Slide 1: Genetics and Evolution - Molecular Basis of Inheritance

• DNA synthesis at the ends of chromosomes
• Telomeres and their function
• Telomerase enzyme and its role in DNA replication
• Telomere shortening and aging
• Telomere lengthening and cancer

Slide 2: Telomeres

• Telomeres are repetitive DNA sequences at the ends of chromosomes
• Consist of tandem repeats of the sequence TTAGGG in humans
• Function to protect the chromosome from degradation and fusion
• Act as a “cap” to maintain chromosome stability
• Play a role in cellular aging and carcinogenesis

Slide 3: Function of Telomeres

• Prevent deterioration of genes near chromosome ends
• Facilitate complete replication of DNA molecules
• Ensure that DNA sequence information is not lost during cell division
• Protect chromosomes from fusing with each other
• Maintain chromosomal stability

Slide 4: Telomerase Enzyme

• Telomerase is a specialized reverse transcriptase enzyme
• Adds repetitive DNA sequences to the ends of chromosomes
• Contains an RNA template that serves as a template for DNA synthesis
• Activated in certain cell types, including stem cells and cancer cells
• Plays a crucial role in replicating telomeric DNA and preventing telomere shortening

Slide 5: Role of Telomerase in DNA Replication

• During DNA replication, conventional DNA polymerases cannot fully synthesize the telomeric DNA at the chromosome ends
• Telomerase binds to the telomere and uses its RNA template to synthesize new telomeric DNA
• Telomerase extends the telomere length by adding repetitive DNA sequences
• Ensures complete replication of the chromosome ends
• Allows cells to divide without any loss of genetic information

Slide 6: Telomere Shortening and Aging

• Telomeres naturally shorten with each round of cell division
• Telomerase activity decreases with age in most somatic cells
• Insufficient telomerase activity leads to progressive shortening of telomeres
• Telomere shortening is implicated in cellular aging and age-related diseases
• Shortened telomeres can trigger cellular senescence or apoptosis

Slide 7: Cellular Senescence

• Cellular senescence refers to the state of a cell where it has ceased dividing
• Cellular senescence can be triggered by telomere shortening
• Senescent cells exhibit altered gene expression patterns
• Senescent cells can accumulate over time and contribute to tissue aging
• Senescence is a protective mechanism against cancer, but also associated with aging-related diseases

Slide 8: Telomere Lengthening and Cancer

• Telomerase activity is often reactivated in cancer cells
• Activated telomerase extends telomeres, preventing or slowing their shortening
• Telomere lengthening allows cancer cells to divide indefinitely
• Telomerase inhibition is a potential therapeutic strategy for cancer treatment
• Developing drugs that target telomerase is an active area of research

Slide 9: Telomerase and Immortal Cells

• Germ cells, stem cells, and cancer cells have higher levels of telomerase activity
• Germ cells and stem cells maintain their telomeres throughout life
• Cancer cells acquire the ability to maintain telomeres through reactivation of telomerase
• Telomerase activation is one of the hallmarks of cancer
• Inhibiting telomerase can selectively target cancer cells while sparing normal cells

Slide 10: Summary

• Telomeres are repetitive DNA sequences at the ends of chromosomes
• Telomeres protect the chromosome and are crucial for genome stability
• Telomerase is an enzyme that extends telomeres
• Telomerase activity is associated with cellular aging and cancer
• Telomere shortening can lead to cellular senescence or apoptosis

Slide 11: DNA Replication

• DNA replication is a process that occurs prior to cell division
• It ensures that each daughter cell receives an exact copy of the genetic material
• The process is semiconservative, meaning each strand of the parental DNA molecule serves as a template for the synthesis of a new strand
• The DNA replication machinery includes enzymes, such as DNA polymerase, helicase, and ligase
• Replication begins at specific sites called origins of replication

Slide 12: Steps of DNA Replication

1. Initiation: Replication begins at the origin of replication, where the DNA double helix unwinds and separates into two strands.

2. Elongation: DNA polymerase synthesizes new DNA strands using the parental strands as templates. One new strand is synthesized continuously (leading strand), while the other is synthesized in short fragments (lagging strand).

3. Priming: Primase synthesizes RNA primers that provide a starting point for DNA synthesis.

4. Okazaki fragments: DNA polymerase synthesizes short DNA fragments on the lagging strand, which are later joined by DNA ligase.

5. Termination: DNA replication is completed when the replication forks meet at the termination sites.

Slide 13: Replication Fork and Replisome

• Replication fork is the Y-shaped structure formed during DNA replication
• It represents the site of DNA unwinding and synthesis
• The replisome is a complex of enzymes and proteins that catalyze DNA replication at the replication fork
• Helicase unwinds the DNA double helix, while DNA polymerase synthesizes new DNA strands
• Other proteins help stabilize the replication fork and coordinate the activities of different enzymes

Slide 14: Leading and Lagging Strands

• The DNA template at the replication fork is read in the 3’ to 5’ direction
• DNA polymerase synthesizes new strands in the 5’ to 3’ direction
• The leading strand is synthesized continuously in the same direction as the replication fork
• The lagging strand is synthesized discontinuously as short Okazaki fragments
• DNA ligase joins the Okazaki fragments to form a continuous strand

Slide 15: DNA Polymerase

• DNA polymerase is the main enzyme involved in DNA synthesis
• It catalyzes the addition of nucleotides to the growing DNA strand
• DNA polymerase requires a primer with a free 3’ end to initiate DNA synthesis
• DNA polymerase extends the primer by adding complementary nucleotides to the template strand
• Different DNA polymerases have specialized functions in DNA replication, repair, and other DNA processes

Slide 16: Proofreading and Repair

• DNA polymerase has proofreading activity to ensure accuracy during DNA replication
• Incorrect nucleotides are removed by the 3’ to 5’ exonuclease activity of DNA polymerase
• Mismatch repair system further corrects errors missed by proofreading
• DNA repair mechanisms ensure the integrity of the genetic material
• Mutations can occur if errors are not corrected, leading to potential genetic disorders

Slide 17: Telomeres and DNA Replication

• DNA replication at the ends of chromosomes is a complex process due to the unique structure of telomeres
• Telomerase plays a crucial role in maintaining telomere length during replication
• Telomerase uses its RNA template to synthesize telomeric DNA de novo
• Telomerase extends the 3’ end of the lagging strand to compensate for incomplete replication at the chromosome ends
• Telomeric DNA synthesis by telomerase ensures the complete replication of the chromosome ends

Slide 18: Regulation of DNA Replication

• DNA replication is tightly regulated to prevent errors and maintain genomic stability
• Initiation of replication is controlled by proteins that ensure proper timing and coordination
• Checkpoints monitor DNA replication and activate repair mechanisms if necessary
• Mutations in replication-associated genes can lead to genetic disorders and cancer
• Understanding the regulation of DNA replication is important for studying cell division and its implications

Slide 19: DNA Replication and Evolution

• DNA replication is a fundamental process that contributes to genetic variation and evolutionary change
• Errors during DNA replication can introduce mutations, which are the raw materials for evolution
• Mutations can lead to changes in an organism’s phenotype, allowing for adaptation to new environments
• DNA replication fidelity and the repair systems have evolved to strike a balance between stability and variability
• Studying DNA replication provides insights into the mechanisms driving genetic diversity and evolution

Slide 20: Summary

• DNA replication is a semiconservative process that ensures the faithful transmission of genetic information
• Replication begins at specific origins of replication and proceeds bidirectionally
• DNA polymerase synthesizes new DNA strands in the 5’ to 3’ direction
• Leading and lagging strands are synthesized differently at the replication fork
• Telomeres require specialized mechanisms for complete replication

Slide 21: DNA Repair Mechanisms

• Different DNA repair mechanisms ensure the integrity of the DNA molecule
• Mismatch repair corrects errors in DNA replication that escaped proofreading
• Base excision repair removes damaged bases and replaces them with correct ones
• Nucleotide excision repair fixes bulky lesions caused by UV radiation or chemicals
• Double-strand break repair restores broken DNA strands
• Defects in DNA repair mechanisms can lead to serious genetic disorders

Slide 22: Mismatch Repair

• Mismatch repair corrects errors missed by DNA polymerase proofreading
• Mismatch repair enzymes recognize and remove bases incorrectly paired during replication
• The mismatched DNA strand is removed and resynthesized using the parental strand as a template
• Mutations in genes involved in mismatch repair can lead to hereditary nonpolyposis colorectal cancer (HNPCC)
• Examples: MutS, MutL, and MutH proteins are key players in mismatch repair

Slide 23: Base Excision Repair

• Base excision repair corrects small DNA lesions, such as damaged or modified bases
• DNA glycosylases recognize and remove the damaged base, producing an abasic site (AP site)
• The abasic site is then processed and replaced with the correct base by other enzymes
• Base excision repair is important for repairing damaged DNA caused by oxidative stress and certain chemicals
• Examples: DNA glycosylases, AP endonucleases, and DNA polymerases

Slide 24: Nucleotide Excision Repair

• Nucleotide excision repair repairs bulky DNA lesions caused by UV radiation or certain chemicals
• A complex of proteins scans the DNA molecule and recognizes lesions/distortions
• The damaged DNA strand is excised, and the gap is filled in by DNA synthesis and ligation
• Nucleotide excision repair ensures the removal of pyrimidine dimers and other bulky adducts
• Examples: XP proteins (XPA to XPG), DNA helicase, DNA polymerases, and DNA ligase

Slide 25: Double-Strand Break Repair

• Double-strand breaks (DSBs) are highly toxic to cells and can result from DNA damage or normal DNA metabolism
• Two major pathways repair DSBs: non-homologous end joining (NHEJ) and homologous recombination (HR)
• NHEJ rejoins the broken ends without using a homologous template, potentially leading to small insertions or deletions
• HR uses a homologous DNA sequence as a template to precisely repair the DNA break
• Double-strand break repair is crucial for maintaining genomic stability and preventing chromosomal rearrangements

Slide 26: Examples of DNA Repair Disorders

• Xeroderma pigmentosum (XP): Defects in nucleotide excision repair leading to extreme sensitivity to UV radiation and a high risk of skin cancer.
• Ataxia telangiectasia (AT): Defective DNA damage response and repair, resulting in progressive neurological problems and increased cancer risk.
• Bloom syndrome: Deficiency in DNA helicase resulting in chromosome instability, growth retardation, and increased cancer risk.
• Werner syndrome: Defects in a DNA helicase leading to premature aging, genomic instability, and increased cancer risk.
• Fanconi anemia: Impaired DNA repair, resulting in bone marrow failure, birth defects, and increased cancer susceptibility.

Slide 27: Human Genome Project (HGP)

• The Human Genome Project aimed to determine the sequence of the entire human genome
• Started in 1990 and completed in 2003, it involved a collaborative effort by scientists worldwide
• The HGP provided a complete reference sequence of the human genome
• The project had many significant outcomes, including the identification of disease-related genes and the understanding of genetic variation
• The HGP laid the foundation for various fields, such as personalized medicine and genetic research

Slide 28: Applications of Genome Sequencing

• Personalized Medicine: Genome sequencing can help predict disease risk and guide customized treatment plans.
• Genetic Counseling: Sequencing genomes can provide information about inherited disorders, helping individuals make informed reproductive decisions.
• Forensic Science: Genome sequencing can aid in criminal investigations by analyzing DNA evidence.
• Evolutionary Studies: Comparative genome sequencing helps understand genetic relationships and the evolution of species.
• Agriculture and Biotechnology: Genome sequencing can improve crop productivity and develop new biotechnological applications.

Slide 29: Ethical Considerations in Genetics

• Privacy: The wide availability of genetic information raises concerns about privacy and data security.
• Genetic discrimination: Genetic information can be used to discriminate against individuals in various areas, such as employment and insurance.
• Informed Consent: Genetic testing requires informed consent, ensuring individuals understand the implications and potential risks.
• Reproductive Choices: Genetic testing raises ethical questions regarding reproductive decisions, including prenatal testing and embryo selection.
• Access and Equity: Ensuring equitable access to genetic testing and treatments for all individuals.

Slide 30: Summary

• DNA repair mechanisms maintain the integrity of the genome by fixing DNA damage caused by various factors.
• Mismatch repair corrects errors in DNA replication, while base excision repair and nucleotide excision repair fix specific types of DNA lesions.
• Double-strand break repair restores broken DNA strands, ensuring chromosomal stability.
• Genetic disorders can result from defects in DNA repair mechanisms.
• The Human Genome Project revolutionized genetics and has wide-ranging applications in medicine, forensics, and research.