AP Bio Unit 6 Study Guide

DNA Structure

• DNA is antiparallel

  • meaning it has two strands that run in opposite directions: (5’ - 3’ & 3’ - 5’)

Prokaryotes

• circular DNA

Eukaryotes

• linear chromosomes

• in nucleus

Base Pairs

RNA → A (adenine) & U (uracil), C (cytosine) & G (guanine)

DNA → A & T (thymine), C & G

DNA Replication
Helicase

• on replication fork

• untwists double helix (breaks through hydrogen bonds)

DNA Polymerase

• requires a primer

  • adds nucleotides

  • only adds from 3’ to 5’ end

• DNA polymerase synthesizes leading strand (the one with nucleotides being added)

  • moves along the old template strand

• has proof reading availability

Primase

• makes primer for DNA polymerase

Ligase

• glues DNA fragments together

Topoisomerase

• sits behind the replication fork

• relieves the strain of twisting the double helix

• prevents supercoiling   

  • when DNA overwinds

Okazaki Fragments

• on the lagging strand

• when DNA is unwound by helicase there are two antiparallel strands

  • one strand can continue in 3’ to 5’ (leading strand)

  • one needs primer to keep moving as the helicase unwinds DNA (5’ to 3’)

    • lagging strand

    • resulting fragments = okazaki fragments

    • ligase ‘glue’ seals the fragments together

Semi-Conservative

• both copies of DNA have one original strand

• one new strand

DNA Transcription

synthesis of RNA using information in DNA

Prokaryotes

• translation occurs in cytoplasm

• translation of mRNA

Eukaryotes

• occurs in nucleus

RNA Polymerase

• catalyzes RNA synthesis

• does not need primer like DNA polymerase

• RNA is complementary to DNA template strand

RNA Processing

• only EUKARYOTES process pre-mRNA

  • before genetic message is sent to cytoplasm

• 5’ end vs. 3’ end

  • 5’ end gets 5’ cap (GTP molecule)

  • 3’ end gets a Poly-A Tail (AAA Adenine)

• Introns → the noncoding stretches of nucleotides that are removed from DNA strands

• Exons → other regions that are eventually expressed

• Spliceosomes → the removal of introns

• Alternative RNA Splicing → some introns contain sequences that regulate gene expression

  • what is an intron of one protein, may be an extron of another protein

DNA Translation

synthesis of a polypeptide, using information in the mRNA

Prokaryotes

• occurs in cytoplasm

• translation of mRNA can begin before transcription has finished

Euryotes

• occurs in cytoplasm

• translation of mRNA cannot begin until transcription is finished

  • nuclear envelope separates between transcription and translation

Codons vs. Anticodons

• Codons → mRNA base triplets read in 5’ to 3’ direction

  • must be read in correct reading frame

  • one codon cannot lead to more than one amino acid

  • located on mRNA molecule

• Anticodon → base pairs with a complementary codon on mRNA

tRNA (transfer RNA)

• reads mRNA to make protein

• transfers amino acids to the growing polypeptide in a ribosome

• each tRNA molecule allows translation of an mRNA codon to a certain amino acid

  • carries a specific amino acid to one end

  • ant-codon on the other end

• consists of a single RNA strand: 80+ nucleotides long

rRNA (ribosomal RNA)

• ribosome is made up of two subunits (large & small)

  • both are composed of protein and rRNA

Initiation

• has to start with mRNA code ‘AUG’

• starts translation

Elongation

• polypeptide grows

• continues until stop codon is reached

Termination

• introduces viral RNA not DNA

• release factors bonds to ribosome

  • released the polypeptide ending translation

all 3 steps occur in 5’ to 3’ direction

Gene Expression

Regulatory Sequenecs: sections of DNA that can help control transcription by interacting with regulatory proteins

Epigenetic Changes: reversible modifications of DNA or histones

• changes are only inheritable when they affect gametes

• DNA wrapped tightly around histones = hard for gene to be expressed (methylation tightens histones)

• DNA wrapped loosely around histones = easy for gene to be expressed (acetylation)

Specific Transcription Factors: genes expressed in a certain order due to certain activators being present

• activators and repressors are specific to each cell = Cell Differentiation

Gene Regulation: occurs in both prokaryotes and eukaryotes

• Prokaryotes: use operons → groups of related genes transcribed into a single mRNA

  • operons can be inducible or repressible

  • Inducible Operons: usually off

    • LAC operon → breaks down lactose

    • lacI turns operon off

    • allolactose → inducer, binds to the repressor protein and turns it off by changing it’s shape when it binds to the protein so it cannot bind to the operon

  • Repressible Operon: usually on

    • TRP operon → usually on

      • when tryptphan is present → binds to the operon and TRP repressor protein and turns it off

    • corepressor = tryptophan

      • when tryptophan is present → binds to the repressor and turns it off allowing the operon to turn on