DNA - Nucleic acids

What are the two types of nucleic acids?

DNA and RNA

What is the role of DNA?

It is the repository of genetic information (the genome) of most life forms

What type of organism uses RNA as its genetic repository instead of DNA?

Some viruses (e.g., retroviruses)

What are the three types of RNA?

mRNA (messenger), rRNA (ribosomal), tRNA (transfer)

What is the role of mRNA?

Carries genetic coding information from the gene to the ribosome for protein synthesis

What is the role of rRNA?

Forms the ribosome along with proteins; site of polypeptide synthesis

What is the role of tRNA?

Transfers specific amino acids to the growing polypeptide chain at the ribosome

What are the three components of a nucleotide?

A phosphate group, a pentose (five-carbon) sugar, and a nitrogenous base

What is the basic monomer/repeating unit of nucleic acids?

A nucleotide

What are the nitrogenous bases in DNA?

Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

What replaces Thymine in RNA?

Uracil (U)

Which bases pair together in DNA?

A pairs with T; G pairs with C

What is the sugar in DNA?

Deoxyribose

What is the sugar in RNA?

Ribose

What is the 5' end of a polynucleotide chain?

The end with a free phosphate group attached to the 5' carbon of the sugar

What is the 3' end of a polynucleotide chain?

The end with a free hydroxyl group (–OH) attached to the 3' carbon of the sugar

What does antiparallel mean in the context of DNA?

The two strands run in opposite directions — one 5'→3', the other 3'→5'

What holds the two strands of DNA together?

Hydrogen bonds between complementary base pairs and hydrophobic stacking interactions

What is chromatin?

The complex of DNA and histone proteins that makes up chromosomes

What are histones?

Proteins that chromosomes are closely associated with in eukaryotes

What percentage of the human genome consists of genes?

About 30%

What is an operon?

A cluster of functionally related genes in prokaryotes controlled by a single promoter

What is polycistronic mRNA?

mRNA that contains coding sequences for several polypeptides; common in prokaryotes

What is the Central Dogma of Molecular Biology?

The flow of genetic information: DNA → RNA → Protein

What three processes describe the flow of genetic information?

Replication, Transcription, and Translation

What is reverse transcription?

The process where retroviruses use reverse transcriptase to convert RNA back into DNA

What enzyme carries out transcription?

RNA polymerase

What is a promoter?

A DNA sequence that signals the initiation of RNA synthesis

What is the TATA box?

A crucial promoter DNA sequence in eukaryotes that helps position RNA polymerase

What strand of DNA does RNA polymerase read during transcription?

The template (antisense) strand, read in the 3'→5' direction

In which direction is the new mRNA strand synthesized?

5' to 3' direction

What is a pre-mRNA (primary RNA transcript)?

The raw, unprocessed mRNA produced after transcription in eukaryotes

What are introns?

Non-coding sequences in pre-mRNA that must be removed before translation

What are exons?

Coding sequences in pre-mRNA that are retained in the finished mRNA and expressed as protein

What are snRNPs?

Small nuclear ribonucleoproteins; particles made of RNA and protein that remove introns

What is a spliceosome?

A complex of snRNPs that removes introns and joins exons together

What is splicing?

The process of removing introns from pre-mRNA and joining exons

What is a codon?

A triplet of nucleotides in mRNA that codes for a specific amino acid

What is the start codon and what amino acid does it code for?

AUG; codes for methionine

What are the three stop codons?

UAA, UAG, and UGA

What is the anticodon?

A set of three nucleotides on tRNA that is complementary to a codon on mRNA

What is an aminoacyl-tRNA?

A tRNA loaded with its specific amino acid, ready for translation

What are the three sites of the ribosome?

A site (aminoacyl), P site (peptidyl), and E site (exit)

What happens at the A site of the ribosome?

New aminoacyl-tRNA enters here

What happens at the P site of the ribosome?

The growing polypeptide chain is held here

What happens at the E site of the ribosome?

Used (uncharged) tRNA exits the ribosome here

What is translocation in translation?

The movement of the ribosome along the mRNA, shifting tRNAs from A→P→E sites

What protein terminates translation?

Release factor, which recognizes the stop codon and releases the polypeptide

What is a mutation?

A change in the DNA sequence

What is a silent mutation?

A mutation that changes a base but does not change the amino acid (e.g., AAA→AAG both = Lysine)

What is a missense mutation?

A base change that results in a different amino acid being incorporated into the protein

What is a nonsense mutation?

A base change that creates a premature stop codon, producing a truncated protein

What is a frameshift mutation?

A mutation caused by insertion or deletion of bases that shifts the reading frame of the mRNA

What is sickle cell disease caused by?

A single missense mutation (A→T) in the globin gene that changes glutamic acid to valine

What are mutagens?

Agents that increase the rate of DNA mutations (e.g., UV radiation, X-rays, certain chemicals)

What is the relationship between mutagens and carcinogens?

Many mutagens have been found to also be carcinogens (cancer-causing agents)

What are Antisense Oligonucleotides (ASOs)?

Short DNA/RNA sequences that bind to specific mRNA to block its translation, silencing a gene

What is RNA interference (RNAi)?

A gene-silencing mechanism that uses small RNA molecules to degrade specific mRNA