Chromatin of eukaryotes is organized into repeating interactions with protein octomers called nucleosomes. Nucleosomes are composed of which class of molecules?

histones
lipids
nonhistone chromosomal proteins
H1 histones
glycoproteins

histones

In contrast with euchromatin, heterochromatin contains more genes and is earlier replicating.

True
False

False

Telomerase is an enzyme involved in the replication of the ends of eukaryotic chromosomes.

True
False

True

The genetic code is degenerate, meaning that a codon can specify more than one amino acid.

True
False

False

(The degeneracy of the genetic code means that an amino acid may be coded for by more than one codon. However, a single codon can only ever specify one amino acid.)

A DNA sequence produces a mutant protein in which several amino acids in the middle of the protein differ from the normal protein. What kind of mutation could have occurred?

An addition and a deletion mutation
A nonsense mutation
An addition or deletion mutation
No mutation

An addition and a deletion mutation

(A single addition or deletion would change the reading frame of the protein, but if another mutation occurred to cancel the effects of the first mutation, only those amino acids between the mutations would change.)

Which of the following statements about eukaryotic transcription is false?

Transcription initiation occurs when RNA polymerase binds to a complex of transcription factors at the TATA box.
The transcripts produced contain both exons and introns.
A polycistronic mRNA may be transcribed if the gene products are used in the same pathway or needed at the same time.
Eukaryotic promoter regions contain a TATA box and a CAAT box.

A polycistronic mRNA may be transcribed if the gene products are used in the same pathway or needed at the same time.

(This statement is false. Polycistronic mRNAs are produced only in prokaryotes. In eukaryotes, a single gene is transcribed at a time.)

What is the initiator triplet in both prokaryotes and eukaryotes? What amino acid is recruited by this triplet?

AUG; methionine
AUG; arginine
UAA or UGA; arginine
UAA, methionine
UAA; no amino acid called in

AUG; methionine

Introns are known to contain termination codons (UAA, UGA, or UAG), yet these codons do not interrupt the coding of a particular protein. Why?

These triplets cause frameshift mutations, but not termination.
Exons are spliced out of mRNA before translation.
UAA, UGA, and UAG are initiator codons, not termination codons.
Introns are removed from mRNA before translation.
More than one termination codon is needed to stop translation.

Introns are removed from mRNA before translation.

When considering the initiation of transcription, one often finds consensus sequences located in the region of the DNA where RNA polymerase(s) binds. Which of the following is a common consensus sequence?

any trinucleotide repeat
GGTTC
TATA
satellite DNAs
TTTTAAAA

TATA

Promoter recognition

RNA polymerase is a holoenzyme composed of a five-subunit core enzyme and a sigma (σ) subunit. Different types of σ subunits aid in the recognition of different forms of bacterial promoters. The bacterial promoter is located immediately upstream of the starting point of transcription (identified as the +1 nucleotide of the gene). The promoter includes two short sequences, the –10 and –35 consensus sequences, which are recognized by the σ subunit.

Chain initiation

The RNA polymerase holoenzyme first binds loosely to the promoter sequence and then binds tightly to it to form the closed promoter complex. An open promoter complex is formed once approximately 18 bp of DNA around the –10 consensus sequence are unwound. The holoenzyme then initiates RNA synthesis at the +1 nucleotide of the template strand.

Chain elongation

The RNA-coding region is the portion of the gene that is transcribed into RNA. RNA polymerase synthesizes RNA in the 5′ → 3′ direction as it moves along the template strand of DNA. The nucleotide sequence of the RNA transcript is complementary to that of the template strand and the same as that of the coding (nontemplate) strand, except that the transcript contains U instead of T.

Chain termination

Most bacterial genes have a pair of inverted repeats and a polyadenine sequence located downstream of the RNA-coding region. Transcription of the inverted repeats produces an RNA transcript that folds into a stem-loop structure. Transcription of the polyadenine sequence produces a poly-U sequence in the RNA transcript, which facilitates release of the transcript from the DNA.