- DNA strucure and terminology.

Chromosomes are thread-like structures located inside the nucleus of cells. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

see the picture which illustrate that also show the true genetic content & non genetic areas....

Promoters: are DNA sequences adjacent to the beginning of the genes and control gene activation.

 

Introns are sequences inside the gene although they do not code for protein sequence they are necessary for correct coding.

 

Exons are the regions inside the gene between introns that contain the coding DNA sequences. 

 

Most of the harmful mutations occur in exons and as a consequence change the structure of the coded protein.

 

Haploid describes a cell that contains a single set of chromosomes.

 

Diploid describes a cell that contains two sets of chromosomes.

 

Allele, also called allelomorph, any one of two or more genes that may occur alternatively at a given site (locus) on a chromosome.

 Alleles may occur in pairs, or there may be multiple alleles affecting the expression (phenotype) of a particular trait. The combination of alleles that an organism carries constitutes its genotype. 

 

If the paired alleles are the same, the organism’s genotype is said to be homozygous for that trait; if they are different, the organism’s genotype is heterozygous. 

 

A dominant allele will override the traits of a recessive allele in a heterozygous pairing. 

 

In some traits, however, alleles may be codominant—i.e., neither acts as dominant or recessive. An example is the human ABO blood group system; persons with type AB blood have one allele for A and one for B. (Persons with neither are type O.)

 

A gene is a region of DNA that encodes function.

 

Components of DNA

 

- DNA is a polymer.

- The monomer units of DNA are nucleotides,

- The polymer is known as a "poly nucleotide ".

- Each nucleotide consists of:-

1. 5-carbon sugar (deoxyribose).
2. nitrogen containing base attached to the sugar.
3. phosphate group.

- There are four different types of nucleotides found in DNA,

differing only in the nitrogenous base.

- The four nucleotides are given one letter abbreviations

for the four bases: -

• A is for adenine.
• G is for guanine.
• C is for cytosine.
• T is for thymine.

- DNA is found in spiral manner around histones

in the chromosome .

- DNA is double stranded formed as two raws of nucleotides

attached to each other by phosphodiester bond .

- One nucleotide from one strand attached to another one

By hydrogen bond through the nitrogenous base of nucleotide.

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