1. Transcription & Translation

2. Compare the structure of RNA and DNA
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

3. Transcription and Translation
4/19/2017
Types of RNA and their functions
G. Podgorski, Biol 1010

4. The genetic code – triplets of bases

5. 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

6. Relationship between genes and polypeptides
genes code for the synthesis of proteins (polypeptides)
one gene codes for one polypeptide chain
one 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

7. Transcription & Translation
Transcription;- 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.

11. Transcription
RNA polymerase is the enzyme that controls transcription process
RNA polymerase binds to a promoter region on the DNA
RNA polymerase unwinds the DNA strands & splits it into two strands
RNA polymerase binds free nucleoside triphosphates to the antisense (template) strand of DNA
as 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 process
transcription continues until RNA polymerase reaches a terminator signal
mRNA detaches from the template strand and DNA rewinds
RNA polymerase detaches from the DNA
many RNA polymerases can follow each other during transcription process
introns are removed & exons spliced (in eukaryotes) to form mature mRNA

12. Structure of ribosomes
ribosomes 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

13. Computer generated model of a ribosome

15. Translation consists 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 attached
First, 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

16. Formation of a dipeptide from two amino acids

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