DNA/DNA replication

The role of DNA

  1. storing information

  2. copying information

  3. gene expression

DNA: Deoxyribose Nucleic Acid

  • found in all living things

genes: segments of DNA, used for traits

DNA structure

  • made of nucleotides

  • 3 parts

    • phosphate

    • sugar

    • base

  • Strands are anti-parallel

    • phosphate and sugar

  • each sides are held up together by hydrogen

  • Base-pairing

Nitrogenous Bases

  • Adenine

  • Thymine

  • Guanine

  • Cytosine

Double Helix Model

  • explains the base-pair rule

  • how the DNA strands are held together

Erwin Chargaff: studied the percentage of each base in the sample

Chargaff’s rule/Base pair Rule: T’s and A’s match, G’s and C’s match

Rosalind Franklin: took photos using X-rays which identified the shape as a helix

Watson and Crick: established the structure of DNA as a double helix

DNA bases are like letters

  • letters form words

  • words for sentences

hemoglobin: red blood cell

DNA replication

DNA replication: DNA makes a copy of itself (cell division)

  • is done before cell division so the daughter cell can have the DNA

    • done in interphase mitosis/meiosis in a eukaryote

DNA polymerase: replicates DNA molecules to build the new DNA

helicase: “unzipping” enzyme that breaks the hydrogen bond that holds the DNA together

primase: makes the primer so DNA polymerase can figure out where to copy. The primer is made of RNA

Ligase: helps glue DNA fragments together

Steps to DNA Replication

  1. starts at the origin, helicase unwinds the DNA and SSB proteins bind the DNA strands to keep them separate and topoisomerase keeps the DNA from supercoiling

  2. Primase then makes RNA primers on both strands

  3. DNA polymerase builds new strands, yet it only can build in the 3’ to 5’ direction, meaning it adds more strands to the 5’ to 3’ direction

    1. 5’ to 3’ are called lagging strands

    2. they lag because the DNA polymerase has to go back as it unwinds which creates gaps

    3. gaps are filled by new nitrogenous bases

  4. Ligase fills the gaps of lagging strands

  5. DNA polymerase goes back and “proof reads”

semi-conservative: ½ of the old strand is saved

DNA polymerase

  • adds new nucleotides to each side

  • nucleotides are adding in different directions

  • includes “proofreading” to correct errors

telomeres: tips of the chromosomes that has short DNA sequences and holds DNA in

Prokaryote Replication

  • one origin of replication

  • forms move away from each other

Eukaryote replication

  • multiple replication forks

  • occurs in the nucleus

DNA

  • double helix

  • deoxyribose sugar

  • found in nucleus

RNA

  • single strand

  • three forms

    • mRNA

    • tRNA

    • rRNA

  • ribose

  • uracil

Enzyme Helicase in DNA Replication: Helicase unwinds the double-stranded DNA helix during replication by breaking hydrogen bonds between base pairs, allowing DNA polymerase to access the separated strands for replication.