L10 - Protein Synthesis Flashcards
Describe the process of transcription.
1) RNA polymerase breaks apart the DNA strands
2) Single strand of mRNA is transcribed from the template strand using the base-pair rule
3) In mRNA the nuclease T is substituted by U
4) Required nucleotides for mRNA synthesis are found freely in the nucleus
What is a requirement of Pre-mRNA?
- Pre-mRNA is not in its final form
- Pre-mRNA requires a 5’ cap
What is the Pre-mRNA cap composed of, and how is it attached?
- Composed of phosphorylated 7-methyl guanosine
- It is added to the 5’ end of the mRNA by guanyltransferase
Why is Pre-mRNA capping necessary?
- Ensures mRNA is exported out of the nucleus
- Blocks degradation of mRNA by 5’ exonucleases
- Promotes translation
What is a second requirement of Pre-mRNA?
Needs a 3’ poly-A-tail
Describe the process of polyadenylation of Pre-mRNA.
- Pre-mRNA is cleaved by an endonuclease near a signal AAUAAA sequence at the 3’ end
- Approx. 200 adenosine residues are added at the cleavage site by poly-A-polymerase
What does the poly-A-tail do?
- Poly-A-tail protects the mRNA from degradation by 3’ exonucleases
- Poly-A-tail also aids in termination of transcription, ensures export from the nucleus and is important in translation
What is a third and final requirement of Pre-mRNA before it becomes mature mRNA?
- Pre-mRNA contains exons and introns
- Introns need to be spliced out to produce the final mRNA
What are exons?
Sequences which code for proteins
What are introns?
Sequences which do not code for proteins
Describe the entire process Pre-mRNA undergoes before becoming mature mRNA.
1) Addition of a 5’ cap
2) Cleavage near signal AAUAAA sequence at the 3’ end
3) Addition of 3’ poly-A-tail
What can alternative splicing do?
Alternative splicing of the pre-mRNA sequence can produce different proteins from the same gene.
What does mature mRNA consist of?
- 5’ cap
- 5’ UTR (Untranslated Region)
- Coding Region (to be translated into a protein)
- 3’ UTR (Untranslated Region)
- Poly-A-tail
What is initiation?
- Binding of the ribosome to 5’ end of mRNA
and - Hydrogen binding of the anticodon of an amino acylated tRNA carrying methionine on the AUG start codon
What is elongation?
Addition of further amino acids to the growing polypeptide chain, brought by corresponding amino acylated tRNAs
(Peptidyl transferase creates covalent peptide bonds between the amino acids)
What is termination?
When the stop codon (UAG, UAA, UGA) is reached and the peptide and ribosomal subunits are released.
What do proteins consist of?
Amino acids covalently linked into a polypeptide chain.
Name the 2 small amino acids.
- Alanine (Ala)
- Glycine (Gly)
Name the 3 nucleophilic amino acids.
- Serine (Ser)
- Threonine (Thr)
- Cysteine (Cys)
Name the 5 hydrophobic amino acids.
- Valine (Val)
- Leucine (Leu)
- Isoleucine (Ile)
- Methionine (Met)
- Proline (Pro)
Name the 5 aromatic amino acids.
- Phenylalanine (Phe)
- Tyrosine (Tyr)
- Tryptophan (Trp)
- Aspartic Acid (Asp)
- Glutamic Acid (Glu)
Name the 2 amides.
- Asparagine (Asn)
- Glutamine (Gln)
Name the 3 basic amino acids.
- Histidine (His)
- Lysine (Lys)
- Arginine (Arg)
What does one DNA triplet represent?
An amino acid
What are base triplets in mRNA called?
Codons
How many different amino acids are there?
20
How many different mRNA codons are there?
64 (4^3)
Explain why the genetic code is referred to as degenerate.
More than one codon can code for each of the 20 amino acids.
Why are the first 2 bases in each codon the most crucial?
Because they give tolerance against mutations.
State an mRNA start codon.
AUG (Methionine)
State the 3 mRNA stop codons.
- UAG
- UAA
- UGA
What are the 2 important processes for protein synthesis?
- Transcription (DNA to RNA)
- Translation (RNA to Protein)
Describe the substitution of bases, and its potential impacts.
- Substitution of bases changing the codon can change the corresponding amino acid
- This can change the protein structure significantly
- This may result in the creation of stop codons in the wrong places
Describe how the sickle cell shape of RBCs occurs.
- A change in the amino acid sequence causes the haemoglobin to crystallise when oxygen levels are low
- This results in the sickle cell shape, which causes RBCs to get stuck in small blood vessels
What can accumulation of sickled cells in small blood vessels lead to?
- Downstream tissue ischaemia, causing pain and infarction
- In severe cases: Organ damage, and ischaemic stroke
What is sickle cell anaemia?
An inherited and chronic disease with periodic painful attacks
Where is sickle cell anaemia most common and why?
- Common in Sub-Saharan Africa where sickled red blood cells provide protection against the malarial parasite
- Individuals with this mutation have a selective advantage
Why is DNA code transcribed into mRNA in the nucleus, but protein synthesis takes place outside the nucleus?
DNA is too big to leave the nucleus, but mRNA is small and mobile
Describe how mRNA leaves the nucleus.
- mRNA leaves the nucleoplasm via nuclear pores in the nuclear envelope, and enters the cytoplasm
- mRNA can then travel to ribosomes for translation of the DNA code into the proteins
Describe the structure of ribosomes.
- Composed of ribosomal RNA (rRNA) and ribosomal proteins
- Consist of 60S subunit and a smaller 40S subunit
- Can be free or attached to the rough endoplasmic reticulum
What is tRNA bound to an amino acid called?
Aminoacyl tRNA or charged tRNA
What is a tRNA molecule which has had its amino acid removed called?
Deacylated tRNA or uncharged tRNA
What is tRNA bound to a growing polypeptide chain called?
Peptidyl tRNA
Describe the fate of a synthesised polypeptide.
1) Has to acquire secondary structure (α-helices and β-pleated sheets)
2) Has to fold into tertiary structure
3) Has to assemble into quaternary structure if appropriate
4) Proteins have to reach their destination
Describe how different proteins are synthesised in different parts of the cell.
- Proteins destined for use within the cytoplasm are synthesised on free ribosomes
- Proteins destined for secretion out of the cell are synthesised on ribosomes attached to the rough endoplasmic reticulum (RER)
Describe the Rough Endoplasmic Reticulum (RER).
- RER is a system of flattened cavities
- Lined by a thin membrane running from the nuclear envelope and into the cytoplasm
- Has many ribosomes on its surface
- Provides a compartment for protein synthesis
What do secreted proteins have?
Special signal sequences which interact with the RER membrane.
What are secreted proteins incorporated into?
They are incorporated into vesicles (small spherical compartments made from RER membranes) for transport to the Golgi apparatus
Describe the journey of a secreted protein.
1) Incorporated into vesicles
2) Vesicles move to the Golgi apparatus
3) Post-translational modification of proteins occurs in the GA cavities
4) The now modified protein traverses the GA and is packaged into secretory vesicles
5) Proteins containing secretory vesicles move to the cell membrane, fuse with it and expel their contents into their extracellular space (exocytosis)
What are lysosomes?
Specialised GA vesicles
What do lysosomes contain?
Enzymes which digest old organelles
