Transcription & Translation

Assessment Statements 3.5.1 Compare the structure of RNA and DNA. 3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase. 3.5.3 Describe the genetic code in terms of codons composed of triplets of bases. 3.5.4 Explain the process of translation, leading to polypeptide formation. 3.5.5 Discuss the relationship between one gene and one polypeptide. 7.3.1 State that transcription is carried out in a direction. 7.3.2 Distinguish between the sense and antisense strands of DNA. 7.3.3 Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator. 7.3.4 State that eukaryotic RNA needs the removal of introns to form mature mRNA. 7.4.1 Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy. 7.4.2 Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites. 7.4.3 State that translation consists of initiation, elongation, translocation and termination. 7.4.4 State that translation occurs in a direction. 7.4.5 Draw and label a diagram showing the structure of a peptide bond between two amino acids. 7.4.6 Explain the process of translation, including ribosomes, polysomes, start codons and stop codons. 7.4.7 State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes.

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

How Spliceosomes Process RNA

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

Protein Synthesis - Animation

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

Free and membrane bound ribosomesFree ribosomes in the

cytoplasm are associated with the synthesis of proteins for internal use in the cell

Ribosomes which are attached to the wall of the endoplasmic reticulum are associated with proteins which will be placed into vesicles and secreted form the cell