molecular biology

1. CONCEPT OF THE GENE
CLASSICAL
Alleles,
Multiple Alleles,
Pseudoalleles,
Complementation Test,
Experiments On Rii Locus
And Lozenge Locus.

2. CONCEPT OF THE GENE
MODERN
Jumping Genes,
Overlapping,
Split Genes,
Nested Genes,
and Fusion Genes.

3. JUMPING GENES OR TRANSPOSONS
• Earlier it was thought that genes were static and had
definite and fixed locus.
• Barbara McClintock in maize (corn) in late 1940s first
predicted by certain genetic elements were moving from
one site to an entirely different site in the chromosome.
• She discovered that in Indian maize corn have cobs
with kernels of different colours.
• Alexander Brink. In late 1960s this phenomenon was
also discovered in bacteria. These controlling elements
were later on called as transposable elements.
• “a DNA sequence that is able to move or insert itself
at a new location in the genome.”
• They constitute a fairly accountable fraction of genome
of organisms like fungi, bacteria, plants, animals and
humans.
The reddish streaks on
these corn grains are
caused by transposons

4. All of them are characterized by
(i) The presence of inverted terminal repeats required for
transposition;
(ii) The ability to create duplication of flanking DNA at the site
of insertion, target site duplications; and
(iii) Presence of open reading frames coding for enzyme
transposase, which catalyzes transposition.

5. Types of Transposons
Examples :
IS-elements,
P-elements in maize,
hobo-elements in
Drosophila etc.
Tn3-elements found in bacteria
may be viral or non-viral.
Retrovirus like elements:
They carry long terminal repeats
(LTR).
Eg:copia, gypsy elements in
Drosophila.
Retroposons:
LTR are absent.
Eg: LINEs and SINEs in humans.

6. Transposons in Maize:
Ac-Ds system:
o by Barbara Mc. Clintock.
o Ac stands for Activator and Ds for Dissociation.
o Barbara found that Ds and Ac genes were sometimes
mobile and moved to different chromosomal locations
thus resulting in different kernel phenotypes.
o Ac and Ds, both exhibit inter-chromosomal as well as
intra-chromosomal movements (transposition).
o Ds element is activated by Ac and on activation it serves
as the site provider for breakage in chromosome. Ac can
move autonomously(the ability of their own excision)
while Ds can move only in the presence of Ac (Fig. 6).
The transposition involving this Ac-Ds system produces
altered kernel phenotypes.
• Other transposable elements of maize are:
i. spm (suppressor mutator) system,
ii. dt (dotted) system,
iii. Mu (Mutator) system, etc.

9. Significance of Transposable Elements:
• 1. Transposons may change the structural and functional characteristics of genome
by changing their position in the genome.
• 2. Transposable elements cause mutation by insertion, deletion, etc.
• 3. Transposons make positive contribution in evolution as they have tremendous
impact on the alteration of genetic organisation of organisms.
• 4. They are useful as cloning vectors also, in gene cloning.
For example, P-elements are frequently used as vector for introducing transgenes into
Drosophila.
• 5. Transposons may also be used as genetic markers while mapping the genomes.
• 6. Transposon-mediated gene tagging is done for searching and isolation of a
particular gene.

10. OVERLAPPING GENES
• Genes which can be read or translated in two different ways to
produce two different proteins.
• In the genome of φ x 174 ,
-The actual number of nucleotide - 5400.
- The estimate of number of nucleotides -exceeds 6000 .
• How these proteins could be coded from a DNA segment which is
not long enough to code the required number of amino acids?.
-----Number of base pairs should be 3 times the number of amino
acids in protein.

11. • In 1976 Barnell and his co-workers discovered
that the genome of φ x 174 consists of 9 cistrons
(A, B, C, D, E, F, G, H, J).
• Cistron E is present between C and J, and that
cistron E overlaps cistron D.
• Again stop codon of gene D is overlapping with
the start codon of gene J (Fig. 14.18).
• Gene B also overlaps with gene A.
• In the base sequence TAATG, the first triplet code
TAA, is the stop codon of gene D and last three
(ATG) is the start codon of gene J (Fig. 14.19).
• Here the middle base (third from left) is the base
where both codons meet.

12. SPLIT GENES
• generally believed that a gene is a
continuous, uninterrupted sequence
of nucleotides which codes for a
single polypeptide chain.
• The sequences of some eukaryotic
genes (globin, ovalbumin) are found
to be interrupted (introns) by
nucleotides that are not represented
within the amino acid sequence of
the protein but present in the same
order in the genome as in the mRNA.
• Intervening sequences were
unequivocally demonstrated for the
first time in the gene for β-globin in
mouse and rabbit and then in chick
ovalbumin.

13. 1. RNA Splicing:
2. Ribozyme:
For the first time Thomas Cech (1986)
discovered that pre-rRNA isolated from
a ciliated protozoa,
Tetrahymena thermophila, is self
splicing.

14. NESTED GENES
• A situation in which one gene
resides within an intron of
another gene.
• Was first demonstrated in
the Gart locus of Drosophila
melanogaster by Steven
Henikoff et al.
• The existence of nested
genes is in contradiction to
the central hypothesis ,that
genes are located in linear
order on the chromosome.

15. FUSION GENE
• A fusion gene is a
hybrid gene formed
from two previously
separate genes.
• It can occur as a
result
of: translocation, inter
stitial deletion,
or chromosomal
inversion.