Transcription, Translation and Replication

What does 'semiconservative' mean in DNA replication?

Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

What enzyme unwinds the DNA helix?

Helicase.

What prevents DNA strands from re-annealing after being separated?

Single-strand binding proteins (SSBs).

What enzyme relieves supercoiling ahead of the replication fork?

Topoisomerase (e.g., DNA gyrase in prokaryotes).

What is the role of primase in DNA replication?

It synthesizes a short RNA primer to provide a starting point for DNA polymerase.

In which direction does DNA polymerase synthesize new DNA?

5' to 3' direction.

What is the difference between the leading and lagging strands?

The leading strand is synthesized continuously, while the lagging strand is synthesized in short Okazaki fragments.

What enzyme joins Okazaki fragments?

DNA ligase.

What is the main DNA polymerase in prokaryotic replication?

DNA polymerase III (for elongation), DNA polymerase I (for primer removal and repair).

What is the role of DNA polymerase I?

Removes RNA primers and replaces them with DNA in prokaryotes.

What is the proofreading function of DNA polymerases?

DNA polymerases have 3' to 5' exonuclease activity to remove incorrect nucleotides.

Which DNA polymerase is responsible for replication in eukaryotes?

DNA polymerase δ (delta) for the lagging strand and DNA polymerase ε (epsilon) for the leading strand.

What is the function of telomerase?

It extends the ends of linear chromosomes (telomeres) in eukaryotic cells to prevent loss of genetic material.

What are the three main types of DNA repair mechanisms?

Base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR).

How does base excision repair (BER) work?

A damaged base is removed by DNA glycosylase, then AP endonuclease cuts the backbone, and DNA polymerase fills the gap.

What type of DNA damage is repaired by nucleotide excision repair (NER)?

Large distortions, such as pyrimidine dimers caused by UV radiation.

What enzymes are involved in mismatch repair (MMR)?

MutS (recognition), MutL (recruitment), and MutH (excision) in prokaryotes.

What is non-homologous end joining (NHEJ)?

A DNA repair process that joins broken DNA ends without a homologous template, often causing mutations.

What is transcription?

The process of synthesizing RNA from a DNA template.

In which direction is RNA synthesized?

5' to 3' direction.

Which enzyme synthesizes RNA from DNA?

RNA polymerase.

What is the template strand in transcription?

The DNA strand that is used as a template to synthesize RNA (also called the antisense or non-coding strand).

What is the coding strand?

The DNA strand that has the same sequence as the mRNA (except thymine is replaced with uracil).

What is a promoter?

A DNA sequence that signals RNA polymerase where to start transcription.

What is the TATA box?

A conserved promoter sequence in eukaryotes (~25 bp upstream of the transcription start site).

What is the role of sigma factor in prokaryotic transcription?

It helps RNA polymerase recognize and bind to the promoter.

What is the transcription initiation complex?

A group of proteins, including transcription factors and RNA polymerase, that initiates transcription.

How many RNA polymerases do prokaryotes have?

One RNA polymerase.

How many RNA polymerases do eukaryotes have, and what are their functions?

Three- RNA polymerase I → rRNA synthesis\n- RNA polymerase II → mRNA synthesis\n- RNA polymerase III → tRNA and 5S rRNA synthesis.

What is an operon?

A cluster of genes regulated together under one promoter (common in prokaryotes).

What is the difference between an inducible and a repressible operon?

Inducible operon (e.g., lac operon) → Normally off, activated when needed.\nRepressible operon (e.g., trp operon) → Normally on, turned off when not needed.

What are transcription factors?

Proteins that help regulate gene expression by enhancing or repressing transcription.

What is an enhancer?

A DNA sequence that increases transcription when bound by activators.

What is an epigenetic modification that regulates transcription?

DNA methylation (silences genes) or histone acetylation (activates genes).