Sunday,26 September 2021, 7 : 18 AM

Fish Genetics

Animal Cell
Cell & Chromosome
Image Credit: Wikipedia


Genetics is a branch of biology which deals with study of heredity and variation.
Heredity relates to that science by which living beings resembles to their relatives and ancestors.
Variation means the science by which living being differs from their relatives and ancestors.
Thus Genetics is the science to study how various characters are transferred from parents and develop into offspring.
These offspring are similar to their parents but are never a carbon copy (clones excluded).
Before we take this up further let us understand some basic terms upon which this branch of science is based.


Animal Cell

Parts of An Animal Cell

All living things are made up of microscopic cells.
Cells are the structural and functional unit of the body.
All living things develop from a pre existing cell.
Some living things can consist of a single cell like Bacteria, yeast, amoeba etc.
However most plants and animals that we see around are made up of millions or billions of cells.
Cells are very small and most of them can be only seen with a microscope.
However some are big enough to be seen with naked eye.
A bird egg is one such example ( the yellow yolk part only).
An ostrich cell just before development is the largest cell in the world.


A Cell has following parts:

  • Cell Membrane:
    Cell membrane is the outer most layer of membrane surrounding the cell.
    It is a semi permeable living membrane, and allows some material pass through while retaining others.
    Plant cells contain a extra layer called "cell wall" and it surrounds the Cell membrane.

  • Cytoplasm : It is the part of the cell inside the cell membrane but outside the Nucleus.
    It is semi liquid in nature and contains other units which are not being discussed here.

  • Nucleus: It is a large somewhat spherical body inside the cell.
    It is surrounded by a double layered nuclear membrane with pores.
    The ground substance of nucleus is a semi solid substance called nucleoplasm.
    It contains one or more round shaped nucleoli.
    The nucleoplasm contains a network of dark coloured fibres called chromatin fibres.
    These chromatin fibres condense into short thick structures called Chromosomes.
    Although every part of the cell contributes to the life of the cell as whole, yet the key regulator of each cell is the nucleus.


As explained above, the nucleus contains thin strands called chromatin fibres.
These fibres condense to form "Chromosomes" just before cell division.
Each Chromosome consists of two &Chromatids" which are joined at some point along the length.
This point of attachment is called as "centromereter".
The number of Chromosomes of an individual of most species is constant.
Also each body somatic cell has same number.
Chromosomes always occur in pairs therefore the chromosome number is always in multiple of 2.
As an example all humans have 46 chromosomes, and they are present in 23 pairs.
In humans, Chromosome pair numbers from 1-22 pair are similar in size and shape and are derived as one each from two parents.
These identical chromosome pairs are called autosomes.
In humans, The 23rd pair is is different and its chromosomes are called Sex Chromosomes which are designated as X and Y.
In females both pairs are similar and called XX while in males the pairs are not similar and designated as XY.
The Chromosome/chromatin material consists of DNA and Histones (proteins).



Structure of An DNA Molecule

DNA stands for deoxyribonucleic acid and is composed of two complementary strands wound around each other like a double helix.

This double helical and each DNA strand in turn winds around a core of 8 histone molecule.
This can be imagined like a football around which a long rope is tightly wound in loops.
Each such complex is called as nucleosome.
DNA is a very large molecule and is composed of two complementary strands wound around each other like a double helix.
Each single DNA strand is composed of repeating unit known as Nucleotides (de-oxy nucleotide).
Each nucleotide is made up of which are made up of three components viz.,
  • a phosphate moity,
  • sugar(de-oxyribose sugar pentose),
  • any one of the nitrogenous base (Adenine, Guanine, cytosine and Thymine).

This structure was worked out by Watson and Crick in 1953.


All species have fixed number of chromosomes but the characteristics of any species including physical appearance, body functions, behavior etc is not simply the outcome of chromosome number but these are result of units called Genes which the chromosomes carry.

Genes are specific part ( DNA segments) of a chromosome, which determine the hereditary characteristics.
In order to understand the concept of gene one must understand how the information encoded(present) in DNA is used to perform a function.
This is explained by what is known as Central Dogma.

In reality DNA itself does not perform any function.
It only acts as a blue-print or map.
The cellular machinery must read and interpret this information to perform all molecular and metabolic processes of a cell.

The first step is called Transcription in which the DNA is used as a template ( blueprint) by a enzyme (protein) called RNA polymerase.
This actually sits on the DNA strand and then scans the length to read the DNA sequence and simultaneously assembles another macromolecule known as RNA (Ribonucleic acid).
RNA is similar to DNA and is mostly single stranded.
It also contains the repeating units or Nucleotides (ribo-nucleotides) made of phosphate, sugar (ribose sugar instead of de-oxy-ribose in DNA) and any one of the nitrogenous base (Adenine, Guanine, cytosine and uracil instead of thymine).

The second step is called Translation in which the RNA molecule formed as above is again used as a template by another protein complex called Ribosomes.
These ribosomes again span the RNA molecule and based on the RNA sequence assemble various amino acids into another polymer known as Proteins.
These proteins are the molecules that perform different functions of a cell.
Some act as enzymes (such as RNA polmerase, ribosomes etc.) others may act as structural function.
Coming back to defining gene, now one can understand that the portion of DNA that can code for a functional RNA and subsequently a functional protein is called a gene.
It is important to point out that not all genes code for a protein.
Some genes only code for a functional RNA which is not translated into a protein.
Thus while all proteins are coded by genes, not all genes code for a protein they may just code for a RNA).