Biology[Module 2:Nucleotides and Nucleic Acids]

DNA

-polymer

-the monomer is a nucleotide

-combines wit proteins(histones)

-packed in chromosomes in eukaryotes

Structure

-double helix:-2 strands each containing a polynucleotide

-phosphate group,deoxyribose sugar, nitrogenous base

difference between ribose and deoxyribose

-on c2 ribose has OH group

Polynucleotide strand formation

-phosphodiester bond between c3 and c5

Double strands

-antiparallel

-complementary bases are held together by hydrogen bond:C=G 3 3 hydrogen bonds

A=T 2 hydrogen bonds

-purine bases(double ring structure):A,G

-pyramidal bases(single ring structure):T,C

DNA replication

-dna makes a copy of itself to:

-repair damages cells

-growth

DNA replication steps

-DNA helices unwinds the double helix and hydrogen bonds holding the strands togehther

-both strands act as a template

-new nucleotides pair up with complementary bases on template strands

-hydroogen bonds form between the complementary bases

-DNA polymerase joins the sugar and phosphate backbone of the new strand by condensation recation

-dna replication is semi-conservative as one strand comes from parent and one is synthesised

RNA

-single stranded

-phospate group, ribose sugar, nitrogenous base(A-U,C-G)

types of RNA

-ribosomal RNA=ribosome

-messenger RNA=carries genetic material from DNA to ribosome(protein synthesis)

-tRNA=

• found in cytoplasm

• carries amino acids tot he ribosome during translation

• it has amino acid binding site at one end(top) and a sequence of 3 bases and the other end(anticodon)

• shape of a clover leaf

Transcription steps

-making mRNA from DNA

-DNA double helix unzips,h-bond are broken by DNA helicase

-2 strands of DNA separate

-one of the strands act as a template

-mRNA nucleotides A-U,G-C now forms complementary base pairs with template strand

-RNA polymerase joins the phosphodiester bonds of single stranded mRNA

-single stranded of mRNA now leaves the nucleus

-DNA double helix zips back into double strands

Translation

-protein synthesis using mRNA

-mRNA is transferred from nucleus to ribosomes in cytoplasm

-2 tRNA molecules carrying specific amino acids bind to the mRNA codons using their complementary codons

-2 amino acids are joined together by a peptide bond

-ATP provides energy

-the ribosome moves along the mRNA codons, forming a polypeptide until a stop codon is reached

Genetic code

-triplet code:3 sequence of dna bases codes for one amino acid

diffrent codes

-degenerate=most aminoacids have more than one codon

-stop codons=3 codons code for no amino acids

-non overlapping=each nucleotide base is read only once

-universal=same codon codes for the same amino acid in all organisms

similarities between dna replication&transcription

phosphodiester bonds

differences

-DNA replication:

copy the entire dna

2 identical dna mol

a-t c-g

entire genome

-Transcription:

produces mRNA for protein synthesis

single stranded DNA

a-u c-g

specific genes only