BIOL 436 Flashcards
What are the two major components of working telomerase?
Telomerase RNA and the telomerase protein (enzyme).
What is the function of telomerase?
Lengthening telomeres.
In what human cell types is telomerase normally expressed?
Germline cells (and possibly pluripotent stem cells).
Why do cells stop replicating when telomeres become too short?
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.
How do cancer cells lengthen their telomeres so that they can replicate indefinitely?
They express a cancer-specific telomerase.
Why can’t we just fight aging by artifically increasing telomerase expression & activity?
Telomerase would affect stem cells and cancer cells, greatly increasing the risk of cancer.
What needs to happen for telomerase RNA to achieve full catalytic activity?
It needs to form a complex with telomerase protein.
What is 7SL RNA?
A component of the signal recognition particle (SRP) for proteins destined to be secreted out of the cell.
What components are needed in a signal sequence for a protein destined for secretion outside the cell?
7SL RNA and an amino acid signal sequence.
What is the difference in signal sequences between proteins destined for places inside the cell and proteins destined for secretion?
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.
What kind of RNA do most human RNA genes encode?
Antisense RNA.
How many antisense RNA genes are in the human genome?
Over 1,500.
How many tRNA genes are in the human nuclear genome?
Around 500.
How many rRNA genes are in the human nuclear genome?
Around 700.
What are the 4 main classes of human nuclear rRNAs?
28S, 18S, 5.8S, and 5S.
How are human nuclear rRNA genes organized?
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.
Why are nuclear rRNA genes found on multiple chromosomes and in huge copy numbers?
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.
Why are there so many pseudogenes associated with rRNA genes?
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.
Which 5 chromosomes are acrocentric?
21, 22, 23, 14, and 15.
Why aren’t deletions on the p arms of acrocentric chromosomes always lethal?
These regions house hundreds of redundant RNA genes, so even if some are deleted, there are backup copies elsewhere in the genome.
What are the 2 main structural regions of precursor miRNA and siRNA?
The hairpin loop and the self-complementary stem region.
True or false: miRNA & siRNA are double-stranded in precursor form, and single-stranded in mature form.
True.
True or false: miRNA & siRNA are ~70 nucleotides long in precursor form, and ~21-23 nucleotides long after processing.
True.
What is the main overall function of miRNA & siRNA?
To regulate gene expression by silencing target genes.
What is the main difference between miRNA and siRNA in terms of processing?
miRNA undergoes 2 cleavages (by Drosha and Dicer), while siRNA only undergoes 1 cleavage (by Dicer).
What are Drosha and Dicer?
Ribonucleases that cleave precursor miRNA (both Drosha and Dicer) and siRNA (Dicer only) to convert them to their mature, active forms.
In what organism was miRNA first identified?
C. elegans.
What feature of amino acid sequence of precursor miRNA & siRNA is responsible for their characteristic hairpin & stem loop structures?
Inverted repeats (i.e. self-complementarity).
How many mature miRNA or siRNA products are created after cleavage by Drosha and/or Dicer?
1 or 2, depending on the RNA gene.
True or false: miRNA & siRNA genes are well conserved in both plant and animal genomes.
True.
How many miRNA & siRNA genes are in the human genome?
~200 miRNA genes and ~500-600 siRNA genes.
Why are miRNA & siRNA genes hard to detect?
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).
What is the most effective mechanism of gene silencing by miRNA & siRNA?
Triggering degradation of mRNA encoded by the target gene.
What condition must be met for miRNA & siRNA to effectively destroy mRNA encoded by a gene targeted for silencing?
There must be strong complementarity between the mi-/siRNA and the target mRNA sequence.
What mechanism of gene silencing via miRNA & siRNA is used to regulate tumour-suppressor genes and oncogenes?
Triggering degradation of mRNA encoded by the target gene.
