Gene Expression

Question 1

What is the difference between **heterochromatin **and Euchromatin?

Answer 1

Heterochromatin:

• Relatively few active genes
• Dark bands
• A + T rich
• replicate late in S phase

**Euchromatin **

• 80% active genes
• Light bands
• G + C rich
• Replicate early in S phase

Question 2

What is the difference between constitutive and regulated gene expression?

Answer 2

Constitutive
•Products made by all cells all the time -often called ‘housekeeping’ genes
•Expression is constant

Regulated
•Time (developmental), place (cell type), amount, in response to signals
•Can be under very tight control

Question 3

Illustrate the different products of gene expression

Answer 3

Next

Question 4

Discuss the role of the spliceosome in RNA splicing of protein-coding genes

Answer 4

The spiceosome is a large ribonucleoprotein complex that splices primary RNA transcripts to remove introns

Splicing occurs between a **donor splice site **and an acceptor splice site downstream. The intervening intronic region between the two genes is loooped and cleaved out of the mRNA transcript at the donor and acceptor splice sites. The two sites are then bound together to form an exonic mRNA

Question 5

What processes can be modulated to alter gene expression?

Answer 5

Question 6

How does chromatin remodelling affect gene expression?

Answer 6

Chromatin remodelling requires chemical modification (epigenetic markers) to histones and DNA

Acetylation promotes open euchromatin conformation

Methylation promotes heterochromatin conformation

DNA methylation suppresses gene expression at CpG islands which surround promotor regions of genes

Question 7

What is required for transcriptional regulation?

Answer 7

Initiation of transcription requires RNA polymerase and other proteins (transcription factors) to bind at promoter regions (egTATA box) at the very minimum.

Question 8

Is there a single promotor region to a gene?

Answer 8

No

Many genes have more than one promotor region that dictates the beginning of transcription

This introduces greater diversity into the genome

Question 9

What post-transcriptional regulation of gene expression can occur?

Answer 9

Alternative splicingof different exonscan lead to different protein products -often tissue specific

Alternatively spliced transcripts can be identified for almost every human gene

Depending upon the splicing regulation; a sequence can be an intron or exon

Question 10

What effects can long non-coding RNAs have of gene expression?

Answer 10

Long non-coding RNAs can regulate gene expression as:

Decoy Molecules:

• act as decoys that titrate away DNA-binding proteins such as transcription factors or RNA polymerase

Scaffold molecules:

• bring or more proteins together into a functional complex of spatial proximity

Guidance molecules:

• recruit proteins to DNA

Enhancer

• Assists in looping the chromosome/DNA sequence and guiding proteins to the loop

Question 11

How can siRNAs degrade mRNA

Answer 11

Short interfering RNA (siRNA) can bind a RNA-induced silencing complex (RISC) within the cytoplasm

Upon binding, the siRNA can bind a complementary sequence of mRNA where the RISC then cleaves at this site

This degrades mRNA

Question 12

What are the effects of miRNAs?

Answer 12

Although the mechanism remains unclear, miRNA molecules can bind RNA-induced silencing complexes and caused repressed translation.

miRNAs direct the complex to complementary mRNA where its presence prevents further translation by ribosomes