topic 5 - final exam Flashcards
- (a) The sequence below shows a template strand found in Gene A.
What is the mRNA sequence? Label the ends and ensure that your answer is presented in the correct orientation of the transcript.
5’ - … TCG GGC TGC TCT GTC GTT AGC CAT TCG TGT … - 3’
As RNA polymerase reads in a 3’ to 5’ direction, reverse your template strand such that it
starts from the 3’ end.
3’ - … TGT GCT TAC CGA TTG CTG TCT CGT CGG GCT … - 5’
Then, transcribe the template strand into an mRNA strand, remembering that the
DNA-dependent RNA polymerase will i) synthesise RNA in a 5’ to 3’ direction and ii)
use U instead of T in RNA.
Final answer:
5’ - … ACA CGA AUG GCU AAC GAC AGA GCA GCC CGA … - 3’
(Always label the ends and present your answer in the 5’ to 3’ orientation)
(b) What is the protein sequence for this transcript? Be sure to label the ends and use the Triplet Code table.
mRNA:
5’… -ACA CGA AUG GCU AAC GAC AGA GCA GCC CGA… - 3’
Identify the start codon from the answer in (a) – as underlined.
5’ - … ACA CGA AUG GCU AAC GAC AGA GCA GCC CGA … - 3’
Translate the sequence starting from the start codon
Final answer:
(NH2 -) Met Ala Asn Asp Arg Ala Ala Arg (- COOH)
The sequence below shows the coding strand found in Gene C.
5’ -… CTA AAG GCC TAC CGA TCA TTA CGG … -3’
What is the mRNA sequence? Label the ends and ensure that your answer is
presented in the correct orientation of the transcript.
As RNA polymerase reads from the template strand (not the coding strand), you
would need to determine the sequence of the template strand. To derive the
sequence of the template strand, complement the coding strand:
or just exchange the T with U
Final answer:
5’ -… CUA AAG GCC UAC CGA UCA UUA CGG … -3
describe the components of DNA
DNA (deoxyribosnucleiic acid)
- DNa strands are antiparralele and froms a double heliec
- Bonds holding structure together
Phosphodiester bonds (sugars to sugar)
Hydrogen bonds (between base pair ) - Complementary bonding only (via h bonds) between certain DNA nucleotides
- A-T base pairs
- C-G base pairs
describe the components of RNA
- single stranded
- Still capable of complimentary base pairing
- Bonds holding structure together:
Phosphodiester bonds (sugars to sugar)
Hydrogen bonds (between base pair )
Complimentary bonding only (via h bods ) between certain RNA nucleotides - A-U base pairs
- C-G base pairs
differences between DNA and RNA
DNA forms double helixes and RNA doesnt
DNA has A- T base pairs RNA has A-U base pairs
similarities of DNA and RNA
C-G base pairs
complimentary boding via H bonds between certian nucleotides
has phosphodiester bonds
Describe the process of transcription
Transcription: initation
RNA polymerase
Enzyme that catalyses the synthesis of RNA from DNA
Unwinds and rewinds DNA
Reads DNA in 3’ to 5’ direction
Synthesizes RNA in 5’ to 3’ direction
Requires nucleoside triphosphates (NTPs)
ATP, GTP, CTP, UTP
Forms phosphodiester bonds between nucleotides
Transcription: Elongation
Chain elongation as RNA polymerase moves along the DNA molecule
- Elongated in 5’ to 3’ direction (nucleotides added to the 3’ growing end)
Transcription : termination
- At the end of the gene, RNA polymerase recieves termnation signal
- Formation of a hairpin structure
- Form due to GC- rich sequence that folds itself into a hair pin loop
- Or protein rho structure
- RNA polymeras detaches from DNA
describe the process of translation
STEPS OF TRANSLATION
INITIATION
- Binding of mRNA and aminoacyl-tRNA [tRNA carrying the first amino acid methionine (Met)] to ribosome
- In bacteria, a modified methionine (n-formylmethionine) is used to initiate translation
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-
ELONGATION
- Polypeptide chain elongation requires peptide bond formation between amino acids
Catalysed by the enzyme peptidyl transferase
the enticodon od an incoming aminoacyl tRNA base pairs with the complementary mRNA codon in the A-site.
an rRNA molecule of the large ribosomal subunit catalyses the formation of a peptide bond between carboxyl end of the polypeptide in the p site and the aa of the new aa in the A site
the ribosome translocates the tRNA in the A site to the p site. at the same time the empty tRNA in the P ssite is moved to the E site where it is released. the mRNA strand then moves along and the next codon is translated in the A site
TERMINATION
- when a ribosome reaches a stop codon on mRNA the A site of the ribosome accepts a release factor instead on aminoacyl tRNA, the release factor promotes hydrolysis of thebond between tRNA in the P site and the last aa of the polypeptide, Thus freeing the polypeptide from the ribosome
roles of ribosomes
Ribosome
- Site of protein synthesis
- Made up of rRNAs and proteins
- Consist of two subunits : small and large
- Prokaryotic vs eukaryotic ribosomes
Involved in protein synthesis (translation)
- Ribosome binding sites
mRNA binding site: binds to a specific nucleotide sequence to hold the mRNA in position
A(aminoscyl) site: holds tRNA carrying the next aa that is to be added to the growing chain
P (peptidyl) site: holds tRNA carrying the growing polypeptide chain
E(exit) site: tRNA leaves the ribosome
role of rRNA, mRNA, tRNA
Ribossomal RNA (rRNA)
- Found in ribosomes
- Relatively stable
Remains in cell in a functional sstate for reasonably long time
- Produced by nucleolus
- Have considerable secondary structures
Messenger RNA (mRNA)
- Highly specific
Codes for the amino acid sequence of a protein
- Much less stable than rRNA and tRNA
Short half-life: rapid turnover allows control of cell function
Cells only manufacture protein if needed
Varies in size: ranging from 200-700 nucleotides
Transfer RNA (tRNA)
- Involves in protein synthesis
> recognizes the coded bases sequence of mRNA
Bring amino acids to site of protein synthesis
One amino acid has at least one tRNA
- Relatively stable
Remains in cell in a functional state for reasonably long time
- Relatively smal molecules : 70-90nucleotides
- Have considerable secondary structure
describe the properties of genetic code
Genetic code
- Decodes mRNA into proteins
- Consists of 20 aa
- Each aa is a triplet of bases (codon)
- Non-overlapping
- Degenerate (most aa coded for r ≥ one codon) is not uniform
- 6 codon encodeing fro aa Ser and Leu
- Two codons encodeing for His and Tyr
- One codeon encodeing for Met and Trp
Code is always read from 5’ to 3’
- Start codon (or initiation ) is mostly the codon AUG, encoding for aa MET
- Three terminator (or stop) codons – UGA,UAA,UAG
The monomeric units of DNA are
Phosphate, deoxyribose sugar and N-containing base
The monomeric units of RNA are
Phosphate, ribose sugar and N-containing base
The genetic code is degenerate because
Most amino acids are coded by more than one codon
