Lecture - 4 DNA Replication

DNA Replication and Structure

Overview of DNA

• DNA (Deoxyribonucleic acid) is the genetic material, distinct from proteins.

• It contains the genetic blueprint that guides development and traits.

• Precise DNA replication ensures genetic transmission from one generation to the next.

• The molecular structure of DNA was described by Watson and Crick, revolutionizing genetic engineering.

DNA Structure

Double Helix Model

• Developed by James Watson and Francis Crick.

• DNA is double-stranded, with strands running antiparallel (5' to 3' and 3' to 5').

• Nucleotide bases pair specifically: Adenine (A) with Thymine (T), and Guanine (G) with Cytosine (C).

• The double helix is stabilized by hydrogen bonds between base pairs.

Components of DNA

• Nucleotides: Composed of a phosphate group, deoxyribose sugar, and a nitrogenous base (C, G, A, T).

• Strands: DNA strands join through hydrogen bonds to form the double helix.

• Purines (A, G) pair with Pyrimidines (C, T).

Functions of DNA

• Replication: Involves unwinding, strand separation, and synthesis of new strands.

• The process is not perfect; repair mechanisms exist to correct errors.

• Telomeres protect the ends of chromosomes from erosion during replication.

DNA Replication Mechanism

Initiation of Replication

• Occurs at specific locations called "origins of replication."

• Formation of replication bubbles allows for bidirectional synthesis of DNA strands.

• Key enzymes involved include helicase, primase, and DNA polymerase.

Leading vs. Lagging Strand

• Leading Strand: Synthesized continuously towards the replication fork.

• Lagging Strand: Synthesized in short segments (Okazaki fragments) away from the replication fork, requiring fragments to be joined later.

• The challenges of antiparallel strands lead to the complexity of replication.

Key Enzymes and Proteins

• DNA Polymerase III: Catalyzes DNA synthesis by adding nucleotides to the 3' end of the growing strand.

• Primase: Synthesizes RNA primers which are necessary for DNA polymerases to start synthesis.

• DNA Ligase: Joins Okazaki fragments on the lagging strand and seals gaps in the sugar-phosphate backbone.

Error Repair in DNA Synthesis

• DNA replication is not perfect, with error rates of 1 in 100,000; permanent mutations occur at 1 in a billion.

• Proofreading: DNA polymerase has the capability to correct errors during replication.

• Nuclease enzymes correct mistakes by removing damaged sections of DNA; polymerases fill in gaps, and ligases seal the backbone.

Telomeres

• Telomeres are repetitive non-coding sequences at the ends of chromosomes, protecting genes from erosion.

• In humans, the sequence TTAGGG is repeated multiple times, safeguarding genetic information during replication.

Summary of DNA Replication Process

1. Separation of Strands: DNA unwinds and separates forming template strands.

2. Addition of Nucleotides: Enzymes synthesize new DNA strands by adding complementary nucleotides.

3. Leading and Lagging Strand Synthesis: Leading strand synthesis is continuous, while lagging strand synthesis occurs in fragments that are later joined.

4. Error Checking and Repair: Enzymes proofread and repair any mistakes during the replication process.

SIMPLIFIED

DNA Replication and Structure

Overview of DNA

• What is DNA?

  • DNA, or Deoxyribonucleic acid, is the molecule that carries the genetic information in living organisms. It is different from proteins, which are another key component of cells.

  • DNA contains instructions that help control how living things grow, develop, and function.

  • When cells divide, it's important that DNA is copied correctly so that the genetic information is passed on to the next generation.

  • The structure of DNA was first described by scientists James Watson and Francis Crick, an important milestone in genetics.

DNA Structure

• Double Helix Model

  • DNA has a shape like a twisted ladder, which scientists call a double helix.

  • It is made of two long strands that run alongside each other in opposite directions, known as antiparallel strands.

  • The rungs of the ladder consist of pairs of molecules called nucleotide bases: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

  • The structure is held together by weak bonds between these base pairs.

Components of DNA

• Nucleotides: The building blocks of DNA, each made up of three parts: a phosphate group, a sugar (deoxyribose), and one of the four types of bases (C, G, A, T).

• Strands: The two strands of DNA are connected by the hydrogen bonds formed between paired bases.

• Pairing: The bases are organized in a way where the purines (A and G) always pair with pyrimidines (C and T).

Functions of DNA

• Replication: The process by which DNA makes copies of itself. This involves unwinding the DNA, separating the strands, and creating new strands.

  • Although this process is generally accurate, mistakes can happen. Nature has mechanisms to correct these errors.

• Telomeres: Special sequences at the ends of chromosomes that help protect DNA from losing important information during replication.

DNA Replication Mechanism

• Starting Replication:

  • DNA replication begins at specific sites known as origins of replication, creating areas that allow for the copying of DNA to happen in both directions.

  • Key proteins and enzymes like helicase (which unzips the DNA), primase (which sets the starting point for copying), and DNA polymerase (which builds new strands of DNA) are involved in this process.

• Leading vs. Lagging Strand:

  • Sometimes, DNA can be copied smoothly (the leading strand) while at other times, it needs to be done in small pieces (the lagging strand), which are later connected together.

Key Enzymes and Proteins

• DNA Polymerase III: The main enzyme that adds new nucleotides to make a new DNA strand.

• Primase: Adds short RNA segments to initiate the DNA synthesis process.

• DNA Ligase: An enzyme that connects the pieces of DNA on the lagging strand and seals any gaps to ensure the strand is continuous.

Error Repair in DNA Synthesis

• Replication has a small error rate, but these mistakes can lead to changes in DNA.

• Proofreading: DNA polymerase can fix some of these errors as it makes new DNA.

• Nucleases: Special enzymes that remove damaged sections of DNA to allow new nucleotides to fill in those gaps, with ligases ensuring everything is properly connected.

Telomeres

• Telomeres are protective caps at the end of chromosomes made of repetitive non-coding DNA. They prevent the vital parts of DNA from being lost each time a cell divides.

• In humans, a specific sequence (TTAGGG) is repeated many times to protect our genetic information during replication.

Summary of DNA Replication Process

1. Strands Separate: DNA unwinds and the strands pull apart, revealing templates for new strands.

2. Nucleotides Added: Enzymes work to build new DNA strands by adding complementary nucleotides.

3. Leading and Lagging Strand Synthesis: The leading strand is created continuously, while the lagging strand forms in segments that are joined later.

4. Checking for Errors: Enzymes help to check and repair any mistakes that happen during copying, helping keep the DNA accurate.