Lecture 12 Chromosomal Mutations: Variation in Number and Structure



    A. Definition of Mutation

    Mutations:  Changes in the genetic material of a cell

    B. Kinds of Mutation

    1.Chromosomal:

    Large-scale mutations, chromosomal rearrangements that affect long segments of DNA during cell divisions, and
    misdistributions of chromosomal numbers during gamete production in meiosis.
 
 

        a) Alterations of chromosome structure:
            Breakage of Chromosome.

     i. Translocation:  broken fragment join a  nonhomologous chromosome

    ii. Deletion:  a chromosomal segment is removed from the chromosome.

   iii. Duplication:   The lost segment may join to the homologous chromosome,
                                                producing a duplication
     iv. Inversion:  The lost fragment could reattach to the original chromosome,
                                         but in the reverse orientation.

        b)  Alterations of chromosome number:  Aneuploidy and Polyploidy

Nondisjunction: the members of a pair of homologous chromosomes do not move apart properly during meiosis I, or in which sister chromatids fail to separate during meiosis II.

Aneuploidy:  If either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number, known as ~.

Polyploidy:  some organisms (Plant Kingdom) have more than two complete chromosome sets.  The general term for this chromosomal alteration is ~.
 

Monosomy:  If a chromosome is missing in the fertilized egg (2n –1 chromosomes), the aneuploid cell is monosomic for that chromosome.
 

Trisomy:  If the chromosome is present in triplicate in the fertilized egg (2n+1 chromosomes), the aneuploid cell is said to be trisomic for that chromosome.

Table 4 listed all the symptoms…
 

2.  Genetic Mutations: (point mutations)

Chemical changes in just one or a few base pairs in a single gene.

Types of point mutations:
a) Base deletion

Normal DNA sequence:  AATGCATAT
Deletion of “T”:               AAGCATAT

b) Base addition

Normal DNA sequence:  AATGCATAT
Addition of “A”:     AAATGCATAT

The above two situation can alter the reading of triplet, it is also known as frameshift mutation.

c) Base substitution

Normal DNA sequence:  AATGCATAT
“T” is replaced by “A”:   AAAGCATAT
 

Spontaneous mutations:  Errors during DNA replication, repair, or recombination can lead to base-pair substitutions, insertions, or deletions.  Such mutations are called ~.
 

Mutagens caused mutations:  Physical and chemical agents interact with DNA to cause mutations.  These agents are called ~.

Physical mutant:  X- Ray, (UV) light…

Chemical mutant:  Chemicals (base analogues to normal DNA bases) interfere with correct DNA replication, and distorting the double helix DNA structure.