D1.2 protein synthesis Flashcards
Describe the process and purpose of transcription (essay)
- In prokaryotic cells, once 1. takes place transcription begins.
- In eukaryotic cells, other transcription factors bind to the promoter region first and attract RNA polymerase for 1. to take place.
- RNA polymerase binds to the promoter sequence (usually upstream of the gene) near the beginning of the gene to be copied, forming a complex with the transcription factors.
- RNA polymerase unzips the double helix structure of DNA (like helicase), breaking the hydrogen bonds between complementary base pairs.
- RNA polymerase moves in a 5’ to 3’ direction, using the ANTISENSE DNA strand as a template
- RNA nucleotides that exist in the nucleus float into place by complementary base pairing, = hydrogen bonds form btwn bases of the antisense template of DNA and RNA nucleotides, 2 phosphates released. Synthesised in a 5’ to 3’ direction
- RNA polymerase reaches terminator region, mRNA molecule released.
DNA adenine pairs to RNA uracil
DNA thymine pairs to RNA adenine
PURPOSE:
to produce mRNA strands to be translated by a ribosome = transcription is a necessary step in GENE EXPRESSION
- hence transcription is a key stage at which the expression of a gene can be turned on or off
Why is DNA stable enough for single strands to be used as templates during transcription?
DNA is stable due to strong covalent phosphodiester bonds formed btwn nucleotide bases = single strands can be used as a template without undergoing any changes in its base sequence
- this is important in SOMATIC cells (cells which do not participate in cell division, e.g. nerve cells, but still need to produce RNA and proteins.)
purpose of mRNA, rRNA and tRNA in translation
rRNA combines with ribosomal proteins to construct cytoplasmic ribosomes (involved in the production of ribosomes)
mRNA brings the genetic code to the ribosome. Has a codon, which is COMPLEMENTARY to the anticodon of tRNA
tRNA brings amino acids to the ribosome. Has an anticodon. The 3 bases in its middle loop are called anticodon bases and they code for which of the 20 amino acids are attached to tRNA = there are 20 different types of tRNA, differentiated by their anticodon.
describe the process of translation (essay)
- (from transcription) mRNA molecule passes through the pores in the nucleus membrane and enters the cytoplasm
- SMALL SUBUNIT of the ribosome binds to the 5’ end of mRNA, ribosome moves along until it reaches the start codon (AUG)
- tRNA with specific amino acid attached to it binds to
the codon via its anticodon by complementary base pairing (forming hydrogen bonds) - LARGE subunit aligns itself to tRNA molecule at the P site, forms a complex with the SMALL subunit
- a second tRNA with its own amino acid binds to the LARGE subunit at the A site (like the first one), attaching by complementary base pairing to mRNA
- An enzyme catalyses a condensation reaction btwn the 2 amino acids, forming a peptide bond
- the 1st mRNA in the P site detaches from its amino acid (becoming deacylated) and from mRNA, floats off to attach to another amino acid of the same type
- the tRNA in the A site carrying the peptide chain moves to the P site
- the ribosome moves down the mRNA template in a 5’ to 3’ direction
- a 3rd tRNA attaches to the A site, pairs with mRNA (like the first 2)
- another peptide bond forms btwn the amino acids, forming an amino acid chain, process repeats until stop codon enters the ribosome (from mRNA)
- a RELEASE FACTOR is recruited (signals for translation to stop)
- the finished polypeptide is released and the ribosome disassembles back into its 2 independent subunits
structure of ribosome
composed of LARGE subunit and SMALL subunit
- mRNA binds to SMALL subunit
- up to 2 tRNA bind to LARGE subunit simultaenously
why are codons made of 3 nucleotide bases (and not 2 or 1?)
- there are 20 amino acids found in all living organisms = the genetic code must be able to code for all of these amino acids
- combining nucleotides into sequences of 3 produces 64 different codons = which code for 20 different amino acids
(e.g. using only combinations of 2 nucleotides would only produce 16 combinations)
Features of genetic code (3)
- genetic code is universal, the same code used in all life forms
- genetic code is degenerate = more than one codon may code for the same amino acid
- there are 3 stop codons which do not code for any amino acid, and terminate translation
Example and causes of a point mutation affecting protein structure
SICKLE CELL ANAEMIA:
- a single base SUBSTITUTION MUTATION for the alpha globin molecule of haemoglobin
- the amino acid change (from Glu to Val) changes the structure of haemoglobin
- Valine is hydrophobic, allowing the beta subunits to join together, causing haemoglobin to polymerize into insoluble fibrous strands.
- insoluble haemoglobin cannot carry oxygen as effectively = individual feels constantly tired
- if a person has 2 copies of the sickle cell allele, they suffer from sickle cell amenia = RBC becomes sickle shaped = block small blood vessels = block blood supply to vital organs
- the sickle cells are destroyed more rapidly than normal cells = lower RBC count = amaenia = bone marrow must replace the cells lost = damage to bone structure
causes:
DNA: change from GAG to GTG on the antisense DNA strand
mRNA: change from GAG to GUG at the 6th codon position
Polypeptide: 6th amino acid for the beta chain of haemoglobin changed from glutamic to valine (glu to val)
4 types of mutations
Point mutation:
- involves a change in only one base of a gene
- alters transcription, changes specific amino acid produced = affect protein structure
Missense mutation:
- Beneficial mutations change the gene sequence to create new variations of the same trait
Nonsense mutation:
- Detrimental mutations change the gene sequence to abrogate the normal function of a trait
Silent mutation:
- mutations that have no effect on the function
How is gene expression regulated?
Transcrption factors are regulator proteins that attach to the promoter region
- they mediate and control whether RNA polymerase can bind to promoter region, thus controlling gene expression
- these transcription factors bind to either the proximal control elements (near the promoter) or the distal control elements (at a distance)
How is the rate of transcription regulated?
Regulatory proteins bind to DNA sequences outside of the promoter (usually the distal control elements) and interact with transcription factors
- Activator proteins bind to enhancer sites and increase the rate of transcription by mediating transcription
- Repressor proteins bind to silencer sequences and decrease the rate of transciption by preventing complex formation
Examples of non-coding sequences in DNA and their functions (4)
Non-coding DNA are sections of DNA that do not code for functional polypeptides.
Controlling gene expression: some base sequences (e.g. promoters, enhancers, silencers) region are sites where regulatory proteins bind to promote or inhibit the transcription of genes
Genes for tRNA and rRNA: transcription of these genes produce tRNA and rRNA
Telomeres:
- repetitive sequences that protect the ends of chromosome, preventing impt genes from being lost, protecting against chromosomal deterioriation
- when eukaryotic DNA is replicated, telomeres are not replicated, so a short part of the telomeres are lost = length of telomeres limit the no. of cell cycles
Introns:
- Introns do not code for a protein and interrupt the gene sequence
- Extrons code for a protein
Hence introns are removed during transcription and not expressed in mRNA
Describe post-transcriptional modification and reasons for it (in eukaryotic cells), essay!
Introns are intruding sequences whereas exons are expressing sequences = introns need to be removed
Conversion of PRIMARY mRNA to MATURE mRNA transcript:
Capping:
- Addition of a methyl group to the 5’ end of the transcribed RNA to protect against degradation by exonucleases, and to allow the transcript to be recognised by the cell’s translational machinery
Polyadentation:
- A poly A-tail (a long chain of adenine nucleotides) added to the 3’ end to stabalise mRNA and facilitate the export of mRNA from nucleus
Splicing:
- Splicososomes attach to both ends of an intron, cutting out the intron
- the exons are spliced together to form MATURE mRNA (to be translated by ribosome to polypeptide)
What does alternative splicing of exons do?
alternative splicing is the selective removal of exons/splicing together different combinations of exons.
- this results in the formation of different polypeptides from a SINGLE gene sequence
= one gene can code for MANY polypeptides by alternative splicing
Functions of the tRNA binding sites (on the large ribosomal subunit)
A site: Aminoacyl site - tRNA brings a new amino acid to be added to the growing polypeptide chain
P site: Peptidyl site - holds tRNA attached to the growing amino acid chain. A peptide bond forms btwn the amino acids of the tRNA molecules at the A and P site. This bonding realeases the tRNA from the P to E site
E site: Exit site - when tRNA enters the E site it exits the molecule
