetic material.

Genes are segments of DNA, that are responsible for the inheritance of a unique Phenotypic character. Genes are present in a particular place on a chromosome called its Locus.

Gene information is passed from one generation to another generation during cell division.

Structure of chromosome with Diagram

It is a thin, long thread-like structure with two identical strands called Sister chromatids. These chromatids are held together by the centromere. Each chromatid is made up of a spirally coiled thin structure called Chromonema.

The chromonema has a number of bead-like structures along the length called Chromomeres. The chromosomes are made up of DNA, RNA chromosomal proteins such as histones and non-histones, and certain metallic ions.

These proteins provide structural support to the chromosomes.

A chromosome consists of:

Primary Constriction

Two arms of a chromosome meet at a point called Primary constriction or centromere. The centromere is the region where spindle fibers attach to the chromosomes during cell division.

Secondary Constriction

Some chromosomes possess secondary constriction at any point of the chromosome. They are known as the nuclear zone or nucleolar organizer (formation of the nucleolus in the nucleus).

Telomere

The end of the chromosome is called a telomere. Each extremity of the chromosome has a polarity and prevents it from joining the adjacent chromosome. It maintains and provides stability to the chromosomes.

Telomeres act as the aging clock in every cell. Telomeres are protective sequences of nucleotides found in chromosomes. As a cell divides every time, they become shorter. Telomeres get too short to do their job, causing our cells to age.

Satellite

Some of the chromosomes have elongated knob-like appendages at one end of the chromosome known as the satellite. The chromosomes with satellites are called the sat-chromosomes.

Types of Chromosomes based on the position of the centromere

Based on the position of the centromere, the chromosomes are classified as Telocentric, Acrocentric, Submetacentric, and Metacentric.

Types of Chromosomes

Telocentric – The centromere is found on the proximal end. They are rod-shaped chromosomes.

Acrocentric – The centromere is found at the one end with a short arm and a long arm. They are also rod-shaped chromosomes.

Submetacentric -The centromere is found near the center of the chromosome. Thus forming two unequal arms and are J-shaped or L-shaped chromosomes.

Metacentric – The centromere occurs in the center of the chromosome and form two equal arms and are V-Shaped chromosomes.

Types of Chromosomes based on function

The eukaryotic chromosomes are classified into autosomes and allosomes. Autosomes contain genes that determine the somatic characters. Male and females have an equal number of autosomes.

Allosomes are chromosomes that are responsible for determining the sex of an individual. They are also called sex chromosomes or hetro-chromosomes. There are two types of sex chromosomes, X and Y-Chromosomes.

The human male has one X Chromosome and One Y chromosome. The human female has a two-X chromosome.

Karyotype

The number of chromosomes in any living organism is constant. In humans, each cell normally contains 23 pairs of chromosomes. Out of which 22 pairs are autosomes and the 23rd pair is the allosome or sex chromosome.

In the body cells of sexually reproducing organisms, the chromosome generally occurs in pairs. This condition is called diploid (2n). The gametes produced by the organisms contain a single set of chromosomes.

Hence, the gametes are said to be haploid(n). The karyotype is the number, size, and shape of chromosomes in the cell nucleus of an organism.

Idiogram is the diagrammatic representation of the karyotype of a species. It consists of all the metaphasic chromosomes arranged in homologous pairs according to decreasing length, thickness, the position of centromere, shape, etc, with the sex chromosomes placed at the end.

Structure of DNA

DNA is the hereditary material as it contains genetic information. It is the most important constituent of a chromosome. The most widely accepted model of DNA is the double-helical structure of James Watson and Francis Crick.

They proposed the 3-D model of DNA on the basis of X-ray diffraction studies of DNA obtained by Rosalind Franklin and Maurice Wilkins.

In appreciation of their discoveries on the molecular structure of nucleic acids Watson, Crick, and Wilkins were awarded the Nobel prize for Medicine in 1962.

Chemical Composition of DNA Molecule

DNA is a large molecule consisting of millions of nucleotides. Hence it is also called a polynucleotide.

Each nucleotide consists of three components.

• A Sugar Molecules – Deoxyribose sugar.
• A Nitrogenous Base. There are two types of nitrogenous bases in DNA. They are
• Purines (Adenine and Guanine)
• Pyrimidines (Cytosine and Thymine)

A Phosphate Group

Nucleoside and Nucleotide

• Nucleoside = Nitrogen base + Sugar
• Nucleotide = Nucleoside + Phosphate

The nucleotide is formed per the purines and pyrimidines present in them.

Watson Crick Model of DNA

According to the Watson crick model of DNA, a DNA molecule consists of two polynucleotide chains. These chains form a double helix structure with two strands that run anti-parallel to one another.

Nitrogenous bases in the centre are linked to sugar-phosphate units which form the backbone of the DNA.

• Pairing between the nitrogenous base is very specific and is always between purine and pyrimidine linked by hydrogen bonds.
  • Adenine (A) links Thymine (T) with two hydrogen bonds (A=T)
  • Cytosine (C) links Guanin (G) with three hydrogen bonds (C=G)
  • This is called the complementary base pairing.

Hydrogen bonds between the nitrogenous bases make the DNA molecule stable. Each turn of the double helix is 34 Å (3.4nm). There are ten base pairs in a complete turn. The nucleotides in a helix are joined together by phosphodiester bonds.

Chargaff Rule of DNA base Pairing

Erwin Chargaff states that in DNA, the proportion of adenine is always equal to that of thymine. The proportion of guanine is always equal to that of cytosine.

Dna Replication

What is DNA Replication?

It is the basic process that happed within a cell. By DNA Replication, a DNA molecule produces exact copies of its own structure during the replication process.

DNA Replication Steps

1.Origin of Replication

The point where the replication begins is the site of origin. The strands open and separate at this point forming the replication fork.

2. Unwinding of DNA molecule

Enzymes involved in dna replication:

Helicase and Topoisomerase are the enzymes involved in DNA replication. The function of Helicase and Topoisomerase are discussed below.

Helicase – Helicase bind to the origin of replication and separates the two strands of DNA.

Topoisomerase – Topoisomerase enzyme separates the double helix above the replication fork and removes the twist formed during the unwinding process.

Each of the separated DNA strands functions as a template.

3.Formation of RNA Primer

An RNA primer is a short segment of RNA nucleotides. The primer is synthesized by a DNA template close to the origin of the replication site.

4. Synthesis of new complementary strand from the parent strand

After the RNA primer formation, nucleotides are added with the help of an enzyme DNA polymerase and a new complementary strand of DNA is formed from each of the parent strands.

The synthesis is unidirectional. In one strand, the daughter strand is synthesized as a continuous strand which is called the leading strand. In the other strand, short segments of DNA are synthesized.

This strand is called the Lagging strand. The short segments of DNA are called Okazaki fragments. The fragments are joined together by the enzymes, DNA ligase. The replication stops when the replication fork of the two sides meets at a site called terminus.

Terminus is situated opposite to the origin of a replication site.

Significance of DNA

DNA is responsible for the transmission of hereditary information from one generation to the next generation.DNA contains information required for the formation of proteins.

It controls the development process and life activities.

Sex determination in Humans

Human has 23 pairs of chromosomes out of which 22 pairs are autosomes and the 23rd pair is the sex chromosome. The female gametes or the eggs formed are similar in their chromosome type (22 + XX).

Therefore human females are homogametic. If the egg (X) is fused by the X-bearing sperm an XX individual (female) is produced.If the egg (X) is fused by the Y-Bearing sperm an XY Individual (male) is produced.

The sperm produced by the father determines the sex of the child. The mother is not responsible for determining the sex of the child.

Fertilization of the egg (22 + X) with a sperm (22 + X) will produce a female child (44 + XX).

Fertilization of the egg (22 + X) with a sperm (22 + Y) will give rise to a male child (44 + XY).

Mutation

The term mutation was introduced by Hugo De Vries in 1901. He observed phenotypic changes in the evening primrose plant, Oenothera lamarckiana.

What is Mutation in Biology?

A mutation is an inheritable sudden change in the genetic material (DNA) of an organism. Mutations are classified into two main types of mutation. That is

Classification of mutation

1. Chromosomal Mutation
2. Gene Mutation

Chromosomal Mutation

The sudden change in the structure or number of chromosomes is called a chromosomal mutation. This results in

Structural changes in chromosomes – This occurs due to error in cell division. Changes in the number and arrangement of genes take place as a result of deletion, duplication, inversion and translocation in chromosomes.

Number changes in Chromosomes – This involves addition or deletion in the number of chromosomes present in a cell. This is called Ploidy.

There are two types of Ploidy.

Euploidy

Aneuploidy

Euploidy

• Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes.
• If an individual has three haploid sets of chromosomes, the condition is called Tri-ploidy (3n).
• Triploid plant and animals are typically sterile.
• If it has four haploid sets of chromosomes, the condition is called tetra-ploidy (4n).
• Tetraploid plants are advantageous , as its increases fruits and flower size.

Aneuploidy

It is the loss or gain of one or more chromosomes in a set.

It is of three types.

• Monosomy (2n-1)
• Trisomy (2n+1)
• Nullisomy (2n-2)

Down’s Syndrome is one of the commonly known aneuploid conditions.

Down Syndrome

This condition was first identified by a doctor named Langdon Down in 1866.

Down Syndrome Causes and Symptoms

Down Syndrome Causes

It is a genetic condition in which there is an extra copy of chromosome 21 (Trisomy 21).

Down Syndrome Symptoms

It is associated with mental retardation, delayed development, behavioral problems, weak muscle tone, vision, and hearing disability are seen in these children.

Gene or Point Mutation

What is Point Mutation?

Gene mutation is the changes occurring in the nucleotide sequence of a gene. It involves substitution, deletion, insertion, or inversion of a single or more than one nitrogenous base. Gene alternation results in abnormal protein formation in an organism.

Sickle Cell Anaemia

Sickle cell Anemia is caused by the mutation of a single gene, which is a mutation in the hemoglobin-Beta gene found on chromosome 11. Alteration in the gene brings a change in the structure of the protein part of the hemoglobin molecule.

Due to the change in the protein molecule, the red blood cell (RBC) that carries the hemoglobin is sickle-shaped.