DNA replication

Learning Outcomes of the Lecture

• Focus on DNA replication and its processes.

• Review of the structure of DNA.

• Understand the semi-conservative nature of DNA replication.

• Steps and enzymes involved in DNA replication.

DNA Structure

• DNA is made of nucleotide monomers.

• Composed of a sugar (deoxyribose), phosphate group, and nitrogenous base.

• Four nitrogen bases: cytosine (C), guanine (G), adenine (A), thymine (T).

• Binding patterns: C pairs with G (3 hydrogen bonds), A pairs with T (2 hydrogen bonds).

• DNA forms a double helix structure, with two antiparallel strands.

• Directionality is marked as 5' (5 prime) and 3' (3 prime) due to carbon numbering in the deoxyribose sugar.

• Phosphodiester Bonds

• Formed between the 3' carbon of one nucleotide and the 5' carbon of another connecting sugars via phosphate.

History of DNA Structure Discovery

• Watson and Crick's publication in "Nature" introduced the structure of DNA, linking base pairing to genetic replication.

• The understanding of DNA structure led to significant advances in molecular biology.

Overview of DNA Replication

• DNA replication is semi-conservative, implying one original strand is retained in each new double helix.

• The process begins with the separation of the double helix strands, allowing each strand to serve as a template for new complementary strands.

Steps in DNA Replication

1. Unwinding the DNA

• Initiated at origins of replication recognized by proteins that create replication bubbles and forks.

• Helicase unwinds the double helix by breaking hydrogen bonds.

• Single-strand binding proteins prevent re-annealing of the separated strands.

• Topoisomerase relieves strain in the unwinding DNA.

1. Primer Synthesis

• Primase synthesizes RNA primers essential for starting replication.

1. Leading and Lagging Strands

• The leading strand is synthesized continuously in the 5' to 3' direction.

• The lagging strand is synthesized in fragments called Okazaki fragments due to its 3' to 5' template direction.

  • Each Okazaki fragment starts with an RNA primer.

  • Completed fragments are joined by DNA ligase.

1. Removal of RNA Primers

• Exonuclease removes RNA primers; DNA polymerase I fills gaps with DNA.

DNA Polymerase

• Enzyme responsible for adding nucleotides in a 5' to 3' direction.

• Requires a 3' hydroxyl group from the primer or existing DNA strand to add a new nucleotide.

• Reaction is facilitated by the energy released from breaking phosphate bonds in incoming nucleotides.

Telomeres and Replication Issues

• Ends of linear chromosomes, known as telomeres, prevent loss of vital DNA during replication.

• Telomerase extends telomeres in germ and stem cells to protect against shortening during replication.

• Shortening of telomeres is linked to aging and limitations on cellular division.

AZT as a Designer Drug

• Analog of thymine used to treat HIV.

• Lacks a 3' hydroxyl group, preventing formation of DNA strands by reverse transcriptase, stopping viral replication.

Summary of Enzymes in Replication

Final Insights

• DNA replication is a complex and organized process allowing for accurate copying of the genome, critical for cellular division and inheritance.

• Understanding these processes opens avenues in genetic research and development of therapeutic strategies.