Molecular Genetics

DNA Synthesis

Semiconservative Replication

  • The two strands of the double helix are used as template strands for synthesizing DNA

  • One synthesizes in the 5’ to 3’ Direction and the other in the 3’ to 5’ direction

Phosphate Groups - found on the 5’ end

OH Groups - found on the 3’ end

Adenine pairs with Thymine - Cystiene pairs with Guanine

Steps

  1. Helicase unzips the two strands of DNA

  2. While the DNA is getting unzipped it can create tension on the front end which gets relieved by Topoisomerase/Gyrase

  3. The SSB proteins prevent the two strands from reattaching while Helicase unzips it

  4. The 3’ to 5’ strand is the template strand so the synthesized strand will read 5’ to 3’ toward the replication fork

  5. The 5’ to 3’ strand is the template strand so the synthesized strand will read 3’ to 5’ away from the replication fork

  6. Primase adds a Primer onto the 3’ to 5’ strand and DNA Polymerase 3 Synthesizes CONTINUOUSLY TOWARDS THE REPLICATION FORK: Leading Strand

  7. Primase adds multiple Primers onto the 5’ to 3’ strand and DNA Polymerase 3 Synthesizes DISCONTIOUSLY AWAY FROM THE REPLICATION FORM: Lagging Strand

  8. Primer will be removed from the Lagging Strand And Gyrase will fill up the gaps between the Okazaki Fragments

Terms

  • Helicase - Unzips the two DNA strands

  • Gyrase/Topoisomerase - Relieves tension in the double helix caused by helicase’s unzipping

  • Primase - Adds primers.

  • Lygase - Fills up the gaps between the Okazaki fragments after Primers are removed

  • DNA Polymerase 3 -Synthesizes DNA strand by adding the bases that match the ones on the template strand - Synthesizes 5’ to 3’ but reads 3’ to 5’

  • DNA Polymerase 1 - Double Checks DNA Poly. 3’s work and corrects any wrong nucleotides

  • Exonuclease Enzyme - Can remove mistakes in the pairings: DNA Polymerase 3 for 5’ to 3’ strands but DNA Polymerase 1 is both strands

  • Primers - Acts as the starting point for DNA Polymerase 3 to synthesize the strand (it can’t synthesize out of thin air). Removed on the lagging strand later

  • SSB Proteins - Prevents The DNA Strands from reattaching after being unzipped by helicase

  • Leading Strand - The strand synthesized continuously on the 3’ to 5’ Template Strand

  • Lagging Strand - the strand synthesized discontinuously on the 5’ to 3’ Template Strand

  • Okazaki Fragments - the fragments made by DNA Poly.3 on the Lagging Strand in between the primers before they were removed

Protein Synthesis: Transcription

  • Converts DNA into mRNA

  • The 5’ end of the m RNA strand is capped by telomeres

  1. Initiation

    RNA Polymerase binds itself to the Promoter Region

  2. Elongation

    RNA Polymerase causes the two DNA strands to separate and adds nucleotides to the growing mRNA Strand on the noncoding strand (The 3’ to 5’ Template Strand)

  3. Termination

    The RNA Polymerase and mRNA strand separate from the DNA template strand and Polymerase A Enzyme caps the 3’ end of the mRNA strand - Poly A tail

Pre mRNA

  • Contains Introns and Exons

  • Introns are removed in RNA Splicing because they are junk and don’t code for anything

Terms

Promotor Region - Short Sequence of DNA (TATAAA / TATA BOX)

RNA Polymerase - synthesizes RNA from the 5’ end to the 3’ end but reads in the 3’ to 5’ end

Coding/Sense Strand: 5’ to 3’ Direction but DOES NOT CODE FOR PROTEINS

Non-Coding/Antisense Strand: 3’ to '5’ Direction, used to synthesize mRNA and CODES FOR PROTEINS

Poly A Tail - The cap on the 3’ end of the mRNA Strand - Prevents the strand from degradation

Introns - Does not code for DNA - junk

Exons - Used to synthesize proteins

  • mRNA Sequence = Coding Strand Sequence

  • tRNA Sequence = Non-coding Strand Sequence

ex. Given Sequence: 3’ - GCATAGTATACG - 5’

? - 3’ to 5’ end - on the Noncoding strand - translate for the coding strand but replace T with U

  • 3’ - CGUAUCAUAGC - 5’ = mRNA strand

Codons

  • Start Codon: AUG - codes for Met

  • Stop Codons: UAG - UAA - UGA

Mutations

  • Frameshift Mutation - When an addition/deletion occurs and it causes a shift in the sequence + changes the rest of the amino acid sequence

  • Point Mutation - When a single nucleotide is added/deleted/changed

    Insertion - When a single nucleotide is added

    Deletions - When a single nucleotide is deleted

  • Silent Mutation - When a single nucleotide is changed but the amino acid stays the same

  • Missense Mutation - When a single nucleotide is changed which also changes the amino acid

  • Nonsense Mutation - the addition of a STOP codon in the middle of the sequence