Essay on Gene Action (1601 Words)
The process of synthesis of messenger RNA from a DNA template is known as transcription.
DNA Transcriptase/ Transcription – mRNA
Mechanism of Transcription:
The process of transcription takes place during interphase and continues upto prophase of cell division. The process of transcription require (i) DNA tempelate (ii) activated precursors (ATP, GTP, UTP and CTP) (iii) Divalent metal ions (Mg++ or Mn++) (iv) RNA Polymerase.
The process of transcription consists of three main steps —
1. Initiation of transcription
2. Elongation of RNA chain
3. Termination of RNA chain.
1. Initiation of Transcription:
i. Start at specific site in a cistron called promotor site.
ii. The binding of RNA polymerase at initiation site results in separation of DNA double helix.
iii. After opening of DNA strands, synthesis of mRNA begins on one of the two strands of DNA.
iv. The RNA chain starts with triphosphate and the first base is always purine.
v. After synthesis of terminal triphosphate, nucleotides are removed by endonucleases.
2. Elongation of RNA chain:
i. The elongation of RNA chain takes place by addition of activated ribonucloside triphosphates like ATP, GTP, UTP, and CTP to one strand of DNA molecule.
ii. RNA synthesis takes place only on one strand of DNA.
iii. The elongation of mRNA chain takes place with the help of core enzyme which moves along the DNA template.
3. Termination of RNA Chain:
i. The chain is terminated either when DNA receives termination signal or an additional protein.
ii. The terminator gene acts as a signal to end transcription.
iii. RNA polymerase joins the nucleotides together to produce mRNA transcript.
iv. When mRNA transcript is detached, the DNA template strand reforms H-bands with its complementary strand resulting in the formation of double helical structure.
The process of protein synthesis from information in mRNA is known as transition.
DNA Transcriptlon > RNA Transcriptlon > protein
Mechanism of Translation:
The translation process requires mRNA, rRNA, ribosomes, 20 kinds of amino acids and their specific transfer RNA’s.
It consists of five major steps —
(i) Activation of aminoacids
(ii) Transfer of amino acids to tRNA
(iii) Chain initation
(iv) Chain elongation
(v) Chain termination.
1. Activation of Amino Acids:
There are 20 different amino acids which are used in protein synthesis. Each amino acid is activated in the presence of ATP by its own activating enzyme called aminoacyl synthetase to form aminoacyl adenylate.
2. Transfer of Amino Acids to RNA:
The aminoacyl adenylate reacts with specific tRNA and gives rise to aminoacyl tRNA complex.
Thus synthetase has two active sites, one for recognizing the specific amino acid and the other for recognizing specific molecule.
3. Chain Initiation:
i. The 30s ribosome sub unit attached to mRNA to form an mRNA 30S complex.
ii. The amino acid tRNA complex binds to the initiation codon AUG on mRNA through its anticodon UAC to form 30S initiation complex.
iii. The larger ribosomal sub unit joins to the 30s initiation complex to form the complete initiation complex.
iv. The larger ribosomai subunit has two binding sites for tRNA i.e. acceptor site and peptide site.
4. Chain Elongation:
i. The second amino tRNA complex now occupies the acceptor site. There is enzymatic recognition of internal codons. This process requires EF-T4, GTP and metallic ions mg++.
ii. Formation of peptide bonds takes place. This process requires two metallic ions mg++ and k+.
5. Chain Termination:
i. The termination codon provides signal to the ribosome for the attachment of release factors.
ii. The release factors interact with peptydyl transferase causing hydrolysis of the bonds between tRNA and the polypeptide chain.
iii. Hydrolysis of GTP results in the dissolution of the release factors from the ribosome.
iii. The released poiypetide chain undergoes processing. The formyl group of starting amino acid methionine is removed. Proteins are synthesized from amino acids with the help of polysomes, three types of RNA, various enzymes and cofactors etc.
Most of the phenotypic characters are controlled by genes present in the chromosomes. But some characters expressed by factors present in the cytoplasm. These factors lying in the cytoplasm are called plasmagenes. The transmission of characters controlled by plasmagenes are called cytoplasmic inheritance.
The first case of cytoplasmic inheritance was reported by Correns in 1909 in four ‘o’ clck (Mirabilis jalapa) for leaf colour.
Important characteristics (or) features of cytoplasmic inheritance:
1. Reciprocal differences:
Characters which are governed by Cytoplasmic Inheritance invariably exhibit marked differences in reciprocal crosses in F1.
2. Lack of Segregation:
In general F2, F3 and the subsequent generation do not show segregation for a Cytoplasmic Inheritance trait. This is because the F1 individuals generally receive plasma genes from one parent only.
3. Irregular Segregation in Biparental Inheritance:
In some cases, plasma genes from both the parents are transmitted to the progeny, this is known as Biparental inheritance.
Biparental inheritance gives rise to irregular segregation ratio in the F1 generation of higher plants.
4. Somatic Segregation:
Plasma genes generally show somatic segregation during mitosis.
5. Association with organelle DNA:
Several plasma genes have been shown to be associated with CP-DNA not DNA.
Some Mutagens e.g. ethidium bromide, are highly specific Mutagens for plasma genes.
It is very difficult to map cytoplasmic genes or prepare linkage map for such genes.
8. Maternal Effects:
In case of cytoplasmic inheritance distinct maternal effects are observed. This is mainly due to more contribution of cytoplasm to the zygote by female parent than male parent.
Table 5. Difference between Mendelian Inheritance and Cytoplasmic Inheritance
1. Governed by Nuclear Genes
2. Exhibits distinct segregation pattern
3. Reciprocal differences are not observed
4. Does not show maternal effects.
5. Genes can be easily mapped on chromosomes
6. Nuclear genes are associated with chromosomes.
1. Governed by plasma genes
2. Does not exhibit distinct segregation
3. Reciprocal differences are observed
4. Exhibits maternal effects
5. Mapping of plasma genes is very difficult.
6. Plasma genes are associated with either chloroplast DNA or mitochandrial DNA.
Significance in Plant Breeding:
i. Studies of cytoplasmic inheritance have played key role in mapping of chloroplast and mitochondrial genome in yeasts, chlamydomonas, maize, humans etc.
ii. Cytoplasmic inheritance has been useful in explaining the role of various cytoplasmic organelles in the transmission of characters in different organisms.
iii. Development of cytoplasmic male sterility (CMS).
CMS lines have been developed in several crops like maize, pearl millet, sorghum, cotton etc.
iv. Availability of CMS lines has facilitated the production of hybrid seed in these crops at a cheaper cost than with hand emasculated pollination method.
v. Mitochondria can be used in the manifestation of heterosis.
vi. Mutation of chloroplast DNA and mitochnodrial DNA leads to generation of new variants. Some of such variants are of special significance especially in ornamental plants.
Sex determination is the process of sex differentiation, which utilises various genetical concepts to decide whether a particular individual will develop into male or female sex.
Mechanisms of Sex Determination:
1. Chromosomal sex determination
2. Monogenic sex determination
3. Environmental sex determination
1. Chromosomal Sex Determination:
(a) Sex Determination by Allosomes:
Sex with similar type of sex chromosomes (xx) is known as homogametic sex and with dissimilar type of sex chromosomes (xy) as heterogam eti ñ sex.
There are four different systems of allosomal sex determination
1. xx – xy female – male system
2. xx – xo female – male system
3. xo – xx female – male system
4. zw – zz female – male system
(b) Diploid-Haploid System:
In honey bees the females have diploid (2n = 32) chromosomes and males have haploid (n = 16) chromosomes unfertilized eggs develop parthenogenetically into haploid.
(c) Genic Balance System:
Sex determining genes are present on both X-chromosomes as well as autosomes.
2. Monogenic Sex Determination:
In some animals and plants, expression of sex influenced by a single gene.
3. Environmental Sex Determination:
In horse tail plant development of sex depends on growing conditions. Plants grown under good conditions develop as females and those grown under stress conditions develop into males.
Sex Linked Characters:
The characters for which genes are located on sex chromosomes are known as sex linked traits. Such genes are called sex linked genes, and linkage of such genes is referred to as sex linkage.
Main features —
Sex linked genes are located on X chromosomes only.
In diploid organism, each homogametic sex (xx or zz) has two copies of linked alleles.
In homogametic sex, a recessive gene can express. In heterogematic sex, a recessive allele can express.
Sex linked genes are transmitted from female to male.
5. Pattern of segregatin:
Inheritance of sex linked characters does not follow normal segregation pattern.
Sex Limited Characters:
The characters which are expressed in one sex only are referred to as sex limited characters.
i. They express in one sex only.
ii. Sex limited genes may be located either in sex chromosome or autosomes.
iii. Sex limited genes control the expression of primary and secondary sex characters.
Sex Influenced Genes:
The dominance expression of some genes depends on the sex of individual. Such genes are known as sex influenced genes.
i. Such genes are located in the autosomes
ii. They express more frequently in one sex than other.
iii. Expression of such characters appears to be governed by sex hormones.