Introduction to Molecular Biology 3

Question 1

Through what does mRNA have to exit to be exported from the nucleus and into the membrane?

Answer 1

Nuclear pore

Question 2

What are three types of gene mutation?

Answer 2

1- Deletion/nonsense
2- Insertion/frameshift
3- Substitution/missense

Question 3

What is deletion?

Answer 3

1- Base pair is deleted

2- Leads to frameshift or nonsense

Question 4

What is insertion?

Answer 4

1- New base pair is inserted

2- Leads to frameshift or nonsense

Question 5

What is substitution?

Answer 5

1- Base pair is substituted or replaced with a different base pair
2- Missense
3- AA replaced

Question 6

What is polymorphism?

Answer 6

1- Minor change in sequence
2- Usually SNP (single nucleotide polymorphism)
3- Present in more than 1% of population
4- Main cause of genetic variation

Question 7

What are the base pairs of the start codon?

Answer 7

1- AUG

2- Methionine (Met)

Question 8

What is the role of tRNA?

Answer 8

1- ‘Adaptor’
2- Carries amino acids which correspond to mRNA sequence
3- Anti-codon, binds to codon of mRNA

Question 9

Describe the structure of tRNA.

Answer 9

1- 80 nucleotides long
2- Clover leaf-like structure
3- Anti-codon present at the end which corresponds to mRNA codon

Question 10

How does the right amino acid get added to the right tRNA?

Answer 10

1- 20 different amino acids
2- 20 different aminoacyl-tRNA synthesases
3- Each synthesase corresponds only to one amino acid and its compatible/cognate tRNAs
4- Amino acid is activated via enzymes and ATP
5- Amino-acyl is added to tRNA

Question 11

How is the amino acid activated and transferred?

Answer 11

1- Activation:
Enzyme + AA + ATP –(Mg++)–> enzyme(aminoacyl-AMP) + PPi
2- Transfer:
tRNA + enzyme(aminoacyl-AMP) —-> aminoacyl-tRNA + enzyme

Question 12

Compare eukaryote and prokaryote ribosomes.

Answer 12

1- Eukaryote: 80s
small subunit is 40s, large subunit is 60s
2- Prokaryote: 70s
small subunit is 30s, large subunit is 50s

Question 13

How many binding sites do ribosomes have?

Answer 13

1- Four binding sites
2- One for mRNA
3- Three for tRNAs

Question 14

What are the three phases of translation?

Answer 14

1- Initiation
2- Elongation
3- Termination

Question 15

Describe the structure of a nuclear pore

Answer 15

A ring of subunits with a nuclear basket connected by nuclear fibrils

Question 16

Where can ribosomes be found?

Answer 16

Free in cytoplasm or found attached to the endoplasmic reticulum

Question 17

What is the difference between base pairs in DNA from RNA?

Answer 17

1- DNA has thymine

2- RNA has uraccil

Question 18

What are the main differences in structure between DNA and RNA?

Answer 18

1- DNA is a double helix
2- RNA is a single strand
3- RNA may have secondary or tertiary structure

Question 19

What is the function of DNA?

Answer 19

Information storage

Question 20

What are the functions of RNA?

Answer 20

• mRNA: genetic messenger
  2- tRNA: adaptor, physical link between mRNA and amino acids
  3- gRNA: guide RNAs for RNA editing
  4- Structural RNA: e.g. Xist, helps inactivate X chromosome
  5- Catalysts: rRNA links amino acids together, snRNAs help with DNA splicing

Question 21

What are promoters?

Answer 21

1- Region of DNA
2- Found before transcription site, signals its beginning
3- 100-200 bases long

Question 22

What are terminators?

Answer 22

1- Region of DNA

2- Marks the end of transcription

Question 23

What are the three different types of RNA polymerases?

Answer 23

1- RNA pol I
2- RNA pol II
3- RNA pol III

Question 24

What RNA does RNA pol I generate?

Answer 24

rRNA

Question 25

What RNA does RNA pol II generate?

Answer 25

Protein coding RNA

Question 26

What RNA does RNA pol III generate?

Answer 26

tRNA

Question 27

What is the TATA box?

Answer 27

1- Promoter sequence 2- Facilitates binding of RNA pol II 3- Facilitates binding of transcription factors such a TBP (TATA binding factor) 4- It is located 25 bases upstream of the starter code 5- TBP distorts shape of DNA once bound which allows binding of RNA pol II

Question 28

How does RNA polymerase bind to DNA and why?

Answer 28

1- RNA polymerase binds to the factor-promoter complex 2- Trans-acting factors bind to cis-acting promoter 3- RNA cannot bind directly to promoter

Question 29

What are enhancers?

Answer 29

1- Short DNA sequences 2- 50-1500 base pairs 3- Are not always located close to the transcription start site as they can work from a distance 4- Bind to transcription factors to increase likelihood of a specific section of DNA being transcripted 5- Opposed by silencers

Question 30

What opposes enhancers?

Answer 30

Silencers

Question 31

What are the two domains of transcription factors?

Answer 31

1- DNA binding domain | 2- Activation domain

Question 32

What are six ways in which transcription can be controlled?

Answer 32

1- Protein is only synthesised when transcription needs to be carried out 2- Protein is inactivated by phosphorylation 3- Protein is inactivated by dephosphorylation 4- Protein is inactivated by ligand binding 5- Protein is inhibited and rendered inactive by an inhibitor 6- Protein has an inactive binding partner which is switched with an active binding partner when transcription needs to be carried out

Question 33

Why must mRNA be processed?

Answer 33

1- To protect from exonucleases 2- To aid their export from the nucleus 3- So it can be identified as mRNA

Question 34

What are exonucleases?

Answer 34

Enzymes which cleave nucleotides

Question 35

What are introns?

Answer 35

A segment of DNA or RNA which does not code for genes

Question 36

What are exons?

Answer 36

A segment of DNA or RNA which codes for genes after introns are spliced

Question 37

What are the steps of transcription?

Answer 37

1- DNA is unwinded by helicase 2- Genes are transcribed into pre mRNA 3- Intorns are removed by RNA splicing, producing mRNA 4- mRNA is exported from the nucleus into the cytoplasm

Question 38

What are the steps of RNA splicing?

Answer 38

1- Splicesome carrying RNAse and snRNA base pairs to the end of intron 2- Intron is spliced at its 5' end 3- 5' end of intron pairs with the downstream branch sequence, forming a lariat 4- The two exons are then ligated together

Question 39

What is Frasier's syndrome?

Answer 39

1- Genetic mutation 2- Autosomal-dominant 3- Mutation of intron site leading to absence of three amino acids

Question 40

What is the importance of post-transcriptional modifications?

Answer 40

Many different proteins can be made from the same DNA

Question 41

How can DNA be unwound?

Answer 41

Through histone-modifying complex or Chromatin remodelling complex

Question 42

What causes aniridia?

Answer 42

A defect in the eye caused by a genetic mutation of the transcription factor PAX6

Question 43

What are the symptoms of aniridia?

Answer 43

Cornea opaque Iris absent Retina degenerate Lens opaque Increased pressure