Biological Science 101 – General Biology
Lecture Three: DNA Replication
Chapter 16, pg. 305-319
DNA: Deoxyribonucleic Acid.
- Genetic material that makes up traits of an individual species.
- Passed on to next generation.
- Contained inside the nucleus.
Structure of DNA Molecule:
- Sugar (Deoxyribose):
- Nitrogen base.
- Phosphate group.
A=T (30% each)
G=C (20% each)
Adenine and Guanine = Purines
Thymine and Cytosine = Pyrimidines
Watson & Crick – 1953:
- Revealed the structure of DNA.
- Discovered the double-helix shape (two DNA strands). DNA Replication:
- Semi-conservative model: Two strands separate, each strand serves as a template
to produce a complementary strand.
Details of Replication:
1) Origins of replication
- Specific DNA sequence.
- Proteins attach.
- Causes strands to separate.
- DNA polymerase (enzyme) adds new nucleotides.
Energy for Replication:
- Nucleotides occur as triphosphates (3 Phosphates), 2
Phosphates are released and energy is produced.
Orientation of DNA Strand:
- Opposite directions. Replication:
1) Nucleotides added only to the 3’ end.
- New strand goes from 5’ -> 3’.
- First strand is called leading strand.
Okazaki Fragments: Fragments that are joined together by DNA ligase.
- Joins fragments of nucleotides from 100-200 nucleotides long.
Roles of polymerase
DNA Polymerase can only add nucleotides to an existing strand.
- It must have a starting point in order to attach nucleotides to the strands of DNA.
Primer: A nucleotide the initiates elongation of the strand, it is added by the primase
Each Okazaki fragment must have a primer.
Involvement of other enzymes
1) Primase – Adds primer.
2) Polymerase III – Adds nucleotides.
3) Polymerase I – Converts RNA into DNA in primer.
4) Ligase – Fuses Okazaki fragments together.
5) Helicase – Unwinds the double-helix.
- Single Stranded Binding Protein = Holds strands apart.
DNA Replication occurs at 50 nucleotides per second.
- Mistakes taking place.
- An error is 1 in 100,000 bases (mismatched bases). - Errors are corrected by DNA polymerase.
- Error rate is then reduced by 1 in 10 billion nucleotides.
- Errors can result in mutation (e.g. Cancer and etc.)
DNA Damaged in the Cell, Causes are:
- Ultra-violet Light.
- Mutagens/Carcinogens (Chemicals).
1) DNA Polymerase checks for damage and repairs DNA in the cells.
2) Nuclease: Cuts out damaged DNA.
3) Polymerase: Adds nucleotides.
4) Ligase: Seals the pieces together.
Telomeres are short repeated nucleotides.
- They are noncoding.
- They provide protection to the ends of the
E.g. TTAGGG – repeated about 100-1000 times.
Telomerase: Adds nucleotides (telomeres) to the ends of
- Active in gametes (sperm and egg cells).
- High in newbor