Biology Module 5

Reproduction

Making a copy or likeness; producing offspring that resemble the parent(s). Necessary because individuals have a finite lifespan, so genetic material must be passed on for a population/species to survive.

Sexual reproduction

Involves two parents, mixing of genes, and offspring that are genetically unique (not identical to either parent).

Asexual reproduction

Involves only one parent; offspring are genetically identical clones of the parent (unless mutation occurs); produced by mitosis.

Gamete

A haploid sex cell (egg or sperm) produced by meiosis in the gonads (ovaries/testes).

Haploid (n)

A cell containing half the normal number of chromosomes (e.g. n = 23 in humans).

Diploid (2n)

A cell containing a full set of chromosomes (e.g. 2n = 46 in humans).

Zygote

The diploid cell formed when two haploid gametes fuse during fertilisation; divides by mitosis to form a new individual.

Fertilisation

The fusion of two haploid gametes (egg and sperm) to form a diploid zygote; in humans this is often called conception.

Advantages of sexual reproduction

Produces genetic variation, enabling adaptability/evolutionary potential in changing environments; facilitates selection of beneficial traits and elimination of unfavourable ones.

Disadvantages of sexual reproduction

Energy needed to produce gametes; finding a mate is time-consuming, costly and risky; competition for mates can be fatal; high parental investment shortens the parent's lifespan.

Advantages of asexual reproduction

Efficient/simple; no mate needed; rapid production of large numbers of offspring; uses mitosis (less demanding); works well when conditions are ideal and stable.

Disadvantages of asexual reproduction

No genetic variation, so the whole population responds to the environment the same way; can lead to overcrowding/competition; disease or a changing environment can wipe out the whole population.

Internal fertilisation

Union of male and female gametes inside the female's body after mating; occurs in mammals and birds; protects the embryo and increases survival rate of fewer offspring.

External fertilisation

Union of male and female gametes outside the body, usually in water; occurs in most aquatic animals and some amphibians; produces many eggs but low survival rate.

Spawning

The process in which eggs and sperm are released into the water at the same time and place, increasing genetic mixing and chance of fertilisation.

Broadcast spawning

Release of eggs and sperm into open water; the only fertilisation mechanism for sessile organisms like sponges; has a low survival rate due to predation, so millions of eggs are produced.

Oviparity

Fertilised eggs are laid outside the female's body and develop there, nourished by the egg's yolk.

Ovoviparity

Fertilised eggs are retained inside the female; the embryo is nourished by the egg's yolk; young are fully developed when hatched.

Viviparity

Young develop inside the female and are nourished via the mother's blood through a placenta; offspring are born alive.

Why internal fertilisation is advantageous

Protects the embryo from dehydration on land, limits predation, enhances fertilisation by a specific male; fewer offspring but higher survival rate than external fertilisation.

Binary fission

Asexual reproduction in which a single parent cell splits into two genetically identical daughter cells; used by bacteria (no nucleus involved) and most protists (has a nucleus that must replicate).

Budding

Asexual reproduction where an outgrowth forms from the parent, smaller than the parent, and eventually detaches (e.g. yeast, Hydra).

Fragmentation

Asexual reproduction where an organism splits into fragments, each developing into a mature individual identical to the parent (may be intentional or from damage).

Spore formation

Asexual (or sexual) reproduction where spores are produced; mitospores are formed by mitosis (asexual, genetically identical); meiospores are formed by meiosis (sexual).

Vegetative reproduction

Asexual reproduction in plants via specialised tissues (e.g. rhizomes, runners, suckers, tubers, bulbs) that grow into a new plant when separated from the parent.

Parthenogenesis

A form of asexual reproduction in some female animals where offspring develop from an unfertilised egg (no male needed).

Mitospores

Haploid asexual spores produced by mitosis; genetically identical to the parent fungus, usually produced in large numbers.

Meiospores

Spores produced sexually by meiosis in fungi.

How bacteria reproduce

Asexually via binary fission; faster than eukaryotic mitosis because bacteria lack a nucleus and organelles and have less DNA; can divide every ~20 minutes in ideal conditions.

How protists reproduce

Mostly asexual binary fission (splitting into two halves); differs from bacterial fission because protists have a membrane-bound nucleus that must be replicated; some protists also reproduce sexually.

How fungi reproduce

Asexually by spore formation (from fruiting bodies) or budding (e.g. yeast); spores can also be produced sexually (meiospores).

How plants reproduce

Sexually via seed cones (gymnosperms) or flowers (angiosperms), or via spores (mosses/ferns); asexually via vegetative propagation (rhizomes, runners, tubers, bulbs, fragmentation).

Pollination

The transfer of pollen from the anther (male) to the stigma (female) in flowering plants, or between male and female cones in conifers.

Fertilisation in flowering plants

Occurs inside the ovary when pollen (male gamete) is transferred to the ovum (female gamete).

Difference between pollination and fertilisation

Pollination is the transfer of pollen to the stigma/female cone; fertilisation is the fusion of male and female gametes that follows, occurring inside the ovary.

Menstrual cycle hormones

Luteinising hormone (LH), Follicle-stimulating hormone (FSH), Oestrogen, and Progesterone; average cycle length is 28 days.

Oestrogen (role)

Produced by the ovaries; regulates the ovarian cycle (maturation of ova) and the menstrual cycle (prepares the uterus for implantation).

Progesterone (role)

Produced by the ovaries/corpus luteum; prepares and maintains the uterine lining for implantation and pregnancy; levels fall if fertilisation doesn't occur, triggering menstruation.

Follicle stimulating hormone (FSH)

Stimulates the maturation of follicles in the ovaries.

Luteinising hormone (LH)

Promotes final maturation of the ovarian follicle, ovulation, and development of the corpus luteum.

Human chorionic gonadotropin (hCG)

The pregnancy hormone produced once the fertilised egg implants; keeps the corpus luteum active so oestrogen and progesterone continue, preventing the uterine lining from shedding.

Ovarian cycle

The cycle controlling the production and maturation of gametes (ova) in the ovaries, regulated by oestrogen and progesterone.

Menstruation

The shedding of the uterine lining (with bleeding) that occurs when fertilisation does not take place and oestrogen/progesterone levels fall.

Corpus luteum

Structure formed from the empty follicle after ovulation; secretes progesterone to maintain the uterine lining and pregnancy for the first 3 months.

Corpus albicans

A mass of fibrous tissue that the corpus luteum degenerates into (about 8-10 days after ovulation) if the egg is not fertilised, after which menstruation occurs.

Stages of implantation (in order)

Cleavage (zygote to morula) → Morula → Blastocyst → Gastrula.

Cleavage

Rapid mitotic cell division that begins after sperm penetrates the egg, producing many cells.

Morula

A solid ball of ~16 cells formed 3-4 days after fertilisation, which enters the uterus.

Blastocyst

Stage where cells begin to differentiate as the morula continues dividing; ready to attach to the uterine wall by day 8-9.

Gastrula

Formed after the blastocyst implants and undergoes gastrulation (~5 days); has three distinct cell layers and will develop into an embryo, then a fetus.

When pregnancy begins

When the embryo implants into the uterine wall.

Role of prostaglandins in birth

Secreted by the uterine wall; initiates labour.

Role of oxytocin in birth

Promotes coordinated contraction of the smooth muscle of the uterus during labour, resulting in birth.

Selective breeding

Humans choosing organisms with desirable characteristics and breeding them together to increase those traits in offspring, traditionally without advanced scientific/technological knowledge.

Hybridisation

Crossing different varieties within one species to combine desirable characteristics; hybrids are often more vigorous/higher-yielding but may be sterile (e.g. triticale, a wheat x rye hybrid).

Polyploidy

A condition where a cell nucleus has more than two sets of chromosomes (e.g. 3n, 4n, 6n); common in selectively bred plants; can restore fertility to a sterile hybrid (e.g. via colchicine treatment of triticale).

Heirloom plants

Traditional cultivated plant varieties maintained by small-scale growers rather than modern large-scale agriculture; preserve genetic variation and provide food security against pests, disease and climate change.

William Farrer

Pioneer of Australian wheat research who used cross-breeding (hybridisation) to improve bread wheat varieties.

DNA (function)

Deoxyribonucleic acid; stores the genetic information that controls cells and the whole organism; found mainly in the nucleus (also in mitochondria and chloroplasts).

Nucleotide (structure)

The monomer unit of nucleic acids, made up of a sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base.

DNA bases

Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

Base pairing rule

A pairs only with T; G pairs only with C (complementary base pairing).

A-T bond

Held together by a double hydrogen bond.

G-C bond

Held together by a triple hydrogen bond.

Watson-Crick DNA model

Describes DNA as a double helix held together by complementary base pairing (A-T, G-C) via weak hydrogen bonds, allowing the strands to separate for replication.

DNA replication (semi-conservative)

Each of the two original DNA strands acts as a template for a new complementary strand, producing two DNA molecules each with one old and one new strand.

Helicase

The enzyme that unwinds and unzips the DNA double helix, separating the paired bases for replication.

DNA polymerase

The enzyme that adds new complementary nucleotides to the template strand during DNA replication.

Leading strand

Replicated continuously in the 3' to 5' direction during DNA replication.

Lagging strand

Replicated discontinuously in short sections during DNA replication.

Okazaki fragments

Short lengths of DNA produced during discontinuous replication of the lagging strand.

Interphase

The phase of the cell cycle where the cell grows, replicates its DNA (S phase), produces proteins, and prepares for division; DNA replication occurs here.

Mitosis

Division of the nucleus into two genetically identical diploid daughter nuclei; used for growth, tissue repair/maintenance, and asexual reproduction.

Purpose of mitosis

Growth and development, maintenance and repair of tissues, asexual reproduction, and genetic stability.

Prophase (mitosis)

Chromosomes condense and become visible as two joined chromatids; the nuclear membrane breaks down.

Metaphase (mitosis)

Chromosomes line up along the centre (equator) of the cell; spindle fibres attach at the centromere.

Anaphase (mitosis)

Sister chromatids separate and are pulled to opposite poles of the cell by spindle fibres.

Telophase (mitosis)

Spindle fibres disappear; a new nuclear membrane forms around each set of chromosomes.

Cytokinesis

Division of the cytoplasm, producing two genetically identical diploid daughter cells.

Chromatid

One of two identical copies of a chromosome formed by DNA replication, joined at the centromere.

Centromere

The region of DNA that joins two sister chromatids together and where spindle fibres attach.

Meiosis

A type of cell division that produces four genetically unique, haploid daughter cells (gametes); occurs only in eukaryotes, for sexual reproduction; involves two rounds of division (Meiosis I and II).

Meiosis I vs Meiosis II

Meiosis I separates homologous chromosome pairs (reducing chromosome number to haploid); Meiosis II separates sister chromatids (like mitosis), producing four haploid daughter cells total.

Homologous chromosomes

A matching pair of chromosomes, one inherited from each parent, that carry genes for the same traits at the same loci.

Crossing over

The exchange of genetic segments between chromatids of homologous chromosomes during Prophase I of meiosis, producing new combinations of genes (recombination) and increasing genetic variation.

Chiasma

The point where crossing over occurs between homologous chromatids.

Law of Independent Assortment

States that the inheritance of one pair of alleles/genes is independent of the inheritance of another pair, because homologous chromosome pairs align randomly at the metaphase plate during meiosis.

Law of Segregation

States that every individual carries two alleles for each trait, and these two alleles separate (segregate) during gamete formation so only one is passed to each gamete.

Nondisjunction

Failure of chromatids/homologous chromosomes to separate properly during meiosis, resulting in gametes/offspring with an incorrect number of chromosomes (e.g. 45 or 47).

Turner Syndrome

A condition caused by nondisjunction where an individual has only one X chromosome (genotype X); features include short stature, webbed neck, and non-functioning ovaries (gonadal dysfunction, sterility).

XYY Syndrome

A condition where a male receives an extra Y chromosome (47,XYY karyotype); occurs in about 1 in 1000 male births; usually has a normal clinical phenotype.

Mitosis vs Meiosis - key differences

Mitosis: 1 division, 2 diploid genetically identical daughter cells, used for growth/repair/asexual reproduction. Meiosis: 2 divisions, 4 haploid genetically unique daughter cells (gametes), used for sexual reproduction and creates variation via crossing over and independent assortment.

Genotype

The alleles that constitute a gene for an organism (its genetic makeup).

Phenotype

The physical/observable expression of the genotype.

Gene

A sequence of DNA located on a chromosome that codes for a phenotype; the functional unit of DNA.

Allele

An alternative form or version of a gene.

Heterozygous

Having two different alleles for a gene.

Homozygous

Having two identical alleles for a gene.

Locus

The specific position of a gene on a chromosome.

Punnett square

A diagram showing the possible allele combinations (and their ratios) resulting from a genetic cross between two parents.