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Biology

Unit 6

DNA and Protein Synthesis

6 topics · 24 practice questions · Free responseNot yet reviewed

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Notes

DNA structure and replication

  • DNA is a double helix of nucleotides with a sugar-phosphate backbone. Bases pair A-T and C-G.
  • Replication (S phase): helicase unzips, DNA polymerase adds matching nucleotides. It is semiconservative.

DNA vs RNA

DNARNA
StrandsDoubleSingle
SugarDeoxyriboseRibose
BasesA, T, C, GA, U, C, G
LocationNucleus (eukaryotes)Nucleus and cytoplasm

Protein synthesis

  • Transcription (nucleus): RNA polymerase copies a gene into mRNA.
  • Translation (ribosome): mRNA codons (3 bases) are matched by tRNA anticodons, which bring amino acids. Starts at AUG, ends at a stop codon.

Mutations

  • Point (substitution): one base changes; may change one amino acid or none.
  • Frameshift (insertion or deletion): changes every codon after it.
  • Only mutations in gametes are inherited.

Gene expression and biotechnology

Cells express different genes. Tools include genetic engineering, PCR (copies DNA), gel electrophoresis (sorts DNA by size), and CRISPR (edits genes).

Topics in this unit

Each topic has its own short notes and practice.

Practice

A gene's DNA template strand is TACAAACCGATT. Transcribe the mRNA.
Using a codon chart, AUG = methionine, UUU = phenylalanine, GGC = glycine, UAA = stop. What protein does the mRNA AUGUUUGGCUAA code for?
Which step happens first in making a protein from a gene?
A mutation changes the codon UAU (tyrosine) to UAC (also tyrosine). What effect does this have on the protein?
Police compare DNA from a crime scene to DNA from suspects. Which technique separates the DNA fragments to make a DNA fingerprint?
What is the job of tRNA?

Free response

Write your answer first, then open the scoring guide.

Free-response question

A short piece of normal DNA template strand reads TAC CCG TTT ATT. In a mutant version, it reads TAC CCG ATT ATT. Use this codon list: AUG = Met, GGC = Gly, AAA = Lys, UAA = stop.

(a) Write the mRNA and the amino acid sequence for the normal DNA.

Show the answer and scoring guide

mRNA: AUG GGC AAA UAA. Amino acids: Met - Gly - Lys, then stop.

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(b) Write the mRNA for the mutant DNA and explain how the protein changes.

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Mutant mRNA: AUG GGC UAA UAA. Translation stops after Met - Gly, so the protein is shorter and missing Lys and anything after it.

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(c) Identify the type of mutation and explain whether it would be passed to offspring if it happened in a liver cell.

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It is a point (substitution) mutation, because one base (T) was replaced by another (A). It would not be passed to offspring, because liver cells are body cells, not gametes.

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