Repair system

And

Mutation

By: Omneya Kamal A.Ellatif.

Mutation

A sudden change in the sequence of bases in DNA or RNA.

Types of Mutations

There are a variety of types of mutations. Two major categories of mutations are germline mutations and

somatic mutations.# Germline mutations occur in gametes. These

mutations are especially significant because they can be transmitted to offspring and every cell in the offspring

will have the mutation. #Somatic mutations occur in other cells of the body. These mutations may have little effect on the

organism because they are confined to just one cell and its daughter cells. Somatic mutations cannot be passed

on to offspring.Mutations also differ in the way that the genetic material

is changed. Mutations may change the structure of a chromosome or just change a single nucleotide.

Types of Mutations

Causes of Mutation

* Mutations have many possible causes. Some mutations seem to happen

spontaneously without any outside influence.

They occur when mistakes are made during DNA replication or transcription.

* Other mutations are caused by environmental factors. Anything in the environment that can cause a mutation is

known as a mutagen. Examples of mutagens are pictured in Figure below. 

Mutations are essential for evolution to occur. They are the ultimate source of all new genetic material in a species.

Most mutation are harmful, but some mutations are beneficial.

(A)Numerical Abnormalities

MACRO Mutation

1) Aneuploidy Refers to a numerical change in

part of the chromosome set.

Monosomic (2n-1)-Lack of one chromosome each two identical chromosome make

bivalent, while the single chromosome not combines with any chromosome so it’s called Univalent. (Turner syndrome)

Nullisomic( 2n-2)-Lack of two chromosomes there complete absence of a pair of

identical chromosomes. Trisomic (2n+1)-Increase in one chromosome, the chromosome repeats 3 times

to form Trivalent. (Down syndrome) Tetrasomic (2n+2)-Increase by adding 2 identical chromosomes, the chromosome

repeated 4 times to form Quatrivalent. (Wheat plant)

2) Polyploidy Refers to a numerical change in the

whole chromosome set.

Polyploidy can be induced in plants by some chemicals as Colchicine. Autopoly ploidy:Are polyploids with multiple chromosomes sets

derived from single species as (Potato & Banana).

Allopolyploidy:Are polyploids with chromosomes derived from

different species as Triticale (6 sets); 4 from wheat(Triticum turgidum) & 2 from rye(Secale cereale).

Examples

Triploid(3 sets) Seedless watermelon/Salamanders. Tetraploid(4 sets) Durum “ macaroni wheat” Hexaploid(6 sets) Bread wheat. Dodecaploid(12 sets) Xenopus genus. In humans it rarely occur in liver cells, heart muscle & in bone marrow.

(B) Structural Abnormalities

Structural abnormalities are mutations that change chromosome structure.

They occur when a section of a chromosome breaks off and rejoins incorrectly or does not

rejoin at all.

Possible ways these mutations can occur are illustrated in Figure below.

Chromosomal alterations are major changes in the genetic material.

MICRO Mutation(A) Point Mutations

A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually

less serious than a chromosomal alteration.

Point mutations can be silent, missense, or nonsense mutations, as shown in Table below. The effects of point mutations depend on how they change the genetic code. 

Type Description

Example

Effect

Silent

Mutated codon codes for the same amino acid

CAA (glutamine) →

CAG (glutamine)

None

Missense Mutated codon codes

for a different amino acid

GAG (glutamic acid) → GTG (valine)

Variable

Nonsen

se

Mutated codon is a premature stop codon

CAA (glutamine) →

UAA (stop) usually

Serious

(B) Frame shift Mutations

A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. 

DNA repair systems are vital to the survival of all organisms.

– Living cells contain several DNA repair systems that can fix different types of DNA alterations.

In most cases, DNA repair is a multi-step process: 1. An abnormality in DNA structure is

detected.2. The abnormal DNA is removed.

3. Normal DNA is synthesized.

Repair Mechanisms

1) Direct Repair System

2-Base excision repair(BER) system

Base excision repair (BER) involves a category of enzymes known as DNA-N-glycosylases :

– These enzymes can recognize a single damaged base and cleavethe bond between it and the sugar in the DNA.– Removes one base, excises several around it, and replaces withseveral new bases using Pol adding to 3’ ends then ligase attachingto 5’ end.

Depending on the species,, this repair system can eliminate abnormal bases such as:

– Uracil; Thymine dimers– 3-methyladenine; 7-methylguanine

3-Nucleotide excision repair(NER) system

This type of system can repair many types of DNA damage, including:

– Thymine dimmers and chemically modified bases NER is found in all eukaryotes and prokaryotes

– However, its molecular mechanism is better understood in prokaryotes.

Several human diseases have been shown to involveinherited defects in genes involved in NER:– These include xeroderma pigmentosum (XP) and Cockaynesyndrome (CS)

" A common characteristic of both syndromes is an increased sensitivity to sunlight “

Proofreading enzymes

Correct errors made during replication

4-Mismatch repair(MMR)system

If proofreading fails, the methyl-directed mismatch repair system comes to rescue. This repair system is found in all species. In E.coli by using (MutH,MutS & MutL) . Methyl-directed mismatch repair recognizes mismatched base pairs, excises the incorrect bases, and then carries outrepair synthesis. Eukaryotes also have mismatch repair, but the mechanism to distinguish the template strand from the new strand is still unclear.– Four genes are involved in humans, hMSH2 andhMLH1, hPMS1, and hPMS2.

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