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Lecture 4

Some useful definitions:

Alleles: different forms of the same gene present on the chromosome

locus

Genotype: All alleles of an individual

Wild type: True breading genotype.

Mutant: Altered (changed) genotype from the wild type.

Dominant Allele: Trait which is expressed in heterozygote

Recessive Allele: Trait which is not expressed in heterozygote

Phenotype: All traits of an organism originated from the genotype.

Homozygote: Adiploid with two like alleles of the same gene

Phenotype: All apparent traits of an organism.

Heterozygote: Diploid with two different alleles of same gene

Incomplete dominance: The case where a heterozygote expresses a phenotype

intermediate between the corresponding homozygotephenotypes.

True-breeding: Individuals that on intercrossing always produce offspring of the

same phenotype. These individuals are homozygous at all loci (The major exception

is sex chromosome differences between males and females).

F1: First generation produced by interbreeding of two individuals.

F2: Generation produced by interbreeding of F1 individuals.

Sometimes an allele has more than one phenotype and may be recessive for

one and dominant for another trait. In such cases, the phenotype must be

specified which trait is dominant or recessive.

An example, the allele for sickle cell hemoglobin in humans designated Hbs.

Heterozygous individuals (Hbs/Hba) are more resistant to malaria, thus Hbs

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is dominant for the trait of malaria resistance. On the other hand, Hbs/Hba

heterozygotes do not have the symptoms of sickle cell disease, but Hbs/Hbs

homozygous individuals do. Therefore, Hbs is recessive for the trait of

sickle cell disease.

Allele and trait

Yeast is known as Saccharomyces cerevisiae, which is the single-celled

microbe used to make the bread. Yeast can exist as haploids of either

mating type a (MATa) or mating type (MAT). Haploid cells of different

mating type when mixed together will mate to make a diploid cell.

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DNA recombination:In meiosis homologous pair of chromosomes are arranged in pairs, sothateach chromosome with two parental sister chromatids are facing eachothers in this arrangement to give a special tetradarrangement of 4chromatids .In this arrangement, the opposite chromatids which come from each chromosome willundergo a process of crossover. That is opposite chromatids exchange pieces of DNA

between each others. This exchange of DNA is called DNA recombination, which allows

mixing of genetic information between gametes that originate from father and mother andproduces 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.

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http://en.wikipedia.org/wiki/File:Chromosomal_Recombination.svg

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Recombination involves the breakage and rejoining of two chromosomes (M and F) to

produce two re-arranged chromosomes (C1 and C2).

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:

- Alleles segregate together on the same chromosome - A and B together and a and b

together2. If there is a cross-over between the two gene loci

- 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

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Lecture 5 ( DNA structure and functions)

DNA structure:

DNA is the largest macromolecule in the cell. In eukaryotic cells, 99% of the cell DNA is

present in linear form folded on itself several times to occupy small space within thechromosomes of the nucleus .Each chromosome contains single DNA molecule. Small

amounts of DNA (about 1%) are circular shape DNA present inside the mitochondria.

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The DNA can be described as a polymer of nucleotides (Polynucleotide).That is a long

chain of repeating nucleotide units connected together strongly (by covalent bonds).

Therefore, nucleotide is the unit of DNA structure that has complex structure made of 3different components:

1 .Nitrogen base

2. Pentose sugar3. Phosphate groups.

Without phosphate groups, the combination of nitrogen base and sugar is called a

nucleoside. The nitrogen bases are heterocyclic (combination of carbon and nitrogenatoms) present in the cell with 5 different types. Two purines adenine and guanine (with 9

atoms of 4 nitrogenes and 5 carbons arranged in two rings) and 3 pyrimidines: Cytosine,

Thymine and Uracil having 6 atoms ring

including 2 nitrogens and 4 carbons.

Purine

In the nucleotide structure, the pentose sugar connects to nitrogen base at carbon one

from one direction and to phosphate group at carbon 5 of the other side.

Pyrimidine

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http://en.wikipedia.org/wiki/File:Nucleotides_1.svg

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There are 4 different types of nitrogen bases present in each nucleic acid. Adenine,

Guanine and cytosine are present in both DNA and RNA .However only RNA contains

Uracil and only DNA contains Thymine nitrogen bases. Also RNA nucleotides have

ribose while DNA has deoxyribose as pentose sugars

The difference is - 2'-OH in ribose and 2'-H in deoxyribose.

In polynucleotide formation each two nucleotides are linked together byPhosphodiester bond formed between carbon 3 of the sugar belongs to one nucleotide

together with the 5 carbon for the sugar of another nucleotide

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In single polynucleotide chain of DNA(and similarly for RNA structure) the chain ismade from covalent linkage of sugar phosphate backbone .In this arrangement the

nitrogen bases are exposed freely to the out side of the backbone structure.

.

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In DNA structure two polynucleotides are twisted around each others in

double helical arrangementso that bases of opposite polyncleotides are

specifically connected by week hydrogen bonds.

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TG and CG base pairing

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Opposite direction of DNA polynucleotide chains:

The two strands have their 3 and 5 terminals at opposite ends

5 terminal: at one end of each DNA strand is a phosphate group linked to carbon

atom 5 of deoxyribose

3 terminal: at one end of each DNA is a hydroxyl group attached to carbon atom

3 of deoxyribose.

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