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

Ch 7 - RNA and the Genetic Code Flashcards

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MCAT 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
Q

What are the repressible systems?

A
  • 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
Q

What do transcription factors do?

A

search for promoter and enhancer regions in the DNA

27
Q

Where are promoters located?

A

within 25 base pairs of the transcription start site

28
Q

Where are enhacers located?

A

more than 25 base pairs away from the transcription start site

29
Q

What do modifications of chromatin structure affect?

A
  • the ability of transcriptional enzymes to access the DNA through histone acetylation (increase accessibility) or DNA methylation (decrease accessibility)
30
Q

If DNA codes for proteins, why does it need mRNA?

A
  • 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
Q

What is an anticodon?

A
  • 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
Q

Why are mutations within an intron not likely to change protein sequence?

A

because introns are cleaved out of the mRNA transcript prior to translation

33
Q

What effect do silent, missense, nonsense, and frameshift mutations have on encoded proteins?

A
  • 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
Q

What is the role of RNA polymerase I, II, and III?

A
  • I: synthesizes mose rRNA
  • II: synthesizes mRNA (hnRNA) and snRNA
  • III: synthesizes tRNA and some rRNA
35
Q

What is alternative splicing and what does it accomplish?

A
  • 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
Q

What are the roles of each site in the ribosome during translation?

A
  • 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
Q

What are the major posttranslational modifications that occur in proteins?

A
  • proper folding by chaperones
  • formation of quaternary structure
  • cleavage of proteins or signal sequences
  • addition of other biomolecules (phosphorylation, carboxylation, glycosylation, prenylation)
38
Q

From 5’ to 3’, what are the components of the operon and what are their roles?

A
  • 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
Q

What is the positive and negative control system?

A
  • 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
Q

What is the difference between cis and trans regulators?

A
  • 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
Q

In an enhancer, what is the difference between signal molecules, transcription factors, and response elements?

A
  • 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
Q

By what histone and DNA modifications can genes be silenced in eukaryotic cells? Would these processes increase the proportion of heterochromatin or euchromatin?

A
  • 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
Q

How is the anticodon determined?

A
  • 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
Q

What type of linkage is created in peptide bonds?

A

amide linkage

45
Q

How is DNA read differently than mRNA?

A

they are both read 5’ to 3’, but mRNA is antiparallel to DNA

MCAT Biochem (12 decks)

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