MCAT BIO/BIOCHEM: Transcription and Translation Flashcards

Question 1

Define Central Dogma.

Accepted Answer

The flow of genetic information in cells from DNA to RNA by transcription, then from RNA to protein by translation; reverse transcriptase is the known exception that copies RNA back into DNA.

Question 2

What is Transcription?

Accepted Answer

The synthesis of an RNA molecule from a DNA template by RNA polymerase, adding ribonucleotides in the 5′ to 3′ direction; in eukaryotes it happens in the nucleus.

Question 3

Define Template Strand.

Accepted Answer

The DNA strand that RNA polymerase reads in the 3′ to 5′ direction; the RNA made is complementary to it. Also called the antisense or noncoding strand.

Question 4

Define Coding Strand.

Accepted Answer

The DNA strand not used as a template; written 5′ to 3′, it matches the RNA transcript except that it has thymine wherever the RNA has uracil. Also called the sense strand.

Question 5

Define RNA Polymerase II.

Accepted Answer

The eukaryotic nuclear enzyme that transcribes protein-coding genes into pre-mRNA, as well as most snRNAs and microRNAs; it needs general transcription factors to start at a promoter.

Question 6

What is Promoter?

Accepted Answer

A DNA sequence just upstream of a gene's transcription start site where RNA polymerase and general transcription factors assemble to begin transcription.

Question 7

Define TATA Box.

Accepted Answer

A thymine- and adenine-rich promoter element about 25–30 base pairs upstream of many eukaryotic genes, bound by TATA-binding protein to position RNA polymerase II.

Question 8

Define Transcription Factor.

Accepted Answer

A protein that binds specific DNA sequences, such as promoters or enhancers, and raises or lowers the rate at which RNA polymerase transcribes a nearby gene.

Question 9

What is Enhancer?

Accepted Answer

A regulatory DNA sequence that binds activator proteins and raises transcription of a gene, even from thousands of base pairs away, upstream, downstream or within an intron.

Question 10

What is Pre-mRNA?

Accepted Answer

The primary transcript of a eukaryotic protein-coding gene, which still contains introns and must be capped, spliced and polyadenylated before it leaves the nucleus as mature mRNA.

Question 11

Define 5′ Cap.

Accepted Answer

A 7-methylguanosine joined to the 5′ end of eukaryotic pre-mRNA by an unusual 5′-to-5′ triphosphate link; it protects the transcript from degradation and helps ribosomes bind.

Question 12

Define Poly-A Tail.

Accepted Answer

A stretch of roughly 200 adenine nucleotides added to the 3′ end of eukaryotic pre-mRNA after cleavage; it aids nuclear export, protects against degradation and promotes translation.

Question 13

What is Intron?

Accepted Answer

A noncoding segment within a eukaryotic gene that is transcribed into pre-mRNA but cut out by splicing before the mRNA is translated.

Question 14

What is Exon?

Accepted Answer

A segment of a gene that remains in the mature mRNA after splicing; exons carry the protein-coding sequence as well as the 5′ and 3′ untranslated regions.

Question 15

What is Spliceosome?

Accepted Answer

A large complex of small nuclear ribonucleoproteins (snRNPs) and proteins that cuts introns out of pre-mRNA and joins the neighboring exons together.

Question 16

Define Alternative Splicing.

Accepted Answer

The joining of different combinations of exons from one pre-mRNA, so that a single gene can encode several related proteins in different tissues or conditions.

Question 17

What is Codon?

Accepted Answer

A sequence of three consecutive mRNA nucleotides that specifies one amino acid or a stop signal during translation; the standard genetic code has 64 of them.

Question 18

What is Anticodon?

Accepted Answer

A three-nucleotide sequence on a tRNA that base-pairs antiparallel with a complementary mRNA codon, matching that codon to the amino acid the tRNA carries.

Question 19

Define Start Codon.

Accepted Answer

AUG, the codon where translation begins in most mRNAs; it is read by the initiator tRNA carrying methionine (formyl-methionine in bacteria) and sets the reading frame.

Question 20

Define Stop Codon.

Accepted Answer

One of the three codons UAA, UAG or UGA, which no tRNA reads; a release factor binds instead and ends translation by freeing the finished polypeptide.

Question 21

Define Degeneracy of the Genetic Code.

Accepted Answer

The property that most amino acids are encoded by more than one codon, since 61 sense codons specify only 20 amino acids; synonymous codons usually differ at the third base.

Question 22

Define Wobble Pairing.

Accepted Answer

Non-standard base pairing allowed between the first anticodon position and the third codon position, such as guanine with uracil; it lets fewer than 61 tRNAs cover every sense codon.

Question 23

Define Reading Frame.

Accepted Answer

The division of an mRNA's sequence into consecutive, non-overlapping triplets, fixed by the start codon; any stretch of sequence has three possible frames on one strand.

Question 24

Define Aminoacyl-tRNA Synthetase.

Accepted Answer

An enzyme that attaches a specific amino acid to its matching tRNAs using ATP; cells usually have one for each amino acid, and their accuracy sets the fidelity of the genetic code.

Question 25

What is Ribosome?

Accepted Answer

A ribonucleoprotein machine of one large and one small subunit that reads mRNA codons and links amino acids into a polypeptide; prokaryotic ribosomes are 70S, eukaryotic ones 80S.

Question 26

Define A Site.

Accepted Answer

The ribosome's aminoacyl site, where each incoming charged tRNA binds by pairing with the next mRNA codon during elongation.

Question 27

Define P Site.

Accepted Answer

The ribosome's peptidyl site, which holds the tRNA attached to the growing polypeptide; the initiator tRNA is the only one that enters it directly.

Question 28

Define E Site.

Accepted Answer

The ribosome's exit site, where a tRNA that has handed over its amino acid sits briefly before leaving the ribosome to be recharged.

Question 29

Define Peptidyl Transferase.

Accepted Answer

The catalytic activity of the large ribosomal subunit that forms each peptide bond; it is carried out by rRNA rather than protein, which makes the ribosome a ribozyme.

Question 30

Define Shine-Dalgarno Sequence.

Accepted Answer

A purine-rich sequence a few bases upstream of the start codon on bacterial mRNA that pairs with 16S rRNA, positioning the small subunit to begin translation.

Question 31

Define Release Factor.

Accepted Answer

A protein that recognizes a stop codon in the ribosome's A site and triggers hydrolysis of the bond linking the finished polypeptide to the tRNA in the P site.

Question 32

Define Signal Sequence.

Accepted Answer

A short hydrophobic stretch, usually at a protein's N-terminus, that sends the ribosome making it to the rough endoplasmic reticulum through the signal recognition particle.

Question 33

What is Polysome?

Accepted Answer

A single mRNA being translated by several ribosomes at once, spaced along its length, which multiplies the protein made from one transcript.

Question 34

Define Missense Mutation.

Accepted Answer

A point mutation that changes a codon to one coding a different amino acid, which may alter protein function depending on the chemistry and position of the swap.

Question 35

Define Nonsense Mutation.

Accepted Answer

A point mutation that turns an amino-acid codon into a stop codon, producing a shortened protein that is usually non-functional and often triggering mRNA decay.

Question 36

Define Silent Mutation.

Accepted Answer

A point mutation that changes a codon to a synonymous codon, so the encoded amino acid and the protein's sequence stay the same.

Question 37

Define Frameshift Mutation.

Accepted Answer

An insertion or deletion of a number of nucleotides not divisible by three, which shifts the reading frame and changes every codon downstream of it.

Question 38

Define Post-Translational Modification.

Accepted Answer

A chemical change made to a protein during or after translation, such as phosphorylation, glycosylation or cleavage, that alters its activity, location or lifespan.