DNA Replication Quiz

Course Information

• Course Title: Foundation Course For Health Sciences II
• Course Code: MEDF 1012A
• Instructor: Dr. Yeung Hang Mee, Po
• Email: [email protected]
• Office: Choh-Ming Li Basic Medical Sciences Building 6/F Room 610P

Learning Objectives

• Describe the three-dimensional structure of DNA and its role in maintaining DNA stability.
• Understand the mechanisms of DNA replication and proofreading.
• Identify the characteristics of the cell cycle and programmed cell death.

Transmission of Genetic Information

• This section covers how genetic information is passed on, including the relationship between DNA, mRNA, and proteins.
• A diagram depicting the flow of genetic information (DNA -> mRNA -> protein) is presented.

Three-Dimensional Structure of DNA

• DNA's stability results from specific arrangements of nitrogenous bases, deoxyribose, and phosphate groups.
• Nitrogenous bases are located inside the DNA helix, shielded from reactive nitrogen species.
• Negatively charged phosphate groups and deoxyribose sugars are positioned on the outside, protecting the inner bases.

Genetic Code During Translation

• Codons are three-letter sequences on mRNA that code for specific amino acids.
• There are 61 codons coding for 20 amino acids - not 64, since some amino acids are coded for by multiple codons
• There is one start and three stop codons in mRNA.
• Some amino acids are coded for by only one codon, whereas others can be coded by many, such as Leucine.

More About Codon Table

• E.Coli and humans share the same codon table. This facilitates their use in recombinant DNA technology.
• The genetic code is almost universal, but there are variations in mitochondria.
• Human insulin production from bacteria employing recombinant DNA technology is an example of application.

Prokaryotic DNA Replication

• Prokaryotic DNA, like that in E. coli, contains approximately five million base pairs.
• Replication initiates at a specific site called the origin (oriC).
• Replication is bi-directional, meaning it proceeds in two opposite directions from the origin.
• Key enzymes involved in replication are DNA polymerase, topoisomerase, and helicase.

Replication of DNA

• During replication, the parental DNA acts as a template.
• Replication proceeds in a semi-conservative manner, creating two DNA molecules each with one new and one original strand.
• The diagram visually displays the semi-conservative process.

Replication Fork

• Two separated strands form a replication fork structure.
• New DNA strands are synthesized in different ways - the leading strand is synthesized continuously, whereas the lagging strand is discontinuously, creating Okazaki fragments.
• Synthesis of DNA occurs in the 5' to 3' direction.
• Replication origin (oriC), leading strand, lagging strand, DNA polymerase, and DNA ligase are involved in the process of replication.

Definitions of 5' and 3' ends

• The 5' and 3' designations refer to carbon numbers on the sugar molecule (deoxyribose) in DNA (or ribose in RNA).

Replication of Prokaryotic DNA

• Details of Helicase, single-strand binding proteins and DNA polymerases and their functions in the process.
• Explains the unwinding process of the DNA template for the replication to commence, revealing the key enzymes required like helicase and polymerase.

Prokaryotic DNA Proofreading and Repair

• DNA polymerase proofreads newly synthesized DNA using its 3' to 5' exonuclease activity.
• This activity removes incorrect nucleotides to maintain accuracy.
• Nucleases remove nucleotides.

The Stages of Mitosis

• Prophase: Chromosomes condense, and the nuclear envelope breaks down. Microtubules emerge from centrosomes.
• Prometaphase: Microtubules attach to chromosomes. Centrosomes move to opposite poles.
• Metaphase: Chromosomes line up at the center of the cell.
• Anaphase: Sister chromatids separate and move to opposite poles.
• Telophase: Chromosomes decondense, new nuclear envelopes form.
• Cytokinesis: The cytoplasm divides, forming two daughter cells.
• Further explanation of each stage.

The Stages of Cell Cycle

• The cell cycle consists of four phases - G0, G1, S, and G2 before mitosis - and involves a series of events that lead to cells duplicating their DNA and dividing.
• The 1st phase – Gap 1 (G1) – is where the cell monitors its environment and size prior to DNA replication.
• Second phase is where DNA replication occurs known as synthesis (S) which prepares it for division.
• Other phases include the gap 2 phase (G2) and mitosis phase (M) to prepare for and complete cell division.
• Cell cycles and cell checkpoints; G1, G2 and the S phase checkpoints

Factors Affecting the Cell Cycle

• Control mechanisms monitor cell division through checkpoints in the different phases.
• Cell cycle checkpoints serve as control points that regulate whether a cell advances to the next phase based on whether specific conditions are met.

Central Control System of the Cell Cycle

• Cyclin-dependent kinases (CDKs) and cyclins are crucial proteins in controlling the cell cycle.
• CDKs phosphorylate target proteins to drive the cell cycle forward.
• Cyclins regulate the activity of CDKs.
• Further explanation of cyclin-cdk complex and their role in regulating the cell cycle.

Mitotic Cyclins

• Mitotic cyclins regulate entry into mitosis.
• Mitotic cyclin is degraded to allow for the cell cycle to progress after entering mitosis.

Cell Senescence

• Most cells stop dividing after a certain number of cell divisions, a process called senescence.
• This is to prevent uncontrolled cell growth, important preventative mechanism for cancer.

Cell Death

• Cell death occurs through apoptosis (programmed cell death) or necrosis
• Apoptosis is an active process of disposing of cells without damage to neighboring cells.
• Necrosis is an uncontrolled cell death that damages surrounding tissues causing an inflammatory response.

Caspases and Procaspases

• Caspases are enzymes crucial for apoptosis.
• Procaspases are inactive precursors of caspases.
• Procaspases are activated in a caspase cascade. Cascade refers to the chain reaction of activation.

Extracellular Activation of Apoptosis

• Extracellular death signals or ligands induce the death receptor activation of the caspase cascade leading to programmed cell death.
• The specific protein or ligand on the cell activates the caspase chain leading to apoptosis.
• The chain reaction further activates more caspases resulting in apoptosis

Intracellular Activation of Apoptosis

• Intracellular signals or stressors trigger the release of cytochrome c from the mitochondria.
• Cytochrome c activates procaspases, initiating the apoptotic cascade.
• Further explanation of the intracellular apoptosis pathway involving the mitochondria, cytochrome c, and adaptor proteins.

Summary

• DNA stability is maintained by specific arrangements of nitrogenous bases and the sugar-phosphate backbone.
• DNA replication is a semi-conservative process.
• The cell cycle comprises distinct phases with checkpoints that regulate its progression.
• Apoptosis is a programmed cell death mechanism, while necrosis is an uncontrolled form of cell death.
• The cascade activation of caspase-type enzymes is crucial for apoptosis.

Test your knowledge of DNA replication. Questions cover the roles of enzymes like DNA polymerase and primase, the function of single-strand binding proteins, and the differences between leading and lagging strand synthesis. Explore conservative and semi-conservative replication methods.