Transcription & Translation

Compare the structure of RNA and DNA

DNA RNA

DNA is double-stranded molecule

RNA is single-stranded molecule

DNA contains deoxyribose sugar RNA contains ribose sugar

There is one form of DNA There are three forms of RNA: tRNA; mRNA and rRNA

DNA has nitrogenous base thymine

RNA has nitrogenous base uracil

DNA & RNA are polymers of nucleotides i.e. both are nucleic acids

DNA & RNA contain four nitrogenous bases: A, G, C, T for DNA & A, G, C, U for RNA

Types of RNA and their functions

The genetic code – triplets of bases

The genetic code it is composed of mRNA base triplets called codons

there are 64 different codons each coding for the addition of an amino acid to a growing polypeptide chain

the genetic code is degenerate i.e. more than one codon can code for a particular amino acid

the genetic code is universal i.e. it is the same in almost all organisms

AUG codes for Methionine & is the start codon

some nonsense codons code for the end of translation i.e. act as stop codons

Relationship between genes and polypeptidesgenes code for the synthesis

of proteins (polypeptides)one gene codes for one

polypeptide chainone gene is transcribed into

one mRNA mRNA is translated by a

ribosome to synthesize a polypeptide

if the information on a gene is changed (i.e. mutated), this may alter the structure of a protein

genetic information transcribed by eukaryotes is edited before it is translated

polypeptides may be altered before they become fully functional proteins

Transcription & TranslationTranscription;- the

synthesis of RNA from a DNA template, DNA base sequence of the gene is copied into messenger RNA (mRNA)

Translation;- base sequence on mRNA is translated into an amino acid sequence in a polypeptide chain

One gene is transcribed and translated to produce one polypeptide chain.

TranscriptionRNA polymerase is the enzyme that controls transcription processRNA polymerase binds to a promoter region on the DNARNA polymerase unwinds the DNA strands & splits it into two strandsRNA polymerase binds free nucleoside triphosphates to the antisense

(template) strand of DNAas it moves along in a 5'-> 3' direction�using complementary pairing (A with U & C with G) between

template strand and mRNA nucleotides nucleoside triphosphates loses two phosphates to release the energy

required for transcription processtranscription continues until RNA polymerase reaches a terminator

signal mRNA detaches from the template strand and DNA rewindsRNA polymerase detaches from the DNAmany RNA polymerases can follow each other during transcription

processintrons are removed & exons spliced (in eukaryotes) to form mature

mRNA

Structure of ribosomesribosomes are

composed of ≈ 60 % ribosomal RNA (rRNA) & 40 % protein

ribosomes consists of two sub-units; small sub-unit (30S) & large sub-unit (50S)

small sub-unit has binding site for mRNA

large sub-unit has three tRNA binding sites: Aminacyl (A), Peptidyl (P) and Exit (E) site

in eukaryotes, ribosomes can be free or bound to rough ER

Computer generated model of a ribosome

Translationconsists of initiation, elongation and termination mRNA translated in a 5' -> 3' direction mRNA binds to small ribosome sub-unit then to large ribosome sub-unit tRNA activating enzymes link correct amino acid to each tRNA activated tRNA

has an anticodon and the corresponding amino acid attachedFirst, initiator tRNA binds to start codon (AUG )on the small subunit of ribosome

Second, tRNA binds to large ribosome subunit ribosome moves down mRNA after a second tRNA binds

amino acid/ polypeptide on first tRNA is transferred & bonded to amino acid on

second tRNA peptide bonds between amino acids catalysed by peptidyl transferase

Translation requires Guanosine-5'-triphosphate (GTP) a source of energy & (or)

substrates

movement of ribosome down the mRNA continues , loss of tRNA and new tRNA binds

Ribosome reaches a stop codon - termination

polypeptide released, the ribosome sub-units detach from the mRNA

Formation of a dipeptide from two amino acids

A diagram showing the structure of a peptide bond between two amino acids