AP Biology Notes: Molecular Genetics

Six Things to Know about the Molecular Genetics: 

  1. Nitrogenous base pairs make up DNA and RNA: adenine pairs with thymine (DNA only) or uracil (RNA only) and cytosine pairs with guanine.
  2. DNA replication is a semiconservative process, in which one of the antiparallel strands of DNA is preserved and the other strand is newly synthesized. New nucleotides are added in a 5′ to 3′ direction.
  3. In transcription, the DNA strands separate and mRNA copies one side. The mRNA takes the information to the ribosome, where protein synthesis occurs.
  4. In translation, tRNA carries amino acids to the mRNA and assembles them into proteins based on the mRNA code. Proteins are often modified after translation, giving them their final structure.
  5. Mutations are the source of genetic change. Types include base-pair substitutions, which affect one amino acid, as well as insertions and deletions, which shift the genetic code and affect many amino acids.
  6. Scientists can modify an organism’s DNA by adding new genes.

Key Topics–Molecular Genetics

Remember that the AP Biology exam tests you on the depth of your knowledge, not just your ability to recall facts. While we have provided brief definitions here, you will need to know these terms in even more depth for the AP Biology exam.

Nucleic Acids

  • DNA: Deoxyribonucleic acid; found in the cell nucleus, its basic unit is the nucleotide; contains coded genetic information; can replicate on the basis of heredity
  • RNA: An abbreviation of ribonucleic acid, a nucleic acid in which the sugar is ribose; a product of DNA transcription that serves to control certain cell activities; acts as a template for protein translation; types include mRNA (messenger), tRNA (transfer), and rRNA (ribosomal)
  • Deoxyribose: A five-carbon sugar that has one oxygen atom less than ribose; a component of DNA (deoxyribonucleic acid)
  • Ribose: A pentose sugar that occurs in nucleotides, nucleic acids, and riboflavin
  • Nitrogenous bases: The five purine and pyrimidine bases found in nucleic acid—adenine, thymine (in DNA only), cytosine, guanine, and uracil (in RNA only
  • Nucleotide: An organic molecule consisting of phosphate joined with a five-carbon sugar (deoxyribose or ribose) and a purine or a pyrimidine (adenine, guanine, uracil, thymine, or cytosine)
  • Adenine: A purine nitrogenous base that pairs with thymine in DNA and uracil in RNA
  • Cytosine: A pyrimidine nitrogenous base that is present in nucleotides and nucleic acids; it is paired with guanine
  • Guanine: A purine nitrogenous base that is a component of nucleotides and nucleic acids; it links up with cytosine in DNA
  • Thymine: A pyrimidine nitrogenous base in nucleic acids and nucleotides; pairs with adenine in DNA
  • Uracil: A pyrimidine nitrogenous base found in RNA (but not in DNA); pairs with adenine
  • Chromosome: A short, stubby rod consisting of chromatin that is found in the nucleus of cells; contains the genetic or hereditary component of cells (in the form of genes)

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