Molecular Genetics Flashcards
What is the Base Pairing Rule in DNA?
A - T T - A
C - G G - C
In transcription what is the Base Pairing Rule in mRNA?
A - U U - A
C - G G - C
What ends are Nucleotides added to?
3’ prime end
Explain Base-Pairing Strength in terms of Bonds.
C - G pairs have 3 hydrogen bonds so they’re 50% stronger than A - T pairs which only have 2 hydrogen bonds.
Explain the steps of Transcription, and where it occurs.
Transcription takes place in the nucleus of the cell.
1) DNA strands unwound and separated by RNA polymerase.
2) The RNA polymerase then runs along the template strand in a 3’ to 5’ direction adding RNA nucleotides against the DNA strand according to the base pairing rule, A-U, C-G.
3) The mRNA separates from the template strand and the DNA joins back up.
4) The mRNA has its introns cut out (RNA processing)
Explain the Steps of RNA Processing, and where it occurs.
RNA processing takes place in the nucleus of the cell.
1) The introns are cut out of the pre-mRNA, and the exons are ligated together
2) A methyl cap is added to the 5’ prime end of the mRNA
3) A poly-A tail is added to the 3’ prime end. (poly-A tail = a long chain of nucleotides with the nitrogen-containing base adenine)
Explain the Steps of Translation, and where it occurs.
1) The mRNA enters the ribosome (5’ first because nucleotides are being added to the 3’)
2) tRNA molecules which have an amino acid at one end and a trinucleotide of 3 bases called an anticodon at the other, pair with specific complementary trinucleotides on the mRNA called codons.
3) Peptide bonds form between between the amino acids. Forming a polypeptide.
Translation takes place in the ribosomes of the cell. if the protein is to be exported from the cell, that ribosome will be located on the rough ER. if used within the cell that makes it, it may be synthesised on a free ribosome, floating in the cytoplasm.
List the differences between prokaryote and eukaryote genes
Prokaryotic genes:
- have no introns
- have no histones
- have no nucleus (so translation starts before translation has finished)
- do not undergo RNA processing
- have a guanine-rich ribosome binding site (RBS) instead of a 5’ methyl cap to which ribosomes bind
- have an operator between the promoter and the start codon (Eukaryotes don’t have an operator)
- sometimes exist in operons; a group of structura genes that share a single promoter
What is gene regulation? provide an example
gene regulation is the process of turning genes on or off, the trp Operon is an example of gene regulation.
How does the trp operon assist in the production of tryptophan?
The trp operon encode an enzyme that contributes to a biochemical pathway that results in the biosynthesis of tryptophan
Why is the trp operon important in tryptophan synthesis?
The synthesising of tryptophan takes a lot of energy and is not a common amino acid. So, the trp operon helps with the production because cells cant afford to waste energy making amino acids that they might not need anyway.
What are the two ways that the expression of the trp operon is regulated.
Repression and Attenuation
What is repression of the trp operon?
repression stops the expression of the trp operon by a repressor protein binding to the operator region that lies between the promoter and structural genes.
if there is a free tryptophan molecule in the cell it binds to the repressor protein, changing its shape so it can fit the operator and bind to that blocking RNA polymerase from transcribing the structural genes.
Explain what happens when repression fails.
from time to time the repressor protein can fall of the operator to which then RNA polymerase will start tryptophan synthesis. The cells back-up plan to stop this is attenuation.
What is attenuation?
Attenuation can stop the expression of tryptophan by causing transcription to fail.
Explain what happens in translation when there IS tryptophan tRNA molecules present in the cell (this al occurs at the leaderm region of the trp operon)
If there are tryptophan-bearing tRNA molecules present in the cell, the ribosome translating the mRNA will run past the pair of tryptophan codons and stop at a stop codon. If the ribosome makes it to the stop codon, it blocks region 2 from forming a hairpin with region 3, so whn it finishes transcribing region 4 it forms a hairpin with region 3. Because region 4 is involved it puts tension on the attenuator and the ribosome pulls away from the DNA. Transcription stops and the structural genes are not transcribed.
Explain what happens in translation when there IS NOT tryptophan tRNA molecules present in the cell (this al occurs at the leaderm region of the trp operon)
If there are NO tryptophan-bearing tRNA molecules then the ribosome will stop at the tryptophan codons and never make it to the stop codon. If the it stops at the tryptophan codons, then RNA polymerase keeps going and the mRNA forms a hairpin loop between regions 2 and 3, preventing ta hairpin forming between regions 3 and 4. because region 4 is not involved the ribosome doesn’t pull away from the attenuator and continues transcription.
