IB BIO TOPIC 3.5 : TRANSCRIPTION & TRANSLATION Flashcards
Where does Transcription and Translation happen ?
Transcription happens in the nucleus of the cell and Translation in the cytoplasm at the Ribosome.
What is transcription ?
In protein synthesis, it’s the copying of a part of the DNA.
What is translation ?
In protein synthesis, it’s the production (cooking) of the copied DNA.
Compare DNA and RNA Structures
DNA RNA
sugar deoxyribose ribose
purines adenine, guanine same
pyrimidines thymine, cytosine uracil, cytosine
strands double single (usually)
Outline DNA transcription in terms of formation of complementary RNA strand by RNA polymerase:
- Initiation
RNA polymerase is an enzyme complex which:
unwinds and unzips DNA double strand
attaches to promoter region of gene, which marks the beginning point for transcription
Outline DNA transcription in terms of formation of complementary RNA strand by RNA polymerase:
- Elongation
RNA polymerase:
uses DNA anti-sense strand as a template
synthesizes a complementary RNA strand using base pairing rules
A = U T = A G ≡ C C ≡ G
Outline DNA transcription in terms of formation of complementary RNA strand by RNA polymerase:
- Termination
RNA polymerase :
reaches termination region of the gene, which marks the end of the coding sequence
terminates transcription by releasing both DNA and RNA
Describe the Genetic Code in terms of codons composed of triplets of bases :
triplet code = 3 nucleotide bases code for one amino acid codon = a group of 3 nucleotide bases is called a codon there are 64 different codons (4 x 4 x4 = 64)
Explain the process of translation, leading to peptide linkage formation :
Initiation :
- mRNA binds to the small subunit of the ribosome
- tRNA anticodon binds to mRNA codon by complementary base pairing
- large ribosomal subunit binds, completing ribosomal structure, and producing two ribosomal binds sites: P site & A site
Explain the process of translation, leading to peptide linkage formation
Elongation : part 1
- tRNA with anticodon complementary to second mRNA codon binds to ribosomal A site, with appropriate amino acid attached to tRNA
- enzymes in ribosome catalyze formation of peptide bond between 1st, P site, and 2nd, A site, amino acids
- P site tRNA, now separated from amino acid, exits ribosome
- ribosome moves one codon (3 nucleotides) along the mRNA, thus shifting previous A-site tRNA to P-site, and opening A-site
Explain the process of translation, leading to peptide linkage formation
Elongation : part 2
- tRNA with anticodon complementary to A-site mRNA codon binds to ribosomal A-site, with appropriate amino acid attached to tRNA terminal
- enzymes in ribosome catalyze formation of peptide bond between 2nd and 3rd amino acids
- P site tRNA, now separated from its amino acid, exits ribosome
- ribosome moves one codon (3 nucleotides) along the mRNA, thus shifting previous A-site tRNA to P-site, and opening A-site
- repetition of process until stop codon is reached
Explain the process of translation, leading to peptide linkage formation :
Termination : Part 1
- when ribosomal A-site reaches a stop codon, no tRNA has a complementary anticodon
- release factor protein binds to ribosomal A-site stop codon
- polypeptide and mRNA are released
- large and small ribosomal subunits separate
- polysomes: several to many ribosomes translating the same mRNA into protein; each moving in the 5’ to 3’ direction
Explain the process of translation, leading to peptide linkage formation :
Termination : Part 2
- start codon: the mRNA triplet codon AUG is universally the start codon used to mark the beginning of the coding sequence of a gene; thus, the tRNA with the anticodon UAC and carrying the amino acid methionine is always the first tRNA to enter the P-site during translation
- stop codon: there are three stop codons in the genetic code; none of these have a corresponding tRNA; instead, when a ribosome encounters a stop codon, a release factor binds to the stop codon, which terminates translation and allows the separation of all of its components
Define degenerate :
two or more codons can code for the same amino acid
Define universal :
all living organisms use the same genetic code, indicating common ancestry; even viruses use the same code
Explain the relationship between one gene and one polypeptide and its significance
- when the relationship between genes and proteins was first discovered it was initially thought that the relationship was one-to-one: one gene coding for one polypeptide
What is a gene ?
- information coded in DNA nucleotide sequences
- transcribed into mRNA
- mRNA translated into a sequence of amino acids joined by peptide bonds to produce a polypeptide
What is a polypeptide ?
- polymer of amino acids joined by peptide bonds
- each polypeptide’s function dependent on its precise sequence of amino acids
What are the exeptions for the genes ?
- some genes produce more than one polypeptide
- there are only about 21,000 human genes, but over 120,000 human proteins
- therefore, many genes produce more than one protein
- this is possible because of post-transcriptional modification, combining exons in various combinations
- example: lymphocyte production of antibodies:
- millions of different antibody proteins are produced from just a few genes
- different lymphocytes splice together parts of these genes in different ways
What are some other exeptions ?
- some genes do not code for protein
- some genes code for tRNA
- not translated into protein
- transports amino acids to ribosomes
- some genes code for rRNA
- not translated into protein
- a component of ribosome structure and function
- some DNA sequences act as regulators of gene expression
- regulatory DNA is transcribed into regulatory RNA
- which then binds to other DNA sequences
- determining whether those genes are transcribed or not
