1B: DNA & RNA
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52 Terms
What did Griffith (1928) show?
A “transforming principle” from heat-killed virulent bacteria could convert harmless bacteria into virulent forms.
What did Avery, MacLeod, and McCarty (1944) conclude?
DNA is the transforming principle and therefore carries genetic information.
What did the Hershey–Chase experiment (1952) show?
Only DNA (not protein) enters bacteria during viral infection, proving DNA is genetic material.
What did Chargaff’s rules show?
A = T and C = G; DNA composition varies between species.
What did Rosalind Franklin’s work show?
DNA is a helical structure with uniform width and regular base spacing.
What is the structure of DNA?
A double-stranded, anti-parallel polynucleotide helix with complementary base pairing.
What is meant by anti-parallel DNA strands?
One strand runs 5’→3’, the other runs 3’→5’.
What are base-pairing rules?
A pairs with T (2 H bonds); C pairs with G (3 H bonds).
Why is complementary base pairing important?
It enables accurate replication and stable DNA structure.
What did the Meselson–Stahl experiment show?
DNA replicates semi-conservatively (each strand acts as a template).
What is semi-conservative replication?
Each new DNA molecule contains one parental strand and one newly synthesized strand.
What models of replication were rejected?
Conservative and dispersive models.
What is the role of helicase?
Unwinds the DNA double helix.
Why are primers needed?
DNA polymerase can only extend from an existing 3’ OH group.
Why does DNA synthesis occur 5’→3’?
DNA polymerase can only add nucleotides to the 3’ end.
What is the leading strand?
The strand synthesised continuously toward the replication fork.
What is the lagging strand?
The strand synthesised discontinuously as Okazaki fragments.
What is the role of ligase?
Joins Okazaki fragments by sealing the sugar-phosphate backbone.
Why is DNA replication highly accurate?
Base pairing, polymerase proofreading, and mismatch repair.
What is proofreading in DNA replication?
DNA polymerase removes incorrect nucleotides using 3’→5’ exonuclease activity.
What is mismatch repair?
Post-replication correction of incorrectly paired bases.
Why are mutations important?
They create genetic variation, which is essential for evolution.
What is the overall evidence that DNA is genetic material?
Griffith, Avery, and Hershey–Chase experiments.
What is the key outcome of DNA structure and replication studies?
DNA is a stable, complementary, semi-conservatively replicating molecule enabling inheritance.
What is a nucleotide?
The basic unit of nucleic acids, consisting of a nitrogenous base, pentose sugar, and phosphate group.
What is the difference between DNA and RNA nucleotides?
DNA has deoxyribose and bases A, T, G, C; RNA has ribose and bases A, U, G, C.
What are purines and pyrimidines?
Purines (A, G) are double-ring; pyrimidines (C, T, U) are single-ring.
What holds nucleotides together within a strand?
Strong covalent (phosphodiester) bonds.
What is the function of single-strand binding proteins (SSBs)?
Stabilise unwound DNA and prevent strands from re-annealing.
Why is DNA replication highly accurate?
Base pairing and DNA polymerase proofreading.
What is proofreading?
DNA polymerase removes incorrect nucleotides during replication.
What is a mutation?
A permanent change in DNA sequence.
What is nucleotide excision repair?
A repair system that removes damaged DNA (e.g. thymine dimers) and replaces it.
What is a genome?
The complete set of genetic material in an organism.
What are key features of mitochondrial DNA (mtDNA)?
Circular, double-stranded, contains 37 genes, codes for respiratory proteins.
How is mtDNA inherited?
Maternally (from the mother only).
Why are mtDNA mutations significant?
They affect energy production and can cause disease.
What did Garrod propose?
Genes control metabolism (“inborn errors of metabolism”).
What did Beadle and Tatum show?
One gene controls one enzyme (one gene–one enzyme hypothesis).
What is mRNA?
Messenger RNA that carries genetic information from DNA to ribosomes.
What is a phenotype?
observable characteristics produced by gene expression.
What is transcription?
Copying DNA into mRNA.
What enzyme carries out transcription?
RNA polymerase
What are the three stages of transcription?
Initiation, elongation, termination
What is translation?
Conversion of mRNA into a polypeptide at the ribosome.
Which occurs first: transcription or translation?
Transcription.
What modifications occur to eukaryotic mRNA?
5′ cap, poly-A tail, and splicing.
What are introns?
Non-coding sequences removed during splicing.
What are exons?
Coding sequences that are joined together.
What are snRNPs?
Molecules that form the spliceosome to remove introns.
Why is mRNA processing important?
It stabilises mRNA and prepares it for translation.