HSC Biology Study Notes: Reproduction and Genetics

Reproduction: Process of producing offspring to ensure species continuity.
Sexual Reproduction: Involves two parents from the same species; each provides half the genetic material through sex cells (sperm/egg). Increases genetic diversity but consumes time and energy.
Asexual Reproduction: Involves one parent; offspring are genetically identical clones. Efficient for fast population growth but results in low genetic diversity.
Animal Fertilisation: - External: Egg fertilised outside the body (e.g., fish and amphibians). Produces high offspring numbers but low survival rates due to environmental factors. - Internal: Egg fertilised inside the female (e.g., mammals, reptiles, birds). Higher survival chances and resource conservation; requires physical mating and produces fewer offspring.
Plant Reproduction: - Asexual: Mechanisms include runners, bulbs, and cuttings (vegetative propagation). - Sexual: Pollination involving the transfer of pollen (male) to ovules (female).
Other Organisms: - Fungi: Asexual via budding or spores; sexual via fusion of spores from different fungi. - Bacteria/Protists: Reproduce asexually through Binary Fission, involving DNA replication, cell growth, and cytoplasm division.

Fertilisation: Fusion of egg and sperm in the oviduct to form a diploid zygote.
Implantation: Attachment of the fertilised egg to the uterine lining. Embryotic development supported by the Amniotic sac (cushioning), Placenta (nutrition/waste), and Umbilical cord.
Hormonal Control: - Oestrogen: Produced by placenta; stimulates ovulation and organ development. - Progesterone: Produced by ovaries/placenta; thickens uterine lining and relaxes the uterus. - Relaxin: Loosens uterine muscles. - Oxytocin: Triggers uterine contractions and milk production.

Mitosis: Division into $2$ identical diploid daughter cells ( $6$ chromosomes). Necessary for growth, repair, and tissue regeneration.
Meiosis: Two-stage division into $4$ non-identical haploid gametes ( $3$ chromosomes). Introduces variation through Crossing Over and Random Assortment.
DNA Structure: Nucleic acid made of deoxyribonucleotides (sugar, phosphate, and nitrogenous bases: $G, C, A, T$ ). Forms a double helix.
RNA: Single-stranded nucleic acid; contains ribose sugar and replaces Thymine with Uracil ( $U$ ).
DNA Replication: Semi-conservative process using DNA helicase to unzip strands and DNA polymerase to bind complementary bases ( $G \rightarrow C$ , $A \rightarrow T$ ).
Prokaryotes vs. Eukaryotes: - Prokaryotes: Small, circular DNA floating in cytoplasm. - Eukaryotes: Large, linear DNA found within a nucleus.

Transcription: mRNA copy of a gene is made in the nucleus. RNA polymerase pairs free nucleotides to the template DNA strand.
Translation: mRNA moves to a ribosome; tRNA molecules with specific anticodons deliver amino acids to form a polypeptide chain.
Genotype vs. Phenotype: Genotype is the genetic makeup (alleles inherited from parents); phenotype is the physical expression influenced by genotype and environment.
Sources of Variation: Fertilisation, mutations, crossing over, and independent assortment. - Example: Drosophila melanogaster experiment showed specific traits (red eyes/straight wings vs. pink eyes/curled wings) linked on chromosomes except for $2\%$ crossover events. - Example: Sickle Cell Anaemia is an autosomal recessive disorder caused by a single amino acid replacement (glutamic acid with valine), affecting $1$ in $500$ African Americans.

Autosomal Inheritance: Traits on non-sex chromosomes; includes dominant/recessive patterns.
Sex-Linked Inheritance: Traits on $X$ or $Y$ chromosomes. $X$ -linked recessive traits affect males more frequently.
Co-Dominance: Both alleles expressed (e.g., Roan cattle: $RW$ ).
Incomplete Dominance: Dominant allele partially masks recessive, creating a blend (e.g., pink Snapdragons: $Rr$ ).
Hardy-Weinberg Equation: Used to calculate genotype frequencies: $p^2 + 2pq + q^2 = 1$ .
SNPs (Single Nucleotide Polymorphism): Variations in a single nucleotide at specific genomic locations used to identify trends.

DNA Sequencing: Determining the exact nucleotide order (e.g., Sanger method using Gel Electrophoresis).
DNA Profiling: Analysing variations (like STRs - Short Tandem Repeats) for identification or parentage testing.
Conservation Management: Using genetics to monitor extinction risk (e.g., Galapagos Finch) and inbreeding.
Human Evolution: Comparing DNA to determine relationships; humans share $99\%$ DNA with chimpanzees and $98\%$ with gorillas.
Disease Studies: GWAS (Genome Wide Association Studies) compare SNPs between healthy and diseased individuals to identify genetic associations.

HSC Question (2022): How does reproduction ensure the continuity of a species?
Response: Sexual reproduction produces unique offspring through the fusion of male and female gametes to form a zygote, splitting characteristics based on dominant and recessive traits. Asexual reproduction, such as budding and binary fission, ensures continuity by producing genetically identical offspring, though spores can occasionally meet to form unique combinations.