DNA cell division /Handouts Part 7
Page 1: Introduction to DNA
DNA (Deoxyribonucleic Acid)
Discovered in the late 19th century by Friedrich Miescher.
Post Gregor Mendel's work on inheritance; however, both were unaware of each other's contributions.
Key Understanding of DNA
Subsequently recognized to be pivotal in heredity (Frederick Griffith, 1928).
Includes components such as:
Sugar-phosphate backbone with phosphodiester bonds.
Four different nitrogenous bases in two complementary pairs (adenine-thymine, guanine-cytosine).
DNA is structured as two complementary strands in an antiparallel configuration.
1950s Breakthrough
Watson and Crick proposed the DNA double helix model, building on Rosalind Franklin's X-ray diffraction studies.
Page 2: DNA Replication Overview
DNA Role
Stores genetic information and transmits it from mother cells to daughter cells during mitosis.
Gametes are produced through meiosis during sexual reproduction.
DNA Replication
Must occur prior to mitosis and meiosis, producing two identical DNA copies by a process known as semiconservative replication.
Each new DNA double helix consists of one old strand and one newly synthesized strand.
Key Enzyme: DNA polymerase.
Page 3: Steps of DNA Replication
Three Major Steps:
Initiation
Occurs at replication origins where initiator proteins bind and open up the DNA helix.
Prokaryotes have one origin; eukaryotes have multiple origins.
Elongation
DNA polymerase synthesizes new strands by reading the template strand from 3' to 5' and synthesizing from 5' to 3'.
Termination
Prokaryotic DNA has a single termination point, while eukaryotic DNA has multiple points characterized by telomeres.
Page 4: Enzymes Involved in Replication
Key Enzymes:
Helicase & DNA Gyrase: Unwind and relieve tension in the DNA double helix.
Primase: Synthesizes RNA primers for initiation.
DNA Polymerases:
DNA pol III: Primary enzyme for DNA synthesis.
DNA pol I: Replaces RNA primers with DNA.
DNA pol II: Proofreads and repairs strands.
Nucleases: Cleave between nucleotides to repair DNA integrity.
DNA Ligase: Joins DNA segments.
Page 5: Mechanisms of DNA Replication
Prokaryotic DNA Replication
Begins at the origin (oriC) with specific nucleotide repeats facilitating the opening of the helix.
Bidirectional replication proceeds with two replisomes moving away from the origin.
Each replisome completes approximately 180° of circular DNA, finishing at the terminus.
Page 6: The Replication Fork
Replication Dynamics:
Leading strand is synthesized continuously from an RNA primer.
Lagging strand is synthesized in fragments (Okazaki fragments) due to its 5' to 3' directionality requirement.
Page 7: Lagging Strand Synthesis
Okazaki Fragments:
Each fragment starts with its own RNA primer.
After removal of RNA primers, DNA ligase is required to stitch fragments together into a continuous strand.
Page 8: Completing the Lagging Strand
Overall Process:
RNA primers are replaced with DNA segments.
Joining of Okazaki fragments leads to a complete strand.
Continuous switching from RNA-primed synthesis to fragment assembly.
Page 9: Eukaryotic DNA Organization
Eukaryotic Chromosomes:
Linear double-helix structures that necessitate added enzymes for DNA replication at multiple origins.
Termination Mechanism:
Specialized sequences called telomeres protect linear chromosome ends from degradation.
Page 10: DNA Repair Mechanisms
Repair Processes:
Errors in DNA must be corrected during duplication and can be prompted by external agents.
Damage is often repaired effectively, preserving genetic integrity.
Specific Repair Mechanism:
Photolyase: Repairs UV-induced thymine dimers using light energy.
Page 11: Nonspecific Repair Mechanisms
Excision Repair:
Involves removal and replacement of damaged DNA segments by specific enzymes.
Note: DNA undergoes proofing and repair; RNA does not.
Page 12: Overview of Mitosis and Meiosis
Cell Division Principles:
Central paradigms state that all cells arise through cell division.
Mitosis:
One cell divides into two genetically identical daughter cells; occurs in prokaryotes via simple cloning.
In eukaryotes, mitosis facilitates growth and repair of tissues while preserving genetic integrity.
Page 13: Meiosis: Sexual Reproduction
Meiosis Process:
Produces gametes (haploid) through two rounds of division but only one DNA replication.
Results in varied genetic combinations crucial for sexual reproduction and evolution.
Page 14: Prokaryotic Cell Division
Process of Prokaryotic Reproduction:
Involves DNA replication, partitioning, elongation, septation, and separation of daughter cells.
Page 15: Chromosome Separation in Prokaryotes
Mechanisms and Proteins:
Chromosome separation resembles eukaryotic mechanisms, hinting at evolutionary connections.
Page 16: Eukaryotic Cell Division Requirements
Cell Division Complexity:
Multiple chromosomes must replicate and segregate properly.
Organelles such as mitochondria and chloroplasts must also be correctly distributed.
Page 17: Overview of Human Genome
Genome Definition:
Complete set of genetic material, organized in chromosomes.
Each chromosome contains vast amounts of information crucial for organism development and function, including approximately 20,000 known protein-coding genes.
Page 18: Chromosomal Structure During Cell Division
Sister Chromatids and Cohesin:
Connection at the centromere is crucial for accurate chromosomal distribution during mitosis.
Page 19: Cell Cycle Phases
Three Main Phases:
G1: Cell growth and function.
S: DNA replication.
G2: Preparation for division.
Page 20: Interphase Activities
Notable Processes:
Synthesis and replication of organelles, chromosomal condensation, and spindle apparatus formation.
Page 21: Mitosis and Stages Overview
M Phase:
Involves prophase, prometaphase, metaphase, anaphase, and telophase in sequence to segregate chromatids.
Page 22: Details of Metaphase and Anaphase
Tension in Microtubules:
Ensures proper segregation of sister chromatids.
Page 23: Telophase and Cytokinesis
Telophase Characteristics:
Chromatids reach opposite poles and nuclear envelopes reform.
Actin filaments facilitate the cleavage of the cell membrane during cytokinesis.
Page 24: Mitosis: A Controlled Process
Importance of Accuracy:
Various checkpoints maintain fidelity of DNA replication, sister chromatid segregation, and organelle distribution.
Page 25: Checkpoints in the Cell Cycle
Significant Checkpoints:
G1/S, G2/M, and spindle checkpoint ascertain readiness and correctness of the cell division process.
Page 26: Meiosis Overview
Meiotic Division Details:
Produces genetic diversity and reduces chromosome number in gametes through two rounds of division.
Page 27: Homologous Chromosomes and Alleles
Genetic Variability:
Homologous chromosomes carry the same genes but can have different alleles, affecting traits such as blood type.
Page 28: Life Cycles and Gamete Formation
Haploid and Diploid Stages:
Organisms transition between haploid and diploid stages through sexual reproduction, with gametes developing from germ-line cells.
Page 29: Meiosis Process Summary
Two Main Steps of Meiosis:
Meiosis I results in haploid cells through random segregation of homologous chromosomes.
Meiosis II focuses on the segregation of sister chromatids.
Page 30: Meiosis II Overview
Key Events:
Sister chromatids segregate in anaphase II, leading to the production of haploid daughter cells.
Page 31: Crossing Over in Meiosis
Significance of Genetic Recombination:
Connection between maternal and paternal chromosomes during prophase I allows for variation in alleles.
Page 32: Genetic Recombination Mechanics
Meiosis I and II Stages:
Emphasizes randomness and the generation of genetic diversity through crossing over and segregation.
Page 33: Conclusion of Meiosis
Final Outcome:
Results in four non-identical haploid cells, each with varied genomes due to recombination steps.
Introduction to DNA
DNA (Deoxyribonucleic Acid)
Discovered in the late 19th century by Friedrich Miescher, DNA was initially isolated from white blood cells. His work set the stage for future genetics research.
Post Gregor Mendel's foundational work on inheritance principles; neither Miescher nor Mendel were aware of each other's contributions, which would later be crucial in understanding heredity and genetics.
Key Understanding of DNA
Recognized as pivotal in heredity by studies conducted by Frederick Griffith in 1928, who demonstrated the phenomenon of transformation, hinting at a genetic material's role in inheritance.
DNA is composed of:
Sugar-phosphate backbone: Forms the structural framework of DNA, linked by phosphodiester bonds that provide stability to the molecular structure.
Four nitrogenous bases: These include adenine (A), thymine (T), cytosine (C), and guanine (G), which pair in complementary fashion (A with T and C with G).
The structure of DNA comprises two complementary strands oriented in an antiparallel configuration (5' to 3' and 3' to 5').
1950s Breakthrough
In the early 1950s, James Watson and Francis Crick proposed the DNA double helix model, heavily relying on Rosalind Franklin's critical X-ray diffraction studies that provided insight into DNA's helical structure.
DNA Replication Overview
DNA Role
DNA serves a dual function: it stores genetic information and transmits it faithfully from mother cells to daughter cells during the cell division process (mitosis). Game change occurs through meiosis, leading to the formation of gametes essential for sexual reproduction.
DNA Replication Process
Prior to both mitosis and meiosis, DNA replication must occur, resulting in two identical DNA copies through a process known as semiconservative replication. This mechanism means that each new DNA double helix comprises one old (template) strand and one newly synthesized strand.
Key Enzyme: The essential enzyme for DNA synthesis is DNA polymerase, responsible for adding nucleotides to the growing chain based on the template strand.
Steps of DNA Replication
Three Major Steps:
Initiation: This step occurs at specific locations in the DNA molecule called replication origins, where initiator proteins recognize and bind to initiate DNA unwinding.
Prokaryotes typically possess one origin of replication, whereas eukaryotes have multiple origins, facilitating simultaneous replication.
Elongation: In this phase, DNA polymerase synthesizes new DNA strands by reading the template DNA in the 3' to 5' direction, which necessitates that the new strand be synthesized in the 5' to 3' direction.
Termination: During this step, the mechanism for terminating replication varies: prokaryotic DNA has a single termination point, while eukaryotic DNA has multiple termination points associated with specialized structures called telomeres.
Enzymes Involved in Replication
Key Enzymes Include:
Helicase and DNA Gyrase: Essential for unwinding the DNA double helix and alleviating the resultant supercoiling tension.
Primase: Synthesizes short RNA primers essential for initiating new DNA strand synthesis, as DNA polymerase requires a primer to start.
DNA Polymerases:
DNA pol III: Functions as the primary enzyme during DNA synthesis.
DNA pol I: Responsible for replacing RNA primers with DNA.
DNA pol II: Involved in proofreading existing DNA strands, ensuring fidelity.
Nucleases: Enzymes that cleave the bonds between nucleotides to facilitate the repair of DNA integrity.
DNA Ligase: This enzyme is critical for joining Okazaki fragments on the lagging strand, thus creating a continuous DNA molecule.
Prokaryotic vs. Eukaryotic DNA Replication
Prokaryotic DNA Replication: Initiation begins at a specific origin (oriC) characterized by specific nucleotide repeats, followed by bidirectional replication with two replisomes spreading away from the origin to complete circular DNA replication at the terminus.
Eukaryotic DNA Organization: Eukaryotic chromosomes feature linear double-helical structures requiring additional enzymes for high-fidelity replication at multiple origins.
DNA Repair Mechanisms
Accurate DNA replication can generate errors or suffer damage from environmental factors. Repair processes are vital to preserving genetic integrity, with the key repair mechanism being photolyase, which repairs UV-induced thymine dimers using light energy.
Excision Repair: A process that involves the removal and replacement of damaged DNA segments. Unlike DNA, RNA does not undergo proofreading or repair, highlighting the significance of DNA's repair mechanisms.