7.2 transcription and gene expression

Question 1

what does the promoter do?

Answer 1

-a sequence near the start of the gene at which the RNA polymerase will bind
- it is important to initiate transcription.

Question 2

what is transcription?

Answer 2

• the formation of a molecule of messenger RNA (mRNA) from a DNA
  template in the nucleus.
• is the first stage of protein synthesis and this process occurs inside
  the nucleus

Question 3

what is the reason for mRNA to leave the nucleus?

Answer 3

DNA is needed to form proteins at the ribosome but it’s to big to leave the nuclear pores.
the mRNA is used to pass the DNA ‘message’ through transcription.

Question 4

what is step one of transcription?

Answer 4

• INITIATION: RNA polymerase binds to a site called the promoter on the template strand of DNA.
• The DNA is unwound by RNA polymerase.

Question 5

what is step two of transcription?

Answer 5

ELONGATION: Free nucleotide triphosphates (NTPs) are used to create the growing strand of mRNA with two phosphate removed as NTPs are converted into RNA nucleotides.
- Breakage of the phosphate group release energy stored in the covalent bond.
- RNA polymerase slides along the DNA, adding the 5’ end of the free RNA nucleotide to the 3’ end of the growing mRNA molecule.
- Therefore mRNA is synthesised in a 5’ to 3’ direction.

Question 6

what is step three of transcription?

Answer 6

TERMINATION: At the 5’ end of the gene, a terminator site is found which stops the
transcription process.
- The mRNA detaches from the DNA and the double helix reforms.

Question 7

what is the point of chemical modification on the tails of histones?

Answer 7

• an important factor in
  determining whether the gene is expressed or not.
• either activate or deactivate genes
  by decreasing or increasing the accessibility of the gene to transcription factors.

Question 8

what are the chemical modifiers for histone proteins?

Answer 8

• acetyl group (acetylation) – neutralises positive histone tail charges to decrease coiling for more transcription.
• methyl group (methylation) – maintains positive histone tail charges to increase coiling and decrease transcription.
• phosphate group (phosphorylation).

Question 9

what are transcription factors?

Answer 9

-proteins that bind to DNA and regulate transcription.
- Heterochromatin is when DNA is supercoiled and not accessible for DNA transcription.
- Euchromatin is the term to describe loosely packed DNA around nucleosomes and so it is accessible to transcription machinery

Question 10

what do introns do?

Answer 10

• non-coding sequences that are removed or spliced to form a mature
  mRNA before it is translated.
• sequences that do not contribute to the formation of the
  polypeptide.

Question 11

what do exons do?

Answer 11

they are fused or spliced together to form mature mRNA.

Question 12

what is post-transcriptional modification?

Answer 12

Post-transcriptional modification is unique to eukaryotic cells (no introns in their RNA).
- After RNA splicing, a 5’ cap (a methyl group) is added before transcription is completed as it protects against degradation by exonucleases and ensures recognition by ribosomes.
- After transcription is complete, a poly-A tail (polyadenylation) is added to the 3’ end other transcript to improve the stability of the transcript.

Question 13

what is alternative splicing of mRNA used for?

Answer 13

• process where a single gene codes for multiple proteins due to multiple exons.
• Sometimes, a particular exon is not included in the mature mRNA at all.
• Therefore, the protein translated from alternatively spliced mRNA will differ in their amino acid sequence and perhaps even in their biological function.
• This increases the number of different proteins an organism can produce.