- Human DNA and RNA.

Human DNA

All nucleic acids are polynucleotides. A nucleotide consists of three components:

1.     A nitrogenous base: (Adenosine and guanine which are based on purine rings, Thymine and cytosine which are based on pyrimidine rings).

2.     A pentose sugar (deoxyribose in DNA and Ribose in RNA) at 3′ end

3.     1-3 phosphate groups at 5′ end

 There are two kinds of nucleic acids:

o   Deoxyribonucleic acid (DNA)

o   Ribonucleic acid (RNA).

•Two polynucleotide chains are linear but antiparallel (i.e. one chain runs in a 5' to 3' direction; the other runs 3' to 5').

•   The sugar and phosphate groups form the linear backbone of the strands with the bases projecting inwards towards their partners held together by hydrogen bonds between the opposing bases:

ü  Adenine (A) pairs only with thymidine (T) (2 hydrogen bonds)

ü  Guanine (G) pairs only with cytosine (C) (3 hydrogen bonds).

· Multiple fragments that, together with a protein skeleton (chromatin), form chromosomes.

· Site: Nucleus and Mitochondria.

  Mitochondrial DNA

Mitochondrial DNA (mtDNA), is a single chromosome, which is circular and 16.5 kb in length, replicates independently of nuclear DNA.

Importance: mtDNA contain 37 genes:

ü  13 genes (enzymes of energy)

ü  24 genes (tRNA, and mt rRNA for protein assembly).

Virtually all mitochondria are supplied by the oocyte, which means that mtDNA is maternally derived.

RNA

•Single-stranded; 90% is present in the cell cytoplasm and 10% in the nucleolus.

Unlike DNA Thymine base is replaced by uracil (U) base.

• Forms and their function:

1.     Messenger RNA (mRNA) is the template for polypeptide synthesis. It has a cap at the 5' end, and a poly A tail at the 3' end.

2.     Transfer RNA (tRNA) brings activated amino acids into position along the mRNA template.

3.     Ribosomal RNA (rRNA) is a component of ribosomes which functions as a non-specific site of polypeptide synthesis.

4.     Micro RNA (miRNA): controls expression of target genes.