Flashcards · 9700 · AS Topic 6

Nucleic acids and protein synthesis (AS Level) flashcards

14 cards on AS Topic 6, subtopics 6.1 to 6.2. Pick a subtopic or study them all.

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Nucleotide, and how ATP differs: A nucleotide is a pentose sugar, a phosphate group and a nitrogenous base. ATP is a phosphorylated nucleotide: adenine, ribose and three phosphate groups.
Purines vs pyrimidines: Purines are double-ring bases: adenine and guanine. Pyrimidines are single-ring bases: cytosine, thymine and uracil.
Base pairing in DNA: A pairs with T (2 hydrogen bonds); C pairs with G (3 hydrogen bonds). So in double-stranded DNA, %A = %T and %C = %G.
Phosphodiester bond and antiparallel strands: A phosphodiester bond joins the phosphate of one nucleotide to the sugar of the next (condensation). Antiparallel: the strands run in opposite directions, one 5′ → 3′, the other 3′ → 5′.
Semi-conservative replication: the steps: Hydrogen bonds break and both strands act as templates. Free nucleotides pair with exposed bases. DNA polymerase forms phosphodiester bonds, adding only to the 3′ end. Lagging-strand fragments are joined by DNA ligase. Each new molecule has one old and one new strand.
Why is one strand made in fragments during replication?: DNA polymerase adds nucleotides only in the 5′ → 3′ direction. The leading strand is made continuously; the lagging strand is made in fragments, which DNA ligase joins.
DNA has 24% cytosine. What are the percentages of the other bases?: G = 24% (pairs with C). A + T = 100 − 48 = 52%, so A = 26% and T = 26%.

Outline transcription: In the nucleus the gene unwinds and hydrogen bonds break. RNA polymerase uses the template strand; RNA nucleotides pair (A–U, T–A, C–G, G–C) and are joined by phosphodiester bonds. Introns are removed and exons joined to form mRNA, which leaves through a nuclear pore.
Outline translation: mRNA binds to a ribosome. tRNA with an anticodon complementary to each codon brings a specific amino acid. Peptide bonds form between adjacent amino acids as the ribosome moves along one codon at a time, until a stop codon is reached.
Codon vs anticodon: Codon: three bases on mRNA coding for one amino acid. Anticodon: three bases on tRNA, complementary to the codon.
Start and stop codons (mRNA): Start: AUG, which also codes for methionine. Stop: UAA, UAG and UGA.
Intron vs exon: Introns are non-coding sequences removed from the primary transcript. Exons are coding sequences joined to form mRNA.
Substitution vs deletion of one base pair: Substitution affects at most one codon: same amino acid, a different amino acid or an early stop codon. Deletion causes a frame shift: every codon after it is read differently.
How many mRNA nucleotides code for a 210-amino-acid polypeptide?: 210 codons + 1 stop codon = 211 codons. 211 × 3 = 633 nucleotides.