Genetics & Molecular Biology: Transcription, Translation, and Chargaff's Rules

Page 1

• Date on slide: 8/25/25.
• Chromosome behavior during M phase of the cell cycle:
  • Chromosomes are condensed and replicated.
  • Each chromosome consists of sister chromatids that are genetically identical.
  • Sister chromatids are produced via DNA replication.
• Throughout most of the cell cycle (interphase):
  • Chromosomes are not condensed.
  • Chromatin is accessible to RNA polymerase.
  • Transcription can occur because genes are being expressed.
• Chromosome structure:
  • A chromosome is joined at the centromere.
  • During mitosis, two chromosomes separate into daughter chromosomes after sister chromatid separation.

Page 2

• Central Dogma of Genetics:
  • DNA → transcription → RNA → translation → protein.
  • Flow: genetic information moves from DNA to RNA to protein through transcription and translation.

Page 3

• DNA double helix and transcription/translation basics:
  • The template strand is read by RNA polymerase in the 3' → 5' direction.
  • The newly synthesized mRNA is built in the 5' → 3' direction.
  • Coding strand: the strand whose sequence corresponds to the mRNA (with U replacing T); the coding strand is read in the opposite orientation to the template strand.
• Provided sequences (as shown on the slide):
  • DNA template strand (3' → 5'): 3' TAC TGG CCG TTA GTT 5'.
  • DNA coding strand (5' → 3'): 5' ATG ACC GGC AAT CAA CTA TAT TGA 3' (note: the slide shows some garbled characters in places but this reflects the intended coding sequence).
  • mRNA transcript (5' → 3'): 5' AUG ACC GGC AAU CAA CUA UAU UGA 3' (note: uracil replaces thymine in RNA).
• Key concepts about transcription/translation:
  • mRNA is synthesized from the DNA template by complementary base-pairing, with U replacing T.
  • The mRNA sequence is similar in content and polarity to the DNA coding strand (except for U vs T).
  • Translation starts at the first start codon: 5' AUG 3' (Methionine, Met).
  • Translation proceeds codon by codon until a stop codon is reached:
  • Stop codons: UAA, UAG, UGA.
  • Example amino acid sequence from the given mRNA (until stop):
  • AUG → Met
  • ACC → Thr
  • GGC → Gly
  • AAU → Asn
  • CAA → Gln
  • CUA → Leu
  • UAU → Tyr
  • UGA → Stop
  • Therefore, the polypeptide would be: Met-Thr-Gly-Asn-Gln-Leu-Tyr (terminated by Stop).
• Additional notes:
  • The backbone concept that A pairs with T via 2 hydrogen bonds and G pairs with C via 3 hydrogen bonds helps explain DNA stability:
  • A–T base pairs have 2 hydrogen bonds.
  • G–C base pairs have 3 hydrogen bonds.

Page 4

• Chargaff's Rules (Chargaff’s Rule):
  • In double-stranded DNA:
  • A = T
  • G = C
  • A useful consequence is that A + T equals G + C in terms of total base pairs when considering each strand’s composition, though the precise percentages can vary between organisms.
• Two expression methods for base composition:
  • Letters: express relationships as A = T and G = C.
  • Numbers: express as percentages, e.g., A = T = 20%, G = C = 30% (an example from the slide).
• Example interpretations from the slide (noting some OCR garbles):
  • A = T = 20%; G = C = 30% would yield AT content of 40% and GC content of 60%.
  • The slide also shows attempts at other combinations (e.g., A + G = C + T, A + T = G + C) which align with the general idea of base-pair balance, though the precise statements can vary in wording.
• Practical takeaway:
  • Chargaff’s rules explain why DNA has complementary base pairing and help predict base composition from one strand to the other.
  • Students can express base composition either with letters (A, T, G, C relations) or with percentages (numbers).

Note on garbled items:

• Some lines in Page 3 and Page 4 appear garbled or OCR-mistyped (e.g., stray characters and incomplete fragments such as '3)', '413', '1/8', etc.). The core concepts above reflect the intended content: transcription/translation flow, strand orientation, start/stop codons, amino acid sequence from the provided mRNA, base-pairing rules, and Chargaff’s rule with both letter- and number-based representations.