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Genes, Chromosomes, Genetic Engineering

Genes, Chromosomes, Genetic Engineering Synopsis

Synopsis

 

Genes

  • The term gene was introduced by Johansson for Mendelian factor. 
  • Genes are located on chromosomes where they occupy a specific position called locus. This was proved experimentally by T. Bovery and W.S. Sutton in 1902. 

Molecular Structure of Gene

Chemically gene is formed of DNA. If consists of the following parts:

Functions of Genes

  • Genes control the functions of DNA and RNA.
  • Every gene comprises of the particular set of instructions for a particular function or protein-coding. Genes vary in size depending on the code or the protein they produce.
  • Genes consist of a particular set of instructions or specific functions. For example, the globin gene was instructed to produce haemoglobin. Haemoglobin is a protein that helps to carry oxygen in the blood.

Chromosomes

  • The chromosome is a DNA molecule capable of self-reproduction and plays a vital role in hereditary, variation and evolutionary development of the species.
  • Chromosomes were discovered by the German scientist Walther Fleming in 1882 in the rapidly dividing cells of the larvae of salamander.
  • Polytene chromosomes found in salivary gland cells of the drosophila larvae and lamp brush chromosomes found in oocytes of many vertebrates and insects are giant chromosomes.
  • Sometimes, chromosomes bear a round, elongated or knob-like structure known as a satellite. Such chromosomes with a satellite are called sat-chromosomes.
  • Some chromosomes bear secondary constrictions at any point along their length. Such chromosomes are called nuclear chromosomes

Structure of Chromosome 

  • Chromosomes are straight, rod-like, coiled or thread-like in structure. They are 0.5–30 µ in length and 0.2–3·0 µ in diameter. 
  • Each chromosome consists of two strands which are called chromatids. 
  • The two chromatids of a chromosome are joined together at a point called the centromere.

Types of Chromosomes

Based on the position of the centromere, there are four morphological types of chromosomes:

 

  1. Telocentric: The centromere is located at the proximal end of the rod-shaped chromosome.
  2. Acrocentric: The centromere is located towards one end due to which one arm of the chromosome is short and the other arm is exceptionally long.
  3. Sub-metacentric: The centromere is located near the centre. In this case, unequal arms give the chromosome a J-shape or L-shape.
  4. Metacentric: The centromere is located at the centre forming two equal arms. These types of chromosomes are V-shaped.
Size and Shape of Chromosomes During Cell Cycle
 
Size of chromosomes greatly very during cell cycle. 
  • Interphase: They form a long thread-like structure called chromatin. 
  • Metaphase: They are thickest and shortened and therefore have a definite shape and size. At this stage chromosomes can be counted easily. 
  • Anaphase: They have a rod-like, J-shaped or V- shaped structure during this phase. 
  • Telophase: They have a thread-like structure.
 
Number of Chromosomes During Fertilisation
  • Each species has a fixed number of chromosomes in its cells. 
  • In human beings, there are 46 chromosomes in each body cell. 46 chromosomes in an ordinary human cell are of 23 different types. So, there are two chromosomes, of each kind. The two chromosomes of each kind are called as homologous chromosomes. 
  • A cell which has the full number of chromosomes with two of each kind is called a diploid cell. A diploid cell has two sets of each type of chromosomes.
  • A cell which has half the number of chromosomes, with one of each kind, is called a haploid cell. A haploid cell has only one set of each type of chromosomes.

 
  • Females consists of two similar gametes and are therefore called homogametic. Males consist of dissimilar gametes and are called heterogametic. 
  • During spermatogenesis, two types of sperm cells will be produced one which contains X chromosome and the other which contains Y chromosome. 
  • During oogenesis each egg will produce two X chromosomes. 
  • If X-chromosome of male fuses with X-chromosome of female, it will produce a female child. 
  • If Y-chromosome of male fuses with X-chromosome of female, it will produce a male child.
 
Properties of Chromosomes
  1. Replication: Synthesis of new DNA molecule which is identical to the parent DNA molecule.
  2. Transcription: Synthesis of RNA molecule from DNA molecule. 
  3. Appearance: Change in morphological appearance during cell cycle. 
  4. Repair: Repair of damaged parts of DNA. 
  5. Mutation: Development of genetic changes.
 
Functions of Chromosomes
  • They carry hereditary characters from parents to offspring. 
  • They help the cell to grow, divide and maintain itself by synthesis of proteins. 
  • They undergo mutation and thus contribute to the evolution of animals. 
  • They guide cell differentiation during development. 
  • They also help in metabolic processes. 
  • They bring about continuity of life.
 
Chromosomal Theory of Inheritance 
  • The chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902.

DNA
  • DNA is the largest biomolecule in the cell.
  • DNA was first isolated by Frederick Meischer from the nucleus of pus cells and called nuclin.
 
Structure of DNA
  • DNA is a macromolecule composed of two complementary strands twisted around each other.
  • Each strand is composed of nucleotides. A nucleotide is made of phosphate, deoxyribose sugar and a nitrogenous base. 
  • There are four nitrogenous bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G). Adenine combines with Thymine (A–T) and Cytosine combines with Guanine (C–G).
  • Phosphate and sugar molecules are arranged lengthwise, and the nitrogenous base is attached inwards with the sugar molecule. This gives the double helical strand a ladder-like appearance.
  • The two polynucleotide chains of DNA have antiparallel polarity, 5'-3' in one and 3'-5' in the other.
  • The backbone of each polynucleotide chain is made of alternate sugar-phosphate groups, with nitrogen bases projecting inwards.
  • The nitrogen bases form complementary pairs.
  • Adenine pairs with thymine by two hydrogen bonds, while guanine pairs with cytosine by three hydrogen bonds.

  • The double chain is right-handed helically coiled. This produces major and minor grooves alternately.
  • The pitch of the helix is 3.4 nm with about 10 base pairs in each turn.
  • Planes of adjacent base pairs are stacked over one another which confer stability to the helical structure.
  • DNA is acidic in nature.
  • The long-sized DNA is accommodated in small areas through packing and compaction.
  • Compaction occurs by folding and attachment of DNA with basic proteins, non-histones in prokaryotes and histones in eukaryotes.
 
DNA Replication
  • The biological process which consists of formation of two identical daughter DNAs from one parent DNA molecule is known as DNA replication. 
  • The process of replication is a complex process, which includes several steps, and occurs with great precision.
  • The DNA replication occurs with the help of three stages, namely initiation, elongation and termination.
  • DNA synthesis starts at initiation points called ‘origins’ which are specific coding regions. 
  • There are a number of origin sites and when replication of DNA begins, it forms the shape of a fork and therefore called DNA replication forks. 
  • DNA helicase is the enzyme that unwinds the double helix and exposes the two individual strands. These two templates are used for replication. 
  • DNA primase enzyme synthesises a small RNA primer that acts as a kick-starter for DNA polymerase.



  • DNA polymerase creates the new strands of DNA and helps in its expansion. 
  • The leading strand is newly formed in a 5’ to 3’ direction for one of the templates that existed in 3’ to 5’ direction. 
  • The other lagging strand will be synthesized in 3’ to 5’ direction from the 5’ to 3’ direction template.
  • Since for lagging strand, continuous DNA synthesis is not possible, DNA synthesis occurs in fragments where RNA primers are added to exposed bases every time and these fragments are called Okazaki fragments. 
  • The expansion of the new DNA strands continues until there is either no more template left to replicate at the chromosomal end or two DNA forks meet and subsequently terminate.
Sex Determination
  • The phenomenon or the process which determines whether the developing embryo will be a male or a female is known as sex determination.
  • Sex chromosomes are also called allosomes or idiosomes. A sex chromosome which determines male sex is termed androsome. 
  • The normal chromosomes other than sex chromosomes are called autosomes. 
  • Individuals with similar sex chromosomes in one sex are called homomorphic individuals. 
  • Individuals with dissimilar sex chromosomes in one sex are called heteromorphic individuals.
 
Mechanism of Sex Determination