Nucleic Acids, DNA Replication, and Gene Expression: Key Concepts for Biology

What are the two types of nucleic acids cells make?

DNA and RNA.

What sugar does DNA contain?

Deoxyribose.

What sugar does RNA contain?

ribose

What are the four DNA nucleotides?

A, C, G, T.

What are the four RNA nucleotides?

A, C, G, U.

Which nucleotides are purines?

A and G (double-ring).

Which nucleotides are pyrimidines?

C, T, and U (single-ring).

What are the main features of the Watson-Crick model?

Double helix, antiparallel strands, strict base pairing (A:T, C:G), H-bonds between bases.

What does "semi-conservative replication" mean?

Each daughter DNA has one old strand and one newly synthesized strand.

What does helicase do

Breaks hydrogen bonds between bases to unwind DNA

What does topoisomerase do?

Relieves overwinding strain ahead of helicase.

What does single-strand binding protein do

Prevents DNA strands from re-annealing

What does primase make

A short RNA primer.

What does DNA polymerase III do?

Extends primer by adding nucleotides to the 3' end.

What does DNA polymerase I do

Removes RNA primer and replaces it with DNA.

What does ligase do

Forms covalent bonds to seal DNA fragments.

What is an origin of replication

A sequence where replication begins

What is a replication bubble

Area where DNA is unwound, with two replication forks

Leading strand synthesis direction

Follows the fork; made continuously

Lagging strand synthesis direction?

Moves away from the fork; made in Okazaki fragments.

Why does the lagging strand form fragments

DNA polymerase can only add to the 3' end, so primase must repeatedly add primers.

What are Okazaki fragments

Short DNA fragments made on the lagging strand.

What is the Central Dogma

DNA → RNA → Protein.

What is the transcription start site?

+1, the first nucleotide of RNA.

Where is the promoter located

Upstream of +1 (negative numbers).

What are the three main gene regions?

Promoter, transcribed region, transcription start site.

How do prokaryotes mainly regulate gene expression?

Negative regulation via repressors.

How do eukaryotes mainly regulate gene expression?

Positive regulation with transcription factors.

What do basal transcription factors do?

Bind promoter and recruit RNA polymerase

What do regulatory transcription factors do?

Act as activators or repressors by binding control elements.

What are enhancers?

Position-independent DNA elements that increase transcription when activators bind.

What enzyme performs transcription

RNA polymerase.

Does RNA polymerase need a primer

No

In what direction does RNA polymerase build RNA?

5' → 3'.

Stages of transcription

Initiation, elongation, termination.

What is pre-mRNA?

The unprocessed RNA transcript made in eukaryotes.

Three key mRNA processing steps?

5' cap, poly-A tail, splicing

What is the role of the 5' cap and polyA tail

Protect mRNA, help export, improve translation efficiency

What does splicing do?

Removes introns and joins exons

What is a codon

A 3-nucleotide sequence on mRNA that codes for an amino acid.

What is an anticodon?

A tRNA sequence complementary to an mRNA codon.

Where does translation occur?

Ribosome

What is the start codon?

AUG (codes for methionine)

What happens at the A-site of the ribosome?

A charged tRNA enters

What happens at the P-site?

Holds the growing peptide chain

What happens at the E-site?

tRNA exits the ribosome

What causes translation to stop

A stop codon binding a release factor.

How do codons demonstrate degeneracy?

Multiple codons can encode the same amino acid