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MCAT Biochem > Ch 7 - RNA and the Genetic Code > Flashcards

Ch 7 - RNA and the Genetic Code Flashcards

1
Q

What is the central dogma?

A

states that DNA is transcribed to RNA, which is translated to protein

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2
Q

What does the degenerative code allow?

A

multiple codons to encode for the same amino acid

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3
Q

What are the initiation and termination codons?

A
  • start: AUG

- stop: UAA (U Are Annoying), UGA (U Go Away), UAG (U Are Gone)

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4
Q

What does redundancy and the wobble allow?

A
  • mutations to occur without effect in the protein

- wobble: third base in the codon often plays no role in determining which amino acid is translated from the codon

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5
Q

What can point mutations cause?

A
  • silent mutations with no effect on protein synthesis (substitution of base in the wobble, introns, or noncoding DNA)
  • nonsense mutations that produce a premature stop codon
  • missense mutations that produce a codon that codes for a different amino acid
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6
Q

What do frameshift mutations result from?

A

nucleotide addition or deletion, and change the reading frame of subsequent codons

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7
Q

How is RNA structurally different from DNA?

A
  • substitution of a ribose sugar for deoxyribose
  • substitution of uracil for thymine
  • it is single stranded instead of double stranded
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8
Q

What are the 3 types of RNA with separate jobs in transcription?

A
  • mRNA: carries the message from DNA in the nucleus via transcription of the gene; it travels into the cytoplasm to be translated
  • tRNA: brings in amino acids and recognizes the codon on the mRNA using its anticodon
  • rRNA: makes up the ribosome and is enzymatically active
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9
Q

What does the helicase do?

A

unwind the DNA double helix

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10
Q

When starting transcription, where does the RNA polymerase bind?

A

binds to the TATA box within the promoter region of the genes (25 base pairs upstream from the first transcribed base)

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11
Q

What does hnRNA do?

A

synthesized from the DNA template strand

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12
Q

What are the major posttranscriptional modifications?

A
  • splicing: removal of introns, joining of exons; uses snRNA and snRNPs in the spliceosome to create a lariat, which is then degraded; exons are ligated together
  • 5’ cap: addition of a 7-methylguanylate triphosphate cap to the 5’ end of the transcript
  • 3’ poly A tail: addition of adenosine base to the 3’ end to protect against degradation
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13
Q

How is splicing done?

A
  • by snRNA and snRNPs in the the spliceosome

- introns are removed in a lariat structure and exons are ligated together

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14
Q

How do prokaryotic cells increase the variability of gene products from one transcript?

A

through polycistronic genes in which starting transcription in different sites within the gene leads to different gene products

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15
Q

How can eukaryotic cells increase the variability of gene products?

A

through alternative splicing by combining different exons in a modular fashion to acquire different gene products

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16
Q

What is tRNA role in translation?

A

translates the codon into the correct amino acid

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17
Q

What are ribosomes role in translation?

A

they are the factories where translation (protein synthesis) occurs

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18
Q

What is the difference between initiation of translation in prokaryotes v eukaryotes?

A
  • initiation in prokaryotes occurs when the 30s ribosome attaches to the Shine-Dalgarno sequence and scans for a start codon; it lays down N-fMet in the P site of the ribosome
  • in eukaryotes, occurs when the 40s ribosome attaches to the 5’ cap and scans for a start codon; it lays down Met in the P site of the ribosome
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19
Q

What occurs during elongation of translation?

A
  • involves the addition of a new aminoacyl-tRNA into the A site of the ribosome and transfer of the growing polypeptide chain from the tRNA in the P site to the tRNA in the A site
  • the now uncharged tRNA pauses in the E site before exiting the ribosome
20
Q

What occurs during termination of translation?

A
  • occurs when the codon in the A site is a stop codon

- a release factor places a water molecule on the polypeptide chain and thus releases the protein

21
Q

Which part of translation help with each step in recruitment and assembly/disassembly of the ribosome?

A

initiation, elongation, and release factors

22
Q

What is the Jacob-Monod model?

A

of repressors and activators, explains how operons work

23
Q

What are operons?

A

inducible or repressible clusters of genes transcribes as a single mRNA

24
Q

What are the inducible systems?

A
  • lac operon
  • bonded to a repressor under normal conditions
  • can be turned on by an inducer pulling the repressor from the operator site
25
What are the repressible systems?
- trp operon - transcribed under normal conditions - can be turned off by corepressor coupling with the repressor and the binding of this complex to the operator site
26
What do transcription factors do?
search for promoter and enhancer regions in the DNA
27
Where are promoters located?
within 25 base pairs of the transcription start site
28
Where are enhacers located?
more than 25 base pairs away from the transcription start site
29
What do modifications of chromatin structure affect?
- the ability of transcriptional enzymes to access the DNA through histone acetylation (increase accessibility) or DNA methylation (decrease accessibility)
30
If DNA codes for proteins, why does it need mRNA?
- mRNA is the messenger of genetic information - DNA codes for protein but cannot perform any of the important enzymatic reactions that proteins are responsible for in cells - mRNA takes the information from the DNA to the ribosomes, where creation of the primary protein structure occurs
31
What is an anticodon?
- during translation, the codon of the mRNA is recognized by a complementary anticodon on a tRNA - the anticodon sequence allows the tRNA to pair with the codon in the mRNA
32
Why are mutations within an intron not likely to change protein sequence?
because introns are cleaved out of the mRNA transcript prior to translation
33
What effect do silent, missense, nonsense, and frameshift mutations have on encoded proteins?
- silent: none - missense: one amino acid is changed in the protein; variable effects on function depending on specific change - nonsense: early truncation of protein; variable effects on function, but usually more severe than missense - frameshift: change in most amino acids after the site of insertion or deletion; usually the most severe of the types
34
What is the role of RNA polymerase I, II, and III?
- I: synthesizes mose rRNA - II: synthesizes mRNA (hnRNA) and snRNA - III: synthesizes tRNA and some rRNA
35
What is alternative splicing and what does it accomplish?
- the ability of some genes to use various combinations of exons to create multiple proteins from one hnRNA transcript - this increases protein diversity and allows a species to maximize the number of proteins it can create from a limited number of genes
36
What are the roles of each site in the ribosome during translation?
- A: binds incoming aminoacyl-tRNA using codon-anticodon pairing - P: holds growing polypeptide until peptidyl transferase forms peptide bond and polypeptide is handed to A site - E: transiently holds uncharged tRNA as it exits the ribosome
37
What are the major posttranslational modifications that occur in proteins?
- proper folding by chaperones - formation of quaternary structure - cleavage of proteins or signal sequences - addition of other biomolecules (phosphorylation, carboxylation, glycosylation, prenylation)
38
From 5' to 3', what are the components of the operon and what are their roles?
- regulator gene: transcribed to form repressor protein - promoter site: site of RNA polymerase binding - operator site: binding site for repressor protein - structural gene: gene of interest; its transcription is dependent on the repressor being absent from the operator site
39
What is the positive and negative control system?
- positive: require the binding of a protein to the operator site to increase transcription - negative: require the binding of a protein to the operator site to decrease transcription
40
What is the difference between cis and trans regulators?
- cis: DNA regulator base sequences (promoters, enhancers, response elements) because they are in the same vicinity as the gene they control - trans: transcription factors because they have to be produced and translocated back to the nucleus; they travel through the cell to their point of action
41
In an enhancer, what is the difference between signal molecules, transcription factors, and response elements?
- signalmolecules include steroid hormones and second messengers, which bind to their receptors in the nucleus - these receptors are transcription factors that use their DNA-binding domain to attach to a particular sequence in DNA called a response element - once bonded to the response element, these transcription factors can then promote increased expression of the relevant gene
42
By what histone and DNA modifications can genes be silenced in eukaryotic cells? Would these processes increase the proportion of heterochromatin or euchromatin?
- histone deacetylation and DNA methylation will both downregulate the transcription of a gene - these processes allow the relevant DNA to be clumped more tightly, increasing the proportion of heterochromatin
43
How is the anticodon determined?
- through base pairing - anticodon and codon are antiparallel and nucleic acid always read 5' to 3' ex. valine (GUU, GUC, GUA, GUG) - anticodon must end with AC (pair the first 2 and switch)
44
What type of linkage is created in peptide bonds?
amide linkage
45
How is DNA read differently than mRNA?
they are both read 5' to 3', but mRNA is antiparallel to DNA

MCAT Biochem (12 decks)

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