Gene Mutations

Gene mutation is a change in the nucleotide sequence of DNA. If this sequence changes, then the codons change and sequence of amino acids in a polypeptide changes.

Frame shift Mutations
The mutation occur most often because one or more nucleotides are either inserted or deleted from DNA. This results in a completely new sequence of codons and a nonfunctional protein.

Point Mutations
These involve a change in a single nucleotide and therefore a change in a specific codon. The occurrence of valine instead of glutamate in the beta chain of hemoglobin results in sickle cell disease. Due to point mutation there is enzyme deficiency and homogentisic acid is not further changed into 4 maleyacetoacetic acid as this leads to alkaptonuria.

Point Mutation

Abnormal Development

The various events of embryonic development follow each other in precise manner. The process of development is under strict control of genome (genetic information). In spite of these, sometimes individuals are born with defective or abnormal organs.

Examples
Microcephally: When the skull becomes small.

Cleft palate: When the upper lip is bifurcated.

Polydactyl: The number of digits may be more in hands or feet then the usual five digit.

Teratology is the branch of biology which deals with the study of abnormal development. The cause, of abnormality can be traced at three levels, (a) Gene level (b) Chromosome level (c) Functioning of the glands.

Gene level: Many abnormalities are due to gene mutation e.g. extra finger in thumb, albinism.

Chromosome level: When the number of chromosome varies than that of usual 46 it may cause many syndrome e.g. Down syndrome, Turner syndrome etc. Sometimes a part of a chromosome may be missing or there is a hairline break in the chromosome can cause abnormalities e.g. cri-du-chat syndrome. When the child cries, it seems that a cat is crying. It is due to a deletion in the short arm of chromosome no. 5


Functioning of the glands: We have already seen in topic of Endocrine glands, that over or under secretion of hormones can cause a variety of abnormalities e.g. gigantism, dwarfism etc.

Mutations

Definition
Mutations (in Latin Mutatus means Change) are permanent changes in genes or chromosomes that can be passed to offspring if they occur in cells that become gametes. There are two main types of mutations.

1. Chromosomal Mutations
2. Gene Mutations

Chromosomal Mutations
The chromosomal mutations include:
a. Changes in chromosome number
b. Changes in chromosome structure

Changes In Chromosome Number
Changes in chromosome number include monosomy, trisomy and polyploidy.

Monosomy and Trisomy
Monosomy occurs when an individual has only one of a particular type of chromosome (2n-1) and trisomy occurs when an individual has three of a particular type of chromosome (2n+1). The usual cause of monosomy and trisomy is non-disjunction during meiosis. Monosomy and trisomy occur in both plants and animals. In humans turner syndrome is a monosomy of the sex chromosomes, the individual inherit a single X chromosome. The most common trisomy among human is Down Syndrome, which involves chromosome 21.

Polyploidy
Some mutants have more than two sets of chromosomes. They are called polyploids. Polyploid organism maybe triploid (3n), tetraploid (4n), pentaploids (5n) and so on. Polyploidy is not seen in animals. It is estimated that 47% of all flowering plants are polyploids. Crops such as wheat, corn, cotton, sugarcane and fruits such as watermelons, bananas and apples are polyploids. Polyploids generally arise by following hybridization.

Changes In Chromosome Structure
There are various agents in the environment, such as radiation, certain organism chemicals or even viruses that can cause chromosome to break. When the broken ends of chromosomes do not rejoin in the same pattern as before this results in a change in chromosomal structure.

Inversion: It occurs when a segment of a chromosome is turned around 180̊.
Translocation: It is the movement of a chromosome segment from one chromosome to another non homologous chromosome.
Deletion: It occurs when an end of a chromosome breaks off or when two simultaneous breaks lead to the loss of an internal segment. An example is cri du chat (cat's cry) syndrome.
Duplication: It is the doubling of a chromosome segment. 
Types of chromosomal mutations



Protein Synthesis

The two main steps of protein synthesis is transcription and translation.

TRANSCRIPTION
The genetic message is in DNA. From DNA template a single strand of mRNA is produced. This process is called transcription (which we have already discussed in the previous blog posts). It is called mRNA because it carries the genetic message from DNA. The triplet of DNA is called code and the triplet of mRNA is called codon.

TRANSLATION
It is the mechanism by which the triplet base of mRNA forms specific sequence of amino acids in a polypeptide chain. The main steps involved in the translation are:

1- Binding of mRNA with the ribosome
2- Amino acid activation
3- Attachment of amino acid to tRNA
4- Polypeptide chain initiation
5- Chain elongation
6- Chain termination

1- Binding of mRNA with the ribosome
The mRNA attaches itself to a 30S ribosome. The 50S subunit joins then 30S to form 80S ribosome. On a fully assembled ribosome there are two sites.
Binding of mRNA with the ribosome


A site = Amino-acyl site
P site = Peptidyl site

Several adjacent ribosomes may become attached to a molecule of mRNA forming a structure called polysome. 

2 - Amino Acid activation
For each amino acid there is an enzyme. With the presence of this enzyme the amino acid becomes attached with AMP and then amino acid is brought into a higher energy state i.e. amino acid is activated. The enzyme is called activation enzyme.
ATP + Amino Acid + Enzyme = Enzyme-Amino Acid-AMP+PP
Amino acid activation 
3 - Attachment of Amino Acid to tRNA
The D loop of the tRNA recognizes the activation enzymes. An amino acid becomes attached to a particular tRNA at the 3' end i.e. A-C-C end, by its carboxyl group.

Attachment of Amino Acid to tRNA

4 - Chain Initiation
As we have already seen that a ribosome has two binding sites for tRNA. One of these is called P (for peptide) site and the other is called the A (for amino acid) site. The AUG is the initiation codon. So methionine carrying tRNA becomes attached with the ribosome at P site. The anticodon UAC of tRNA matches with the codon AUG. The ribosome exposes the codon on the mRNA immediately adjacent to the initiating AUG codon. The tRNA where anticodon will match with this codon become attached with the ribosome at A site. The two amino acids will react and form the peptide bond. The methionine tRNA will be released.

 
Chain initiation
5 - Chain Elongation
In the process of elongation, ribosome will move along the mRNA molecule a distance corresponding to the three nucleotide. Another tRNA carrying a specific amino acid links up with codon. The amino acid becomes linked to the previous amino acid and tRNA at site P is released. The process of ribosomal movement continues along the mRNA and a polypeptide is formed.

6 - Chain Termination
When the ribosome comes to a codon signaling "stop", the joining of amino acid does not take place, as no tRNA will come and match with this codon. The terminating codon are UAA, UAG and UGA. At this stage polypeptide chain leaves the ribosome and the translation is completed. 

Genetic Code

The sequence of nucleotide in DNA specifies (and a copy of this sequence in mRNA directs) the order of amino acids in a polypeptide. This relation between the amino acid and the bases is called genetic code. It would be seen that there must be a code for each of the 20 amino acids. But can four nucleotide provide enough combinations to code for 20 amino acids? If each code word called a codon, were made of one base, the protein could contain only 4 amino acids. If the combination of two base codes for one amino acid then 16 amino acid could be specified into protein molecules. But if each codon were made up of three bases there would be 64 codons more than enough to code for 20 amino acids.

Genetic Codons 
Finding the Genetic Code
In 1961, Marshall Nirenbery and J. Heinrich Matthei constructed synthetic RNA which composed only of Uracil, and the protein that resulted was composed of only amino acid phenylalanine. Therefore a codon for phenylalanine was known to be UUU. Later a cell free system was developed by Nirenberg and Philip Leder in which three nucleotides at a time were translated; in that way it was possible to assign an amino acid to each of the RNA codons.

Important Properties of Codon
Degenerate: The genetic code is "degenerate". This means that most amino acids have more than one codon.
Unambiguous: The genetic code is unambiguous. Each triplet codon has only one meaning.
Initiation codon: There is only one initiation codon which is AUG.
Punctuation codon: There are three stop signal. These are UAA, UAG, UGA.
Universal Triplet of Bases: The triplet code for an amino acid is universal in all organisms.
Non-Overlapping: mRNA sequence beginning AUGAGCGCA is not read as AUG/UGA/GAG. It will be read as AUG/AGC/GCA.