Code of life: expression of the genome

Question 1

genome

Answer 1

total genetic content, the DNA of an organism

Question 2

gene

Answer 2

piece of DNA (genetic information) that contains the code for a protein (polypeptide)

Question 3

central dogma of molecular biology

Answer 3

DNA → transcription → RNA → translation → protein

Question 4

transcription: RNA synthesis

Answer 4

production of a single-stranded RNA molecule from a double-stranded DNA template; the first step in gene expression - involves copying a gene’s DNA sequence to make a RNA molecule
- the top strand contains the coding strand (5’→3’)
- the bottom (complementary) strand is the non-coding or template strand
- the mRNA that is made from this piece of DNA will be synthesized on the template DNA strand and will have the same sequence (in RNA) as the coding DNA strand

Question 5

RNA polymerase I - genes transcribed

Answer 5

most ribosomal RNA (rRNA) genes

Question 6

RNA polymerase II - genes transcribed

Answer 6

all protein-coding genes (mRNA)

Question 7

RNA polymerase III - genes transcribed

Answer 7

transfer RNA (tRNA) genes (5S rRNA gene)

Question 8

stages in transcription - initiation

Answer 8

1. RNA polymerase binds to promoter
2. polymerase unwinds the DNA strands
3. RNA synthesis is initiated at start point of the template strand
4. RNA polymerase works with transcription factors

Question 9

stages in transcription - elongation

Answer 9

1. polymerase moves downstream, unwinding DNA
2. RNA transcript is elongated 5’ to 3’
3. DNA strands re-form a double helix after transcription

Question 10

stages in transcription - termination

Answer 10

1. RNA transcript released
2. polymerase detaches from DNA

Question 11

stages in transcription for prokaryotes

Answer 11

1. sigma factor released
2. polymerase clamps down on DNA
3. termination and release of both polymerase and completed RNA transcript
4. sigma factor rebinds

Question 12

RNA capping and polyadenylation

Answer 12

• pre-mRNA is the immediate product of transcription
• pre-mRNA undergoes post transcriptional modifications, such as, 5’ cap (guanine) and poly-A tail (adenine) resulting in messenger RNA (mRNA)

Question 13

messenger RNA

Answer 13

code for proteins

Question 14

ribosomal RNA

Answer 14

form the core of the ribosome’s structure and catalyze protein synthesis

Question 15

microRNA

Answer 15

regulate gene expression

Question 16

transfer RNA

Answer 16

serve as adaptors between mRNA and amino acids during protein synthesis

Question 17

other noncoding RNA

Answer 17

used in RNA splicing, gene regulation, telomere maintenance etc.

Question 18

translation: protein synthesis

Answer 18

production of a protein from a mRNA molecule

protein synthesis is catalyzed by ribosomes, which can:
- bind to and move along the mRNA
- capture tRNA-amino acid molecules that are complementary to mRNA codons
- covalently link the amino acids to form a polypeptide chain (protein)

Question 19

amino acids

Answer 19

building blocks of proteins, coupled by formation of a peptide bond

Question 20

deciphering the RNA code

Answer 20

• DNA and RNA each consist of only 4 nucleotides
• proteins consist of 20 amino acids
• mRNA is read in groups of 3 or triplets
  
  • each triplet is called a codon
  • each codon corresponds to a single amino acid
• the genetic code shows redundancy:
  
  • triplets of DNA bases define 20 different amino acids
  • multiple triplets encode the same amino acid

Question 21

tRNA - intermediate between mRNA and protein

Answer 21

an intermediate molecule used to translate the triplets into amino acids

• they are attached to a specific amino acid at one side (CCA-3’)
• single-stranded RNA that folds onto itself, resulring in base pairing and hair pin loops
• they have an anticodon on the other side
  
  • the anticodon can form basepairs with a codon in mRNA
• the mRNA codon and the tRNA anticodon interact specifically through base pairing
• the mRNA is read 5’→3’

Question 22

tRNA - amino acid interaction

Answer 22

specific tRNA’s are coupled to their specific amino acid partner by aminoacyl-tRNA synthetases (one for every amino acid)

Question 23

coupling of amino acids

Answer 23

stepwise growth of polypeptide chain from amino (N) to carboxy (C)-terminus through peptide bond formation

Question 24

ribosomes structure

Answer 24

a ribosome consists of a large complex of:
- numerous small proteins; 1/3 of total weight
- several RNA molecules - rRNAs; 2/3 of total weight
- rRNA is responsible for overall structure and catalysis of protein synthesis

Question 25

ribosomes structure - sites

Answer 25

• E-site = (exit) uncharged tRNA
• P-site = (peptidyl) holds the tRNA which is linked to the growing polypeptide chain
• A-site = (aminoacyl) of a ribosome is a binding site for charged tRNA molecules during protein synthesis

Question 26

how does protein synthesis start?

Answer 26

1. translation of an mRNA begins with the codon AUG (start codon)
2. a special (initiator) tRNA is required to initiate translation
3. this initiator tRNA always carries the amino acid methionine

Question 27

translation initiation

Answer 27

• only an initiator tRNA can bind to the P site of the small ribosomal subunit
• protein elongation follows
• translation is terminated at a stop codon of the mRNA (UAA, UAG or UGA)

Question 28

location of transcription and translation - bacterial cells

Answer 28

in a bacterial cell, which lacks a nucleus, mRNA produced by transcription is immediately translated without additional processing

Question 29

location of transcription and translation - ekukaryotes

Answer 29

1. the nucleus provides a separate compartment for transcription
2. the original RNA transcript, called pre-mRNA is processed in various ways before leaving the nucleus as mRNA