S1-L8: Protein Synthesis Flashcards
What is the transcription process and how does it occur? (refer to figure 1)
- RNA polymerase breaks DNA strand apart
- single mRNA strand transcribed from template strand via single base pair ruling
- In mRNA nucleobase T substituted by U
- required nucleotides for mRNA synthesis found free in nucleus
Outline what pre-mRNA capping is (figure 2)
- early mRNA (pre-mRNA) not in final form
- pre-mRNA needs 5’ cap
- ->cap composed of phosphorylated 7-methyl guanosine which added to 5’ end of mRNA guanyl transferase
Why is pre-mRNA capping important?
- ensures mRNA transported out of nucleus
- blocks degradation of mRNA by 5’ exonucleases
- promotes translation
What is polyadenylation of pre-mRNA? (figure 3)
- pre-mRNA needs 3’ poly-A-tail
- pre-mRNA cleaved (split) by endonuclease near signal AAUAAA sequenced at 3’ end
- ->approx. 200 adenosine residues then added at cleavage site by poly-A-polymerase
In appropriate detail explain the importance of polyadenylation
- this poly-A-tail protects mRNA from degradation by 3’ exonucleases
- poly-A-tail also aids in termination of transcription/ ensures export from nucleus AND important in translation
What is pre-mRNA splicing? (refer to figure 4)
- pre-mRNA contains exons (code for proteins) AND sequences not coding for proteins (introns)
- ->introns need to be spliced out to produce final mRNA
Define alternative splicing
-alternative splicing of pre-mRNA sequence able to produce different proteins of same gene
Outline the composure of mature mRNA (refer to figure 5)
- 5’ cap
- 5’ UTR (untranslated) region
- coding region- to be translated in to protein
- 3’ UTR region
- poly (A) tail
What is the composition of a protein? (figure 6)
-consists of amino acids covalently linked into polypeptide chain
Which amino acids fall in to each of the following categories?: (refer to figure 7)
1-small 2-nucleophilic 3-hydrophobic 4-aromatic 5-acidic 6-amide 7-basic
1-Glycine (Gly)/ Alanine (Ala)
2-Serine (Ser)/ Threonine (Thr)/ Cysteine (Cys)
3-Valine (Val)/ Leucine (Leu)/ Isoleucine (Ile)/ Methionine (Met)/ Proline (Pro)
4-Phenylamine (Phe)/ Tyrosine (Tyr)/ Tryptophan (Trp)
5-Aspartic acid (Asp)/ Glutamic acid (Glu)
6-Asparaginine (Asn)/ Glutamine (Gln)
7-Histidine (His)/ Lysine (Lys)/ Arginine (Arg)
How is the specific order of amino acids in protein encoded in DNA/mRNA? (refer to figure 8)
1-coding of DNA & template DNA 2-transcription of template DNA 3-mRNA formed 4-translation of mRNA 5-peptide formed
How are amino acids represented by DNA?
-DNA base triplets represent each amino acid
Define a codon
-base triplets in mRNA called codons
State the number of amino acids AND mRNA codons there are
- 20 amino acids
- 64 (4^3) mRNA codons
Define the term degenerate to describe the genetic code
-more than one codon codes for each of 20 amino acids
Why are the first two bases in codons most crucial?
- possibly give tolerance against mutations
- example: UCA/UCC/UCG/UCU all code for serine
How do some mRNA codons have special roles? (figure 9)
- start codon- AUG (methionine)
- stop codon- UAG/UAA/UGA
What determines the sequence of amino acids in proteins?
-order of bases in genetic code in DNA codes for appropriate sequence of amino acids in proteins
Outline the two important processes in protein synthesis
- transcription (DNA to RNA)
- translation (RNA to protein)
Briefly explain what substitution means and its possibly effect (refer to figure 10)
- of bases changing codon can change corresponding amino acid
- ->this may change protein structure significantly AND possibly result in stop codon being created in wrong place
How may mutations in sickle cell anaemia occur? (figure 11)
- DNA sequence- T substituted with A and A substituted with T
- ->amino acid changes from glutamic acid to valine (mutant)
- ->amino acid sequence changes
What is the effect of the change in the amino acid sequence to the red blood cells?
- change in amino acid sequence causes hemoglobin to cyrstallise when O2 levels low
- ->causes sickle shape which gets stuck in small blood vessels
Explain the following clinical consequence of the mutation:
accumulation of sickle cells in small blood vessels
- downstream tissue ischaemia- causes pain AND infarction
- inherited & chronic disease with periodic painful heart attacks
How does the sickle-cell trait affect the African population and the Africa-American population in the U.S?
- common in Sub-Saharan Africa- sickled red blood cells provide protection against malarial parasites
- ->gives individual with mutations selective advantage
- In US malaria not endemic-sickle cell trait slowly disappearing from Africa-American population
What may happen if a deletion/ insertion in the DNA sequence occurs?
- could result in significant changes of protein sequence
- ->frameshift mutation
Outline the steps taken for protein synthesis (refer to figure 12)
- DNA code transcribed into mRNA in nucleus
- protein synthesis takes place outside nucleus
- DNA too big to leave nucleus BUT mRNA small + mobile
- ->mRNA leaves nucleoplasm via nuclear pore in nuclear envelope & enters cytoplasm
- ->mRNA then able to travel to ribosomes for translation of DNA code in to proteins
What is the composure of ribosomes? (figure 13)
- composed of ribosomal RNA (rRNA) & ribosomal proteins
- consists of 60S subunit AND smaller 40S subunit
How may you find ribosomes with the rough ER?
-can be free OR attached to the rough endoplasmic reticulum
Explain how tRNA molecules work (figure 14)
- tRNA molecules have complementary base triplets which match up to mRNA code
- there is tRNA for each codon
- tRNA molecules have amino acids attached
- matching tRNA & mRNA means amino acids assembled in correct sequence
Outline the structure of tRNA molecules (figure 14)
- tRNA bound to amino acid Aminoacyl tRNA (AKA charged tRNA)
- tRNA molecules which had amino acids removed known as deacylated OR uncharged tRNA
- tRNA molecule bound to growing polypeptide chain known as peptidyl tRNA
Define the term initiation
-binding of ribosomes to 5’ end of mRNA AND hydrogen binding of anticodon of aminoacylated tRNA carrying methionine on AUG start codon
What is elongation?
- further amino acid addition to growing polypeptide brought corresponding aminoacylated tRNA’s
- peptidyl transferase creates covalent peptide bonds between amino acids
Outline what termination is
-when stop codon (UAG, UAA and UGA) reached AND peptide & ribosomal subunits released
What happens in translation? (refer to figure 15)
- translation process incorporates 20 different amino acids in precise sequence
- ->dictated by 3-base codons built from 4 bases from alphabet
- this process in ribosome builds polypeptide chains- which will become proteins
Once polypeptide chain has been synthesised what happens to it
1-has to acquire secondary structure
–>a-helices AND B-pleated sheets
2-needs to fold into tertiary structure
3-then has to assemble into quaternary structure if appropriate
-proteins have to reach their destination
–>proteins destined for use within cytoplasm synthesised on free ribosomes
–>proteins destined for secretion out of cell synthesised on ribosomes attached to rough ER
Describe the rough endoplasmic reticulum (RER)
- system of flattened cavities
- ->lined by thin membrane running from nuclear envelope AND into cytoplasm & with many ribosomes on it’s surface
- ->provides compartment for protein synthesis
Explain the first stage of the synthesis- the protein on ribosomes attached to ER (figure 16 is a visual representation)
- secreted proteins have special signal sequence- these interact with RER membrane- synthesis on ribosomes
- proteins incorporated into vesicles for transport to golgi apparatus
- ->small spherical compartments made from RER membrane
Describe the golgi apparatus
- system of flattened plate-like cavities
- ->lined by thin membrane
Explain what happens in the second stage of synthesis where modification occurs (figure 17)
- vesicle moves to golgi apparatus (golgi complex)
- post-translation modification occurs of protein in GA cavities
- ->like glycosylation of membrane spanning proteins
After the modification has happened what happens to the protein (refer to figure 17)
- modified protein transverses golgi apparatus AND is packed in to secretory vesicles
- protein containing secretory vesicles moves to cell membrane
- ->fuses with it and expels it’s content into extracellular space (exocytosis)
- some specialised golgi apparatus vesicles- called lysosomes contain enzyme which able to digest old organelles
