ATAR Notes: Module 5 Heredity

Inquiry Question: How Does Reproduction Ensure the Continuity of a Species?

1.1 Sexual and Asexual Reproduction Methods

• Reproduction Overview: The process of creating offspring via sexual or asexual biological processes to ensure species continuity.

1.1.1 Animals

• Sexual Reproduction Processes:

  • Gamete Production: Produced by parents through meiosis.

  • Chromosome Count: Each gamete is haploid, containing half the necessary number of chromosomes ($n$).

  • Fertilisation: Fusion of the male gamete (sperm) and female gamete (ovum/egg).

    • Internal Fertilisation: Occurs inside the body (e.g., mammals).

    • External Fertilisation: Occurs outside the body in the environment (e.g., fish like salmon).

  • Zygote Formation: The fusion results in a diploid zygote ($2n$) containing combined genetic material from both parents.

• Comparison of Fertilisation Methods:

  • Sexual Reproduction (General):

    • Advantages: Increased variation due to chromosome combination assists survival.

    • Disadvantages: Requires mating (syncing fertility cycles); slower and less prolific than asexual methods.

  • External Fertilisation:

    • Advantages: Large number of gametes leads to more offspring; simpler behavior (no mating rituals).

    • Disadvantages: Requires large energy to produce many gametes; requires a watery environment; high predation/environmental risk.

  • Internal Fertilisation:

    • Advantages: High likelihood of fertilisation (close proximity); protection from the environment leads to higher survival rates for offspring.

    • Disadvantages: Fewer offspring; difficult to bring sexes together; risk of sexually transmitted infections (STIs).

1.1.2 Plants

• Sexual Reproduction:

  • Reproductive Organs: Flowers (stamen, anther, filament, sepal, petal, stigma, style, ovary, pistil).

  • Mechanism: Pollen (male gamete) is transferred to female ovules (ovary) via pollination (wind or insects). Fertilisation leads to seeds inside a fruit, which disperse when ripe.

  • Advantages: Genetic diversity; higher disease resistance; adaptation to change.

  • Disadvantages: Favourable genes (recessive) may be masked; impossible for isolated individuals.

• Asexual Reproduction:

  • Mechanism: Structural modifications to stems or roots result in new individuals without seeds/spores.

  • Vegetative Propagation: New individuals develop from multicellular structures that detach from the parent (clones).

  • Advantages: Favourable traits passed directly; less energy intensive; rapid population increase in suitable habitats.

  • Disadvantages: Pathogens spread easily; low genetic diversity increases susceptibility to disease; reduced evolution.

1.1.3 Fungi

• Sexual Reproduction:

  1. Plasmogamy: Two genetically different cells fuse.

  2. Karyogamy: The nuclei of the fused cells fuse.

  3. Meiosis: Gametes generate spores distributed into the environment.

  • Advantages: Increases genetic variability; offers a choice of propagation method.

  • Disadvantages: High energy; requires a findable mate (fungi are immobile).

• Asexual Reproduction:

  • Fragmentation: Hyphae pieces separate to form new colonies.

  • Budding: Nucleus divides, a bulge forms, and is split off by cytokinesis.

  • Spores: Mitosis produces identical cells distributed by wind/vectors.

  • Advantages: Wide distribution (colonisation); large numbers produced quickly with little energy (even under stress).

  • Disadvantages: Offspring may be suited to only one specific habitat.

1.1.4 Bacteria

• Asexual Reproduction (Binary Fission):

  1. DNA Replication: Genetic information is copied.

  2. Separation: DNA is divided.

  3. Cytokinesis: Cell elongates and splits into two identical daughter cells (clones).

  • Advantages: Rapid (e.g., $E. coli$ can replicate every $20$ minutes in right conditions); requires only one organism.

  • Disadvantages: Lack of diversity. (Note: Overcome by high mutation rates and Horizontal Gene Transfer/HGT via plasmids).

1.1.5 Protists

• Sexual Reproduction:

  • Haploid Protists: Two $1n$ cells fuse to form a zygote ($2n$), which then undergoes meiosis to form new $1n$ cells.

  • Diploid Protists: Adults undergo meiosis to produce $4$ gametes, which fuse to form a $2n$ zygote.

  • Advantages: Evolutionary advantage; sexual recombination allows greater variation.

  • Disadvantages: Fewer offspring; energy intensive; requires a mate.

• Asexual Reproduction:

  • Binary Fission: Predominant method (same as bacteria).

  • Budding: New organism grows from the parent body to form a colony.

  • Advantages: Rapid reproduction makes them effective pathogens; low energy cost.

  • Disadvantages: Limited adaptation; host immunity may easily target identical clones.

1.2 Fertilisation, Implantation, and Hormonal Control in Mammals

Definitions and Process

• Fertilisation: Fusion of gametes to initiate a new organism. Occurs in the Fallopian tube (oviduct) within a $12-24$ hour window after ovulation.

• Implantation: Fertilised egg adhering to the uterus wall. Occurs approximately $7$ days after fertilisation.

• The Path of Development:

  1. Zygote (Two-cell: $30$ hrs; Four-cell: $40$ hrs).

  2. Morula (Early: $80$ hrs; Advanced: $4$ days).

  3. Blastocyst (approx. $5$ days): Embeds in the endometrium.

1.2.1 Hormonal Control of Pregnancy

• Chorionic Gonadotropin (hCG): Produced by the early embryo/placenta. Thickens uterine lining; triggers oestrogen and progesterone release; ceases menstruation. Production drops off after approx. $10$ weeks.

• Progesterone: Produced by the corpus luteum, then the placenta. Strengthens uterine lining; encourages nutrient-producing glands; prepares body for breastfeeding; relaxes the uterus to prevent early labor. Levels rise throughout gestation.

• Oestrogen: Thickens lining; assists foetal organ development; develops breasts/milk ducts. Works with progesterone.

• Secondary Hormones:

  • Prolactin: Milk production; mammary gland enlargement (starts rising in 2nd trimester).

  • Oxytocin: Causes uterine muscle contractions (labor); stimulates milk let-down; bonding hormone.

  • Relaxin: Loosens uterine muscles; relaxes the cervix during labor.

  • Human Placental Lactogen: Provides nutrients to foetus.

1.2.2 The Ovarian Cycle and Feedback Loops

• Follicular Phase (Days 1–14):

  • FSH (Follicle-Stimulating Hormone): Released by the pituitary; stimulates maturation of primary follicles.

  • Negative Feedback (Days 1–10): Low oestrogen inhibits LH release.

  • Positive Feedback (Late Follicular): High oestrogen triggers a large surge in LH (and small surge in FSH).

  • Ovulation (Day 14): LH surge triggers egg release.

• Luteal Phase (Days 15–28):

  • Corpus Luteum: Formed from the degraded follicle; secretes Progesterone, Oestrogen, and Inhibin.

  • Inhibin: Exerts negative feedback to inhibit FSH (prevents new follicle maturation).

  • End of Cycle: If no fertilisation, corpus luteum degenerates, progesterone falls, and the endometrium is shed (menstruation).

1.3 Manipulating Reproduction in Agriculture

• Selective Breeding: Mating plants/animals with desirable hereditary traits (e.g., Jersey or Angus cows).

• Artificial Insemination:

  • Method: Detection of oestrus; semen collection; insemination via a gun into the cervix.

  • Benefits: Synchronised births; passing of favourable traits (meat/milk quality); increased yield.

• Artificial Pollination:

  • Method: Pollen removed from stamen and applied to the stigma.

  • Benefits: Cross-breeding; self-pollination for specific traits; high crop yield.

• Genetic Engineering:

  • Examples: Bt cotton (insect resistance); Golden rice (nutritional value); Strawberries (frost resistance).

  • Impact: Estimated $170.3$ million hectares of GM crops grown globally in $2012$.

2.1 Cell Replication: Mitosis and Meiosis

2.1.1 Mitosis (Identical Daughter Cells)

• Interphase: DNA replication prepares the cell ($G_1$, $S$, $G_2$).

• Prophase: Chromosomes condense; mitotic spindle (microtubules) forms.

• Prometaphase: Nuclear envelope breaks down.

• Metaphase: Sister chromatids line up at the equator.

• Anaphase: Chromatids pulled to opposite poles via kinetochores.

• Telophase: Two new nuclear envelopes form.

• Cytokinesis: Tightening ring of proteins squeezes the cell into two.

2.1.2 Meiosis (Four Haploid Gametes)

• Meiosis I:

  • Prophase I: Homologous chromosomes pair and undergo crossing over (recombination at same loci).

  • Metaphase I: Homologous pairs line up independently.

  • Anaphase I: Homologous pairs separate (independent assortment/segregation).

  • Telophase I/Cytokinesis: Two diploid daughter cells form.

• Meiosis II (similar to Mitosis):

  • Anaphase II: Sister chromatids separate.

  • Result: Four non-identical haploid ($n$) cells.

2.1.3 DNA Replication (Watson and Crick Model)

• Structure: Double helix; sugar-phosphate backbone; complementary base pairs ($A-T$, $C-G$) held by hydrogen bonds; antiparallel strands ($5'$ to $3'$ and $3'$ to $5'$).

• Enzymes and Steps:

  1. Initiation: Helicase unwinds DNA.

  2. Elongation: RNA primers signal starting points. DNA Polymerase adds nucleotides in the $5'$ to $3'$ direction.

     • Leading Strand: Continuous replication.

     • Lagging Strand: Discontinuous replication.

  3. Termination: Polymerase reaches the end. Strands recoil. Nuclease proofreads for errors.

• Continuity of Species:

  • DNA Replication: High-fidelity transfer of hereditary information.

  • Mitosis: Essential for multicellular growth, tissue repair, and asexual reproduction.

  • Meiosis: Introduces variation through crossing over, independent assortment, and random segregation; creates haploid gametes for sexual reproduction.

3.1 Genetic Material Storage: Eukaryotes vs. Prokaryotes

• Eukaryotes:

  • Location: Nucleus.

  • Structure: Linear DNA wound around histones; forms nucleosomes and supercoils (chromosomes).

  • Complexity: Larger genomes; many non-coding and repetitive sequences.

• Prokaryotes:

  • Location: Cytoplasm (nucleoid region).

  • Structure: Free-floating circular chromosomes; not bound by histones.

  • Additional DNA: Plasmids (extra-chromosomal; used for horizontal gene transfer).

3.2 Polypeptide Synthesis

3.2.1 Transcription (DNA to mRNA)

1. RNA Polymerase binds to the promoter sequence.

2. Small region of DNA unwinds.

3. Polymerase reads the template strand, matching complementary nucleotides (NTPs); Uracil ($U$) replaces Thymine ($T$).

4. mRNA chain is synthesised until a terminator sequence is reached.

5. Splicing: Introns are removed; exons are joined.

3.2.2 Translation (mRNA to Polypeptide)

1. mRNA docks with a ribosome.

2. Ribosome matches tRNA molecules to codons (sets of $3$ nucleotides) via anti-codons.

3. Adjacent amino acids form peptide bonds.

4. The polypeptide chain elongates until a stop codon is reached.

5. Chain folds into a mature protein.

• Complexity: Humans have approx. $20,067$ genes but $~1,000,000$ proteins due to mRNA rearrangement and post-translational modifications.

3.3 Protein Structure and Function

3.3.1 Structure

• Primary: Amino acid sequence.

• Secondary: Folds like $\alpha$-helices and $\beta$-sheets (hydrogen bonding).

• Tertiary: complex 3D folding due to ionic, covalent, and hydrophobic interactions between R-groups.

• Quaternary: Multiple polypeptide chains combining.

3.3.2 Function

• Structure: Cytoskeleton, hair, nails.

• Transport: Membrane proteins; storage (e.g., Ferritin stores iron).

• Enzymes: Biological catalysts for reactions.

• Antibodies: Immune response.

• Messengers: Hormonal signaling.

4.1 Predicting Genetic Variation

• Sources of Variation:

  • Meiosis: Crossing over creates new chromatid combinations; independent assortment.

  • Fertilisation: Random combination of parental alleles.

  • Mutation: Errors during DNA replication.

4.2.2 Types of Inheritance

• Autosomal: On chromosomes $1-22$. Traits passed equally to both sexes.

• Sex-linked: On X or Y chromosomes.

  • X-linked Recessive (e.g., Haemophilia): Males more frequently affected as they lack a second X to mask the gene.

• Co-dominance: Both alleles fully expressed (e.g., AB blood type).

• Incomplete Dominance: Blended phenotype (e.g., pink carnations from red/white cross).

• Multiple Alleles: Many available alleles for one trait (e.g., rabbit fur colour: black, chinchilla, Himalayan, albino).

4.2.9 Punnett Squares and Pedigrees

• Pedigree Rules: Circles (Female), Squares (Male); Shaded (Affected).

  • Dominant traits cannot skip generations.

  • Recessive traits can skip generations.

4.3 Genetic Data and Population Trends

• SNPs (Single Nucleotide Polymorphisms): Single nucleotide changes (substitution, insertion, deletion) at specific genome positions.

  • Account for >90\% of human differences.

  • Found once every $300$ nucleotides on average.

5.1 DNA Sequencing and Profiling

• DNA Profiling: Uses VNTRs (Variable Number Tandem Repeats).

  1. Digestion: DNA cut by restriction enzymes (e.g., EcoR1 at $GAATTC$ sequence).

  2. Separation: Gel electrophoresis separates fragments by size.

  3. Visualisation: Unique band patterns (fingerprints) compared.

5.2 Data Analysis and Population Genetics

• Conservation Management: Uses genetics to protect species (e.g., FAO report on Animal Genetic Resources).

• Inheritance of Disease:

  • Haplotype: Group of alleles inherited together.

  • Haplogroup: Haplotypes sharing a common ancestral SNP.

  • HapMap Project: Global map of genetic variation (SNPs).

• Human Evolution:

  • Mitochondrial Eve: Tracing maternal lines via mtDNA (approx. $200,000$ years ago).

  • Aboriginal Australians: mtDNA analysis identifies various ancient haplogroups (>40,000 years), supporting hypotheses of multiple entry points and long-term isolation.