dna genomics ll (translation and transcription) Flashcards
what is a gene
specific sequence of nucleotides in a DNA molecule which codes for a specific sequence of amino acids in one polypeptide chain
features of the genetic code
- it is a universal code
- it is a degenerate code (more than one codon codes for the same amino acid)
- it is a non-overlapping code
- it is a continuos code
- it is a triplet code
- it includes start and stop codons
components of a gene
promoter
termination sequence
transciption unit
definition of transcription
process where by the nucleotide sequence of a gene is used as a template to direct synthesis of RNA made up of complementary base sequences
function of promoter
DNA sequences that function as a recognition site for binding of RNA polymerase and regulatory proteins to initiate transcription
determine which one of the 2 strands of DNA molecule is used as a template for transcription
events of transcription
initiation
elongation
termination
post transciptional modfication
what happens in initiation (transcription)
- RNA polymerase attaches to promoter with the aid of protein factors
- RNA polymerase breaks hydrogen bonds between complementary base pairs = unzipes and separates the two strands of the DNA helix
- only one strand, with promotor, is used as the template to synthesise complementary mRNA strand
what happens in elongation
- free ribonucleotides will bind to deoxyribonucleotides on DNA template strand by complementary base pairing
- adenine forms 2 hydrogen bonds with uracil, thymine forms 2 hydrogen bonds with adenine, cytosine forms 3 hydrogen bonds with guanine
- RNA polymerase catalyses formation of phosphodiester bonds between adjacent ribonucleotides = form sugar-phosphate backbone of growing mRNA
- mRNA strand is synthesised in 5’ to 3’ direction as new ribonucleotides are added to 3’ OH end of growing mRNA strand
- as RNA polymerase continues down the template strand, region of DNA that has just beem transcribed reanneals
what occurs in termination
after RNA polymerase transcribes through the termination sequence, the mRNA chain is released and the RNA polymerase will dissociate to terminate transcription
what happens in post transciptional modification
- addition of 5’ cap
- RNA splicing
- polyadenylation
what happens in addition of 5’cap
cap of a methylated guanosine nucleotide will be added to the 5’ end of the pre-mRNA
what happens in RNA splicing
by a spliceosome, intron sequences are excised out and the exons are joined together to form the mature mRNA
what happens in polyadenylation
- pre-mRNA is cleaved by an endonuclease 10-35 nucleotides downstream of the polyadenylation signal = AAUAA sequence
- poly-A-polymerase adds many adenine nucleotides downstream of the signal
what is the function of mRNA
- takes information out of the nucleus via the nuclear pore to the cytoplasm where translation takes place
- acts as template for translation
- sequence of codon within coding region of mRNA determines polypeptide sequence
what are the functions of tRNA
bring in specific amino acids in a sequence corresponding to sequence of codons in mRNA to growing polypeptide
facilitates translation by
1. its ability to bind to a specific single amino acid during amino acid activation
2. the ability of its anticodon to base-pair with mRNA codon during translation
what is the least number of different tRNAs that should exist in any cell
at least 20 different tRNAs, each corresponding to 1 of the 20 different amino acids
function of rRNA
- associates with a set of proteins to form ribosomes
- main constituent of the interface between the large and small subunits of ribsome: allows mRNA to bind to small ribosomal unit by complementary base pairing between the rRNA in the mRNA binding site in small ribosomal unit and mRNA
- main constituent of the P and A site on large ribosomal subunit: enables binding of aminoacyl-tRNAs to P and A site by complementary base pairing
- part of rRNA molecule on large ribosomal subunit acts as ribozyme peptidyl transferase, catalysing formation of peptide bond between 2 amino acids
what are teh different sites in the large subunit of the ribosome
aminoacyl site: holds incoming tRNA carrying next amino acid to be added
peptidyl site: holds tRNA carrying the growing polypeptide chain
exit site: where tRNA leaves the ribosome
define translation
process by which a sequence of ribonucleotides in an mRNA molecule is used as a template to synthesize a sequence of amino acids in a polypeptide chain
what are the events of translation
- initiation
- codon recognition
- peptide bond formation
- translation
- termination
what happens before translation for it to occur
amino acid activation
a specific amino acid is covalently attached to the 3’ CCA stem of a specific tRNA with a specific anticodon to form a aminoacyl tRNA
catalysed by a specific aminoacyl-tRNA synthetase
how is tRNA activated with the correct amino acid
- there are 20 different aminoacyl tRNA synthetases, one for each amino acid
- the enzyme have specific active sites that are complementary in shape and charge to specific amino acid and the different tRNAs through their specific acitve sites at their acceptor stem and/or anticodon loop of the specific tRNA molecule
what happens in initiation
- translation initiation factors will facilitate binding of small ribosomal subunit and initiator tRNA carrying methionine to newly synthesised mRNA strand
- anticodon UAC of initiator tRNA will complementary base pair with start codon AUG of mRNA
- binding od large ribosomal subunit will complete ribosome forming translational initiation complex
4.formation of complex positions initiator tRNA at P side, leaving A site vacant for incoming aminoacyl-tRNA molecules
what happens in codon recognition
a second aminoacyl tRNA with a specific anticodon and corresponding amino acid complementary base pairs with a specific mRNA codon at the A site by forming hydrogen bonds
what happens in peptide bond formation
- a peptide bond is formed between adjacent amino acids, catalysed by peptidyl transferase on the large ribosomal subunit
- methioine dissociates from initiator tRNA
- methioine remains bound to second amino acid at A site
what happens in translocation
- ribosome translocates in 5 to 3 direction, shifting first tRNA to E site and allowing it to be released into cytosol
- tRNA with growing polypeptide chain is now at P site
- A site will hold a new incoming aminoacyl tRNA with an anticodon complementary to next codon on mRNA
- process repeats until a stop codon is reached
what happens in termination
- termination begins when stop codon reaches A site
- release factors will enter A site, causing hydrolysis of covalent ester bond between polypeptide chain and tRNA in P site
- polypeptide is released from ribosome and will fold into its secondary and tertiary structures
- ribosome dissembles into its subunits
what are the stop and start codons
start: AUG
stop: UAA/UAG/UGA
