1. DNA Reccombination

2. 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 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.

3. Robin Holliday ( )
A British molecular biologist. Holliday described a mechanism of DNA-strand exchange that attempted to explain gene-conversion 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.

4. 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.

5. Recombination
Two DNA molecules can recombine with each other to form new DNA molecules that have segments from both parental molecules.

6. 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.

7. 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.

8. 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.

9. 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.

12. 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

13. 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.

14. 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.

15. Recombination involves the breakage and rejoining of two chromosomes (M and F) to produce two re-arranged chromosomes (C1 and C2).

16. 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.

17. Recombination of the V and J gene segments of the human IgG kappa light chain.

18. The light chain can combine
with any of 5,000 possible
heavy chains to produce an
antibody molecule

22. 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.

23. Chromatin present in two forms
Euchromatin: the part of chromatin that, during
interphase, are uncoiled (decondensed) and non-
stained 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.

24. 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).

25. 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.

27. 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

28. If no cross-over between the two gene loci
Behavior of 2 different genes at different positions on the same chromosome
When chromosomes go through meiosis, there are two possible situations:
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).

29. 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

32. 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% .

33. 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.

34. 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.

35. 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.

36. 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.

37. 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.

38. 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.

39. 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.

40. 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

41. Some types of satellite DNA in humans are
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