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DNA Reccombination
Barbara McClintock, (1902–1992)
An American scientist and cytogeneticist who was awarded
the 1983 Nobel Prize in Physiology. She received her Ph.D
from Cornell University in 1927. She studied
chromosomes and how they change during reproduction in
maize. One of those ideas was the notion of genetic
recombination by crossing-over during meiosis—a
mechanism by which chromosomes exchange information.
She produced the first genetic map for maize, linking regions
of the chromosome to physical traits. She demonstrated the
role of the telomere and centromere, regions of the
chromosome that are important in the conservation
of genetic information. She was recognized among the best in
the field, awarded prestigious fellowships, and elected a
member of the National Academy of Sciences in 1944.
Robin Holliday
(1932-2014)
A British molecular biologist. Holliday
described a mechanism of DNA-strand
exchange that attempted to explain geneconversion events that occur during meiosis
in fungi. That model first proposed in 1964
and is now known as the Holliday Junction. In
1975 he suggested that DNA methylation
could be an important mechanism for the
control of gene expression in higher
organisms, and this has now become
documented as a basic epigenetic mechanism
in normal and also cancer cells.
A Holliday intermediate formed between two bacterial
plasmids in vivo, as seen with the electron microscope.
Note that such intermediates can form only when the
nucleotide sequences of the two parental duplexes are very
similar or identical in the region of recombination because
specific base pairs must form between the bases of the two
parental duplexes.
Recombination
Two DNA molecules can recombine with each other
to form new DNA molecules that have segments
from both parental molecules.
Classes Of Recombination
1. Homologous genetic recombination
(also called general recombination)
This involves genetic exchanges between any two
DNA molecules (or segments of the same
molecule) that share an extended region of nearly
identical sequence. The actual sequence of bases
is irrelevant, as long as it is similar in the two
DNAs.
2. Site-specific recombination :
The exchanges occur only at a particular
DNA sequence.
3. DNA transposition :
It is usually involves a short segment of DNA
with the remarkable capacity to move from
one location in a chromosome to another.
These “jumping genes” were first observed
in maize in the 1940s by Barbara McClintock.
Functions of genetic recombination
1. They include roles in specialized DNA repair
systems.
2. Specialized activities in DNA replication.
3. Regulation of expression of certain genes.
4. Facilitation of proper chromosome segregation
during eukaryotic cell division.
5. Maintenance of genetic diversity in a population.
6. Implementation of programmed genetic
rearrangements during embryonic development.
Recombination during meiosis
Model of double-strand break repair for
homologous genetic recombination. The two
homologous chromosomes involved in this
recombination event have similar sequences.
Each of the two genes shown has different
alleles on the two chromosomes. The DNA
strands and alleles are colored differently so
that their fate is evident.
Meiosis in eukaryotic germ-line cells.
Diploid
germ-line cell
Replication
Prophase I
separation of
Homologous pairs
first
Meiotic division
second
Meiotic division
Haploid gametes
DNA recombination
In meiosis homologous pair of chromosomes are
arranged in pairs, so that each chromosome with
two parental sister chromatids are facing each
others in this arrangement to give a special tetrad
arrangement of 4 chromatids .
In this arrangement, the opposite chromatids which
come from each chromosome will undergo a
process of crossover. That is opposite chromatids
exchange pieces of DNA between each others.
DNA recombination
Exchange of DNA which allows mixing of genetic
information between gametes that originate from
father and mother and produces new combinations
of genes. Without this phenomenon, the new
gametes will have exactly the same genetic
information as the original parents and no genetic
variations occur.
Recombination involves the breakage and rejoining of
two chromosomes (M and F) to produce two rearranged chromosomes (C1 and C2).
Crossing over. (a) Crossing over often produces an
exchange of genetic material. (b) The homologous chromosomes
of a grasshopper are shown during prophase I of meiosis. Many
points of joining (chiasmata) are evident between the two
homologous pairs of chromatids. These chiasmata are the
physical manifestation of prior homologous recombination
(crossing over) events.
Recombination of the V and J
gene segments of the human IgG kappa light
chain.
The light chain can combine
with any of 5,000 possible
heavy chains to produce an
antibody molecule
• Chromosome is highly folded form of the
interphase chromatin (extended or relaxed
threads of the same nucleoprotein
structure).The chromosome form appears
during cell division, particularly in the
metaphase stage.
Chromatin present in two forms
1. Euchromatin: the part of chromatin that, during
interphase, are uncoiled (decondensed) and nonstained but containing high concentrations of
transcribed genes (transcriptionally active)
2. Heterochromatin: The part of the chromatin that
remains tightly coiled (condensed) and intensely
stained during interphase, but inactive in gene
expression.
Yeast is known as Saccharomyces cerevisiae,
which is the single-celled fungus used to make
bread. Yeast is haploids with a genome of 16
chromosomes single set .
The gene for mating has a locus on one
chromosome which makes the yeast exists either
in dominant mating type (G) or recessive mating
type (g).
The yeast has two types of cell division, asexual
in which the cell divide to produce two haploid
cells with the same mating type.
A second type of cell division called sexual in
which two cells conjugate together during
mating to
produce new hybrid cell .The hybrid cell is a
diploid having pair of two alleles, similar to the
pair of alleles arrangements in diploid system of
human
cells.
In diploid system
The gene is responsible for certain trait in
phenotype expression(color of eyes, color of
skin).
The allele is a variant of the gene (in eye color
expressed as black or brown or blue ) so that
certain people have a specific allele of that
gene, which results in the trait variant. The
genes code for proteins, which might result in
different traits, but it is the gene, not the trait,
which is inherited
Behavior of 2 different genes at different positions
on the same chromosome
• When chromosomes go through meiosis, there are
two possible situations:
1. If no cross-over between the two gene loci
occurs (if they are present in short distance from
each other' on the same chromosome):
- Alleles segregate together on the same
chromosome - A and B together and a and b
together
2. If there is a cross-over between the two gene loci
(when they are present far distance from each other's
on the chromosome).
Alleles segregate from each other in Meiosis II
Two recombinant products:
- A and b now together in one meiotic product
- a and B now together in one meiotic product
Two parental products
the other two meiotic products are still AB and
ab
Gene density
Not all the DNA genome encodes protein, but
usually it contains coding and non-coding
sequences. The percentage of coding sequence
in total genome is called gene density.
In bacteria the gene density is about 90 % .
In human this density is only about 2% .
• Therefore, majority of eukaryotes DNA contain
non-coding DNA, or regions of DNA that serve
no obvious function. Simple single-celled
eukaryotes have relatively small amounts of
such DNA, whereas the genomes of complex
multicellular organisms, including humans
contain an absolute majority of DNA without
an identified function.
• This non-coding sequences include repetitive DNA
sequence (satellite DNA) , introns and regulatory
regions which occupies 98% of total human genomic
DNA.
• The genomic non-coding DNA sequences are
components of an organism's DNA that do not
encode protein sequences. Some noncoding DNA is
transcribed into functional noncoding RNA molecules
(e.g. transfer RNA, ribosomal RNA, and regulatory
RNAs), while others are not transcribed or give rise to
RNA transcripts of unknown function.
Functions of noncoding DNA
1. Protection of the genome:
Noncoding DNA separate genes from each other
with long gaps, so alteration in one gene or part of
a chromosome does not extend to the whole
chromosome. In high genomic complexity like in
case of human genome, not only different genes,
but also inside one gene there are gaps of introns
to protect the entire coding segment to minimize
the changes caused by changing part of the gene.
2. Genetic switches:
Some noncoding DNA sequences function as
genetic switches that decide when and where
genes are expressed.
3. Regulation of gene expression:
Some noncoding DNA sequences
quantitatively determine the expression levels
of various genes.
4. Act as regulatory sites for gene actions:
• Some non-coding sequence can be
promoters, operators, and enhancers
….etc.
• Repeated sequences are patterns of DNA
that occur in multiple copies throughout
the genome.
Satellite DNA
• Very large sequence consists of tandem repeating,
non-coding DNA. It is the main component of
functional centromeres, and forms the main
structural constituent of heterochromatin. A
repeated pattern can be between 1 base pair long
(a mononucleotide repeat) to several thousand
base pairs long, and the total size of a satellite
DNA block can be several mega bases (Unit of
length for DNA fragments equal to 1 million
nucleotides and roughly equal to 1 cm) without
interruption.
• Most satellite DNA is localized to the
telomeric or the centromeric region of the
chromosome. The nucleotide sequence of
the repeats is fairly well conserved across a
species. However, variation in the length of
the repeat is common. The difference in how
many of the repeats are present in the
region (length of the region) is the basis for
DNA fingerprinting in forensic medicine.
• Tandem repeats: copies which lie adjacent to
each other, either directly or inverted
• Minisatellite - repeat units from about 10 to
70 base pairs, found in many places in the
genome, including the centromeres.
• Microsatellite - repeat units of less than 10
base pairs (typically have 6 to 8 base
• repeat units) mainly found in telomeres
Some types of satellite DNA in humans are
Type
Size of repeat unit (bp)
Location
α (alphoid DNA) 171
All chromosomes
β
68
Centromeres of
chromosomes 1, 9, 13,
14, 15, 21, 22 and Y
Mini-satellite
25-48
Centromeres and other
regions in
heterochromatin of
most chromosomes
Micro-satalite
5
Most chromosomes
telomers