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
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
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
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