Cell Replication Basics

Reproduction plays a pivotal role in ensuring the survival of species and maintaining ecological balance through the transfer of genetic information. There are two primary types of reproduction: sexual and asexual.

Understanding Reproduction

Role in Species Continuity

• Transfer of Genetic Information:
  • Facilitates adaptability within species through genetic variation.
  • Example: Pea plants acquire pest resistance through recombination and mutation processes.

Sexual vs. Asexual Reproduction

• Sexual Reproduction: Promotes genetic diversity, thereby aiding in adaptation.
• Asexual Reproduction: Enables rapid population expansion under stable environmental conditions.

Aspect	Sexual	Asexual
Genetic Diversity	High	Low
Parental Involvement	Involves two organisms	Involves one organism
Examples	Animals, plants	Bacteria, fungi

Sexual Reproduction

Fertilisation:

• Internal Fertilisation:
  • Examples include mammals and reptiles.
  • Offers protection, leading to fewer but more robust offspring.
• External Fertilisation:
  • Examples include fish and amphibians.
  • Facilitates the production of a large number of offspring.

Plant Reproduction

• Pollination Agents:
  • Bees, wind, and insects contribute to genetic diversity and resilience.

Asexual Reproduction

Plant Methods

• Methods such as runners, tubers, and cuttings allow for rapid colonisation of habitats.

Fungi

• Yeast and Spores: Utilise budding and spore dispersal for adaptation.

Bacteria

• Binary Fission: Enables quick reproduction and significant ecological impact.

Mitosis

Mitosis is essential for growth and repair, resulting in two identical daughter cells.

Stages of Mitosis

• Prophase: Chromosomes condense and spindle fibres begin to form.
• Metaphase: Chromosomes are aligned along the equatorial plate.
• Anaphase: Separation of sister chromatids occurs.
• Telophase: Nuclear membranes reform around separated chromatids.
• Cytokinesis: Final division, producing two separate cells.

Meiosis

Meiosis reduces chromosome numbers and creates genetic diversity in gametes.

Subphases of Meiosis I and II

• Meiosis I: Homologous chromosomes pair up and separate.
• Meiosis II: Involves the separation of sister chromatids.
• Crossing Over: Enhances genetic variation during Prophase I.

DNA Replication

Watson and Crick Model

• Describes the double helix structure and antiparallel strands ensuring high replication fidelity.

Replication Process

• Initiation: DNA helicase unwinds the DNA strands.
• Elongation: DNA polymerase synthesises new DNA strands by adding nucleotides.
• Termination: DNA ligase seals any nicks in the sugar-phosphate backbone.

Worked Example: Mismatch Repair

When a DNA replication error occurs, like inserting an A opposite a G instead of a C, mismatch repair enzymes identify this error.

1. The mismatch repair enzyme locates the error
2. It excises the incorrect nucleotide
3. It replaces it with the correct complementary base
4. DNA ligase seals the nick

This process maintains genetic stability by correcting errors before they become permanent mutations.

Hormonal Control

Key Hormones

• Progesterone: Crucial for maintaining the uterine lining.
• Oestrogen: Plays a role in foetal development.
• hCG: Prevents the onset of menstruation.

Techniques

Selective Breeding and Artificial Pollination

• Selective Breeding: Selects for and enhances desirable traits.
• Artificial Pollination: Involves manual pollen transfer to control breeding.

Genetic Modification and Cloning

• Genetic Modification: Involves altering DNA to promote beneficial traits.
• Cloning: Produces genetically uniform organisms.

Advantages of Manipulation

• Increased Yields: Aids in ensuring food security.
• Disease Resistance: Reduces the need for chemical interventions.

Worked Example: Ethical Considerations

Question: Evaluate the ethical implications of genetic modification in agriculture.

Solution:

1. Benefits include increased crop yields and improved nutritional content
2. Concerns include potential ecological impacts on biodiversity
3. Economic considerations: GM crops may create dependencies for farmers
4. Balanced perspective: GM technology requires careful regulation and transparent labelling

This framework helps assess both advantages and potential drawbacks when considering biotechnology applications.

Importance of Exact Replication

Genetic Stability

• Ensures genetic fidelity, which is crucial for species continuity.

Common Replication Errors

• Point Mutations, Insertions, Deletions:
  • Can alter the DNA sequences significantly.

Mechanisms Ensuring Accuracy

• Proofreading and Repair: Essential to minimise errors during DNA synthesis.

Research Highlight

• Research: Indicates that replication errors contribute to biodiversity, fostering adaptability.

Worked Example: Analysing Replication Errors

Question: How might a point mutation in DNA replication affect an organism?

Solution:

1. If the mutation occurs in a coding region, it could alter the amino acid sequence of a protein
2. A silent mutation may have no effect if it produces the same amino acid
3. A missense mutation changes one amino acid and may affect protein function
4. A nonsense mutation creates a premature stop codon, potentially producing a truncated protein

For example, if DNA sequence CAG (coding for glutamine) mutates to CCG (coding for proline), the protein structure could be significantly altered, potentially affecting its function.