BIOL 436

Question 1

What are the two major components of working telomerase?

Answer 1

Telomerase RNA and the telomerase protein (enzyme).

Question 2

What is the function of telomerase?

Answer 2

Lengthening telomeres.

Question 3

In what human cell types is telomerase normally expressed?

Answer 3

Germline cells (and possibly pluripotent stem cells).

Question 4

Why do cells stop replicating when telomeres become too short?

Answer 4

Telomeres serve as a primer for elongation of DNA strands. If they are too short to work as primers, the DNA can’t be replicated again.

Question 5

How do cancer cells lengthen their telomeres so that they can replicate indefinitely?

Answer 5

They express a cancer-specific telomerase.

Question 6

Why can’t we just fight aging by artifically increasing telomerase expression & activity?

Answer 6

Telomerase would affect stem cells and cancer cells, greatly increasing the risk of cancer.

Question 7

What needs to happen for telomerase RNA to achieve full catalytic activity?

Answer 7

It needs to form a complex with telomerase protein.

Question 8

What is 7SL RNA?

Answer 8

A component of the signal recognition particle (SRP) for proteins destined to be secreted out of the cell.

Question 9

What components are needed in a signal sequence for a protein destined for secretion outside the cell?

Answer 9

7SL RNA and an amino acid signal sequence.

Question 10

What is the difference in signal sequences between proteins destined for places inside the cell and proteins destined for secretion?

Answer 10

Proteins moving within the cell have a signal sequence made of amino acids only. Proteins destined for secretion have a signal sequence made of amino acids and 7SL RNA.

Question 11

What kind of RNA do most human RNA genes encode?

Answer 11

Antisense RNA.

Question 12

How many antisense RNA genes are in the human genome?

Answer 12

Over 1,500.

Question 13

How many tRNA genes are in the human nuclear genome?

Answer 13

Around 500.

Question 14

How many rRNA genes are in the human nuclear genome?

Answer 14

Around 700.

Question 15

What are the 4 main classes of human nuclear rRNAs?

Answer 15

28S, 18S, 5.8S, and 5S.

Question 16

How are human nuclear rRNA genes organized?

Answer 16

The 28S/18S/5.8S transcript gene is found in tandemly repeated clusters on chromosomes 21, 22, 13, 14, and 15. 5S RNA genes are found as 200-300 repeats on chromosome 1.

Question 17

Why are nuclear rRNA genes found on multiple chromosomes and in huge copy numbers?

Answer 17

We need lots of ribosomes expressed all the time to make proteins constantly. A higher gene dosage results in a greater number of gene products, i.e. more available rRNA to make more ribosomes.

Question 18

Why are there so many pseudogenes associated with rRNA genes?

Answer 18

rRNA genes are highly redundant, with many clustered repeats. A greater number of repeats means a greater number of replication events and a greater chance for the generation of pseudogenes over time.

Question 19

Which 5 chromosomes are acrocentric?

Answer 19

21, 22, 23, 14, and 15.

Question 20

Why aren’t deletions on the p arms of acrocentric chromosomes always lethal?

Answer 20

These regions house hundreds of redundant RNA genes, so even if some are deleted, there are backup copies elsewhere in the genome.

Question 21

What are the 2 main structural regions of precursor miRNA and siRNA?

Answer 21

The hairpin loop and the self-complementary stem region.

Question 22

True or false: miRNA & siRNA are double-stranded in precursor form, and single-stranded in mature form.

Answer 22

True.

Question 23

True or false: miRNA & siRNA are ~70 nucleotides long in precursor form, and ~21-23 nucleotides long after processing.

Answer 23

True.

Question 24

What is the main overall function of miRNA & siRNA?

Answer 24

To regulate gene expression by silencing target genes.

Question 25

What is the main difference between miRNA and siRNA in terms of processing?

Answer 25

miRNA undergoes 2 cleavages (by Drosha and Dicer), while siRNA only undergoes 1 cleavage (by Dicer).

Question 26

What are Drosha and Dicer?

Answer 26

Ribonucleases that cleave precursor miRNA (both Drosha and Dicer) and siRNA (Dicer only) to convert them to their mature, active forms.

Question 27

In what organism was miRNA first identified?

Answer 27

C. elegans.

Question 28

What feature of amino acid sequence of precursor miRNA & siRNA is responsible for their characteristic hairpin & stem loop structures?

Answer 28

Inverted repeats (i.e. self-complementarity).

Question 29

How many mature miRNA or siRNA products are created after cleavage by Drosha and/or Dicer?

Answer 29

1 or 2, depending on the RNA gene.

Question 30

True or false: miRNA & siRNA genes are well conserved in both plant and animal genomes.

Answer 30

True.

Question 31

How many miRNA & siRNA genes are in the human genome?

Answer 31

~200 miRNA genes and ~500-600 siRNA genes.

Question 32

Why are miRNA & siRNA genes hard to detect?

Answer 32

They are very small (~70 bp), and they can be hidden within noncoding regions of DNA or within introns of other genes (coding or noncoding).

Question 33

What is the most effective mechanism of gene silencing by miRNA & siRNA?

Answer 33

Triggering degradation of mRNA encoded by the target gene.

Question 34

What condition must be met for miRNA & siRNA to effectively destroy mRNA encoded by a gene targeted for silencing?

Answer 34

There must be strong complementarity between the mi-/siRNA and the target mRNA sequence.

Question 35

What mechanism of gene silencing via miRNA & siRNA is used to regulate tumour-suppressor genes and oncogenes?

Answer 35

Triggering degradation of mRNA encoded by the target gene.