Biology Module 5 – Heredity & Reproduction Review

Question-and-Answer flashcards covering key concepts in reproduction, cell division, DNA replication, polypeptide synthesis, genetics, and agricultural manipulation, designed for exam revision.

What is the primary biological purpose of reproduction?

To produce offspring, ensuring the continuous transfer of genetic information across generations and the perpetuation of the species’ lineage.

Define sexual reproduction.

A biological process involving two parent organisms, each contributing genetic material (gametes) that fuse during fertilisation to create offspring genetically distinct from either parent. This process typically leads to increased genetic variation within a population.

Define asexual reproduction.

A form of reproduction involving a single parent organism that produces genetically identical offspring (clones) without the fusion of gametes. This process relies on mitotic cell division.

What is fertilisation?

The crucial event in sexual reproduction where a haploid male gamete (e.g., sperm, pollen) fuses with a haploid female gamete (e.g., egg, ovule) to form a diploid zygote, which then develops into a new organism.

Where does external fertilisation occur?

It occurs outside the body of the female parent, typically in an aquatic environment (e.g., water), where both eggs and sperm are released into the surroundings for fusion.

Give two animal groups that use external fertilisation.

Fish and amphibians (e.g., frogs, salamanders) are two animal groups that commonly exhibit external fertilisation.

State one major advantage of external fertilisation.

A major advantage is the production of a large number of offspring, which increases the likelihood that some will survive to maturity and contributes to greater genetic diversity through random gamete fusion in a less confined space.

State one major disadvantage of external fertilisation.

A significant disadvantage is the low fertilisation success rate due to factors such as predation on dispersed gametes and embryos, environmental hazards (e.g., desiccation, temperature fluctuations, dilution of gametes), and lack of parental care.

Where does internal fertilisation occur?

Internal fertilisation occurs safely within the reproductive tract of the female parent, providing a protected environment for gamete fusion and early embryonic development.

Name two groups that use internal fertilisation.

Mammals (e.g., humans, dogs) and reptiles (e.g., snakes, crocodiles) are two animal groups that primarily use internal fertilisation.

Give one advantage of internal fertilisation.

One advantage is the increased protection of embryos from external dehydration and predation, as development occurs within the mother's body, providing a stable and secure environment.

Give one disadvantage of internal fertilisation.

A main disadvantage is the greater energy and time investment required by parents to locate compatible mates and perform courtship rituals, which can be metabolically costly and increase exposure to predators.

Which type of fertilisation typically involves copulation?

Internal fertilisation typically involves copulation, which is the physical act of transferring sperm from the male reproductive tract to the female reproductive tract.

Which fertilisation method generally has a higher chance of gamete union?

Internal fertilisation generally has a higher chance of gamete union because the gametes are released and fused within a confined, protected space, increasing their proximity and reducing external risks.

List the male reproductive structures in flowering plants.

The male reproductive structures in flowering plants, collectively called the stamen, consist of:

• The anther: The part that produces and contains pollen (male gametes).
• The filament: A stalk that supports the anther, positioning it to release pollen.

List the female reproductive structures in flowering plants.

The female reproductive structures in flowering plants, collectively called the pistil (or carpel), consist of:

• The stigma: The receptive tip that collects pollen.
• The style: The stalk that connects the stigma to the ovary.
• The ovary: The enlarged basal part containing ovules.
• The ovule: Contains the female gamete (egg cell) and, after fertilisation, develops into a seed.

What is pollination?

Pollination is the vital process of transferring pollen grains (which contain the male gametes) from the anther (male reproductive part) of a flower to the stigma (female receptive part) of the same or another flower, enabling fertilisation.

Name two common pollination agents.

Two common pollination agents are wind (e.g., in grasses and conifer trees) and animals (e.g., insects like bees, butterflies; birds; and sometimes bats), which transfer pollen physically.

Describe fertilisation in plants.

In plants, fertilisation occurs after pollination, when a pollen grain lands on the stigma and germinates, forming a pollen tube that grows down the style to the ovule. A sperm nucleus from the pollen then fuses with an egg cell inside the ovule to form a zygote, which develops into a seed. In flowering plants, a second sperm nucleus fuses with central cell nuclei to form the endosperm, providing nourishment to the embryo (known as double fertilisation).

Define a plant runner.

A plant runner (or stolon) is a specialised, horizontal stem that grows along the ground surface, producing roots and new, genetically identical plants (plantlets) at its nodes. Strawberries are a classic example of plants that reproduce using runners.

What is a tuber?

A tuber is a swollen, fleshy, underground stem or root (e.g., potato) that serves as a storage organ for food (starch) and possesses 'eyes' or buds from which new, genetically identical plants can sprout, facilitating asexual reproduction.

Give an example of a bulb.

An onion is a classic example of a bulb. A bulb is an underground storage structure consisting of a short stem base with fleshy leaves (scales) that store food and a central bud from which a new plant can grow asexually.

What is budding in plants?

Budding in plants refers to a form of asexual reproduction where new, genetically identical plantlets develop from small outgrowths or buds on the parent plant's stem, leaves, or roots, eventually detaching to form independent organisms (e.g., Kalanchoe).

Explain binary fission in bacteria.

Binary fission in bacteria is a simple, highly efficient asexual reproduction method where a single circular prokaryotic cell grows in size, replicates its single chromosome, and then divides into two genetically identical daughter cells through inward growth of the cell membrane and cell wall.

Why is binary fission not considered mitosis?

Binary fission is not considered mitosis because mitosis is a complex process specific to eukaryotic cells involving the division of a membrane-bound nucleus and its multiple linear chromosomes, while binary fission is a simpler process occurring in prokaryotic cells (bacteria and archaea) that lack a nucleus.

Describe binary fission in protists.

In protists (unicellular eukaryotes), binary fission involves the division of the nucleus (often similar to mitosis in its own way) followed by the division of the cytoplasm (cytokinesis) to form two identical daughter organisms, each with a complete set of organelles and genetic material.

What is budding in protists?

Budding in protists, exemplified by yeast (a unicellular fungus, often studied with protists), involves the formation of a small outgrowth or bud on the parent cell. This bud receives a copy of the parent's nucleus before it eventually detaches, growing into a new, genetically identical individual that is initially smaller than the parent.

List one advantage of sexual reproduction.

One significant advantage of sexual reproduction is that it dramatically increases genetic variation within a population through independent assortment of chromosomes, crossing over, and the random fusion of unique gametes. This variation enhances the population's adaptability and resilience to changing environmental conditions, disease, and selection pressures.

List one disadvantage of sexual reproduction.

One major disadvantage of sexual reproduction is that it requires considerable time and energy investment to find a mate, engage in courtship rituals, and establish reproductive opportunities. This process can expose individuals to predators and may not always guarantee successful reproduction, making it less efficient in stable environments.

List one advantage of asexual reproduction.

One key advantage of asexual reproduction is the ability for rapid population growth, as only a single parent is needed to produce multiple offspring quickly. This efficiency is particularly beneficial in stable or favourable environments where resources are abundant, allowing for rapid colonisation.

List one disadvantage of asexual reproduction.

A significant disadvantage of asexual reproduction is the low genetic variation among offspring, as they are genetically identical to the parent. This lack of diversity can make entire populations highly vulnerable and less adaptable to sudden environmental changes, new diseases, or drastic shifts in selective pressures, potentially leading to mass extinctions.

What is implantation?

Implantation is the critical process in early human development where the blastocyst, a hollow ball of cells formed about five to seven days after fertilisation, firmly attaches and embeds itself into the endometrium (the inner lining) of the uterus, marking the official beginning of pregnancy.

Name the three layers a sperm penetrates during human fertilisation.

During human fertilisation, a sperm must successfully penetrate three protective layers surrounding the egg: the outermost corona radiata (follicle cells), the inner, thick glycoprotein layer called the zona pellucida, and finally the egg plasma membrane.

What reaction prevents polyspermy in mammals?

The cortical reaction is the crucial mechanism that prevents polyspermy (fertilisation by multiple sperm) in mammals. Upon the entry of a single sperm, the egg releases cortical granules that alter the zona pellucida, causing it to harden and become impenetrable to other sperm.

Define blastocyst.

A blastocyst is a hollow ball of cells, typically formed around five days after fertilisation, comprising an outer layer of cells called the trophoblast (which will form the placenta) and an inner cell mass (which will develop into the embryo). It's the stage that implants into the uterine lining.

What structure connects embryo and placenta?

The umbilical cord is the vital structure that connects the developing embryo (and later fetus) to the placenta. It contains blood vessels that facilitate the exchange of nutrients, oxygen, and waste products between the mother and the fetus.

Which hormone is detected in pregnancy tests?

The hormone detected in most commercial pregnancy tests is human chorionic gonadotropin (hCG). This hormone is produced by the trophoblast cells of the blastocyst (and later the placenta) shortly after implantation, and its presence indicates pregnancy.

State one function of progesterone during pregnancy.

One critical function of progesterone during pregnancy is to maintain the uterine lining (endometrium), making it thick and vascular to support the implanted embryo. It also inhibits uterine contractions, preventing premature labour and miscarriage.

What is the role of relaxin?

The role of relaxin is to prepare the female's body for childbirth. It specifically relaxes the pelvic ligaments and softens the cervix, making it more flexible and dilatable to facilitate the passage of the baby during delivery.

Which pituitary hormone stimulates milk production?

The pituitary hormone that primarily stimulates milk production (lactogenesis) in the mammary glands after childbirth is prolactin. It also plays a role in sexual gratification and various metabolic processes.

Name the four key menstrual hormones.

The four key menstrual hormones, produced by the pituitary gland and ovaries, are:

• FSH (Follicle-Stimulating Hormone): Stimulates follicle growth in the ovary.
• LH (Luteinising Hormone): Triggers ovulation and corpus luteum formation.
• Oestradiol (Oestrogen): Promotes uterine lining growth and secondary sexual characteristics.
• Progesterone: Maintains uterine lining and inhibits LH/FSH after ovulation.

What agricultural technique inserts sperm directly into the female tract?

The agricultural technique that involves inserting semen (containing sperm) directly into the female reproductive tract is called artificial insemination. It is widely used to selectively breed livestock and improve genetic traits without the need for natural mating.

Give one benefit of artificial pollination.

One significant benefit of artificial pollination is that it ensures fertilisation and seed production even when natural pollinators are absent, scarce, or unreliable. It also allows for precise control over breeding to enhance specific desired traits in crops.

What modern tool enables precise gene editing and cloning?

CRISPR technology (Clustered Regularly Interspaced Short Palindromic Repeats) is a revolutionary modern molecular tool that enables highly precise and efficient gene editing by targeting specific DNA sequences. It also holds potential for advanced cloning applications by manipulating genes with unprecedented accuracy.

State one risk of cloning or GM techniques in agriculture.

One significant risk associated with cloning or genetically modified (GM) techniques in agriculture is the potential loss of biodiversity. Over-reliance on a few genetically identical or modified strains can reduce the genetic robustness of a species, making entire populations susceptible to new diseases or environmental changes, and may lead to unforeseen ecological or environmental effects.

Why is exact DNA replication crucial?

Exact DNA replication is crucial because it ensures faithful transmission of genetic information from parent cells to daughter cells and from one generation to the next. This precision prevents accumulation of mutations that could lead to cell malfunction, genetic diseases, developmental abnormalities, and ultimately compromise the continuity and survival of a species.

What type of cells divide by mitosis?

Mitosis is the type of cell division undergone by somatic (body) cells for growth, repair, and asexual reproduction. Examples include skin cells, muscle cells, and epithelial cells.

How many daughter cells result from mitosis?

Mitosis typically results in two genetically identical diploid (2n) daughter cells. Each daughter cell contains the same number and type of chromosomes as the original parent cell.

Name the six main stages of mitosis in order.

The six main stages of mitosis, in order, are:

1. Interphase: Cell grows, copies its DNA and prepares for division (G1, S, G2 phases).
2. Prophase: Chromosomes condense, spindle fibres begin to form, nuclear envelope breaks down.
3. Metaphase: Chromosomes align at the metaphase plate (equator of the cell).
4. Anaphase: Sister chromatids separate and move to opposite poles of the cell.
5. Telophase: Chromosomes decondense, nuclear envelopes reform around two sets of chromosomes.
6. Cytokinesis: The cytoplasm divides, resulting in two distinct daughter cells.

During which mitotic stage do chromosomes align at the equator?

During the mitotic stage of metaphase, the condensed chromosomes align precisely along the central plane of the cell, known as the metaphase plate, ensuring proper separation into daughter cells.

What happens in anaphase of mitosis?

In anaphase of mitosis, the sister chromatids (identical copies of a chromosome attached at the centromere) separate from each other. They are then pulled apart by the shortening spindle fibres towards opposite poles of the cell, becoming individual chromosomes.

How many divisions occur in meiosis?

Meiosis involves two sequential rounds of cell division: Meiosis I (a reductional division, where homologous chromosomes separate) and Meiosis II (an equational division, where sister chromatids separate, similar to mitosis).

What is the ploidy of gametes produced by meiosis?

Gametes produced by meiosis have a haploid (n) ploidy. This means they contain half the number of chromosomes of the parent diploid cell (2n), ensuring that when two gametes fuse during fertilisation, the resulting zygote restores the diploid chromosome number.

Define crossing over.

Crossing over (or recombination) is a genetic process occurring during prophase I of meiosis. It involves the physical exchange of genetic material (DNA segments) between non-sister chromatids of homologous chromosomes, leading to new combinations of alleles on the chromosomes and increasing genetic diversity.

Why does meiosis promote genetic diversity?

Meiosis promotes genetic diversity through two primary mechanisms: independent assortment of homologous chromosomes during Meiosis I (random alignment at the metaphase plate) and crossing over (recombination) between homologous chromosomes during prophase I. Both processes create new combinations of alleles in the resulting gametes.

What are gametes often called ‘vehicles of inheritance’?

Gametes are often called 'vehicles of inheritance' because they literally carry and transmit the genetic material (chromosomes containing genes) from one biological generation to the next. They are the means by which hereditary traits are passed down from parents to offspring.

Which phase precedes both mitosis and meiosis?

The phase that precedes both mitosis and meiosis is Interphase. During interphase, the cell grows, replicates its DNA (S phase), and synthesises proteins, preparing for cell division. Interphase consists of three sub-phases: G1 (growth), S (DNA synthesis), and G2 (further growth and preparation for division).

Name the enzyme that unwinds DNA during replication.

Helicase is the enzyme primarily responsible for unwinding the double-stranded DNA helix during replication. It breaks the hydrogen bonds between complementary base pairs, separating the two strands to create a replication fork.

What is meant by semi-conservative replication?

Semi-conservative replication refers to the process where each new DNA molecule formed after replication consists of one original (template) strand from the parent DNA molecule and one newly synthesised strand. Thus, the original duplex is conserved in each newly formed DNA molecule.

State the complementary base pairings in DNA.

The complementary base pairings in DNA, according to Chargaff's rules, are: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds, and Cytosine (C) always pairs with Guanine (G) via three hydrogen bonds.

Which enzyme synthesises new DNA strands?

The primary enzyme that synthesises new DNA strands by adding nucleotides complementary to the template strand is DNA polymerase III in prokaryotes, and DNA polymerase \delta (delta) and DNA polymerase \epsilon (epsilon) in eukaryotes. These enzymes always synthesise DNA in the 5'\rightarrow3' direction.

What are Okazaki fragments?

Okazaki fragments are short, newly synthesised DNA segments that are formed discontinuously on the lagging strand during DNA replication. Because DNA polymerase can only synthesise DNA in the 5'\rightarrow3' direction, and the lagging strand template runs in the 3'\rightarrow5' direction, synthesis must occur in fragments moving away from the replication fork.

Which enzyme joins Okazaki fragments?

DNA ligase is the enzyme responsible for joining the Okazaki fragments on the lagging strand. It catalyses the formation of phosphodiester bonds between the 3'-hydroxyl end of one fragment and the 5'-phosphate end of the adjacent fragment, creating a continuous DNA strand.

What primer is required to start DNA synthesis?

A short RNA primer, synthesised by an enzyme called primase, is required to initiate DNA synthesis. DNA polymerases cannot start a new DNA strand from scratch; they can only add nucleotides to an existing 3'-hydroxyl (3'\text{-OH}) group, which the RNA primer provides.

Name one enzyme that removes RNA primers.

One enzyme that removes RNA primers during DNA replication is DNA polymerase I in prokaryotes, which has 5'\rightarrow3' exonuclease activity. In eukaryotes, RNase H removes the RNA primer, and then DNA polymerase fills in the gap.

What is topoisomerase's role in replication?

Topoisomerase's crucial role in DNA replication is to relieve the supercoiling and torsional stress that builds up in the DNA helix ahead of the replication fork as helicase unwinds it. It does this by cutting, unwinding, and rejoining the DNA strands, preventing tangles and allowing replication to proceed smoothly.

Where is prokaryotic DNA located?

In prokaryotes, the main genetic material (a single circular chromosome) is located in the nucleoid region of the cytoplasm. Unlike eukaryotes, prokaryotes do not have a membrane-bound nucleus.

Do prokaryotes possess histones?

No; prokaryotes do not possess histones (the proteins around which eukaryotic DNA is wrapped). Instead, their circular DNA is supercoiled and organised by different DNA-binding proteins, though these are not homologous to histones.

Describe the structure of eukaryotic chromosomes.

Eukaryotic chromosomes are complex structures composed of multiple linear DNA molecules tightly coiled and folded around histone proteins, forming fundamental units called nucleosomes. This intricate packaging allows the long DNA molecules to fit within the nucleus and plays a role in gene regulation.

What is an operon?

An operon is a functional unit of prokaryotic DNA containing a cluster of genes that are involved in the same metabolic pathway and are transcribed together under the control of a single promoter. It typically includes a promoter, an operator (where a repressor binds), and structural genes.

Define transcription.

Transcription is the initial process in gene expression where a specific segment of DNA (a gene) serves as a template for the synthesis of a complementary messenger RNA (mRNA) molecule by the enzyme RNA polymerase, effectively converting genetic information from DNA to RNA.

Where does transcription occur in eukaryotes?

In eukaryotes, transcription, the synthesis of mRNA from a DNA template, occurs exclusively within the nucleus, where the DNA is located. The newly synthesised pre-mRNA then undergoes processing before being exported to the cytoplasm for translation.

What modification is added to the 5′ end of eukaryotic pre-mRNA?

A 5′ cap (a modified guanine nucleotide, 7-methylguanosine) is added to the 5′ end of eukaryotic pre-mRNA immediately after transcription. This cap is crucial for mRNA stability, protection from degradation, and efficient binding to the ribosome for translation.

What removes introns from pre-mRNA?

The spliceosome, a complex molecular machine composed of small nuclear ribonucleoproteins (snRNPs) and other proteins, is responsible for removing non-coding regions called introns from eukaryotic pre-mRNA and ligating the coding exon sequences together, creating mature mRNA ready for translation.

Define translation.

Translation is the biological process by which ribosomes read the sequence of codons on a messenger RNA (mRNA) molecule and, guided by transfer RNA (tRNA) molecules carrying specific amino acids, assemble these amino acids into a polypeptide chain, thereby synthesising proteins.

What molecule carries amino acids to the ribosome?

tRNA (transfer RNA) molecules are the adapters, each carrying a specific amino acid to the ribosome. They have an anticodon that base-pairs with a complementary codon on the mRNA, ensuring the correct amino acid is incorporated into the growing polypeptide chain.

What is the start codon and which amino acid does it specify?

The start codon, which initiates translation, is AUG (Adenine-Uracil-Guanine). It universally codes for the amino acid methionine (or fMet in prokaryotes), signifying the beginning of protein synthesis.

What signals the end of translation?

The end of translation is signalled by the presence of a stop codon (UAA, UAG, or UGA) on the mRNA molecule. These codons do not code for any amino acid but are recognised by release factors, which prompt the ribosome to terminate polypeptide synthesis and release the newly formed protein.

Why is polypeptide synthesis essential?

Polypeptide synthesis is essential because it produces proteins, which are the workhorses of the cell. Proteins perform a vast array of crucial functions, including providing structural support, acting as enzymes (catalysing biochemical reactions), functioning as hormones (signalling molecules), transporting substances, and regulating gene expression, vital for the cell's function, growth, and survival.

Explain genotype.

Genotype refers to the specific genetic makeup of an organism, representing the complete set of alleles (different forms of a gene) an individual possesses for a particular trait or all traits. It describes whether an individual is homozygous (two identical alleles) or heterozygous (two different alleles) for a gene.

Explain phenotype.

Phenotype refers to the observable physical or biochemical characteristics of an organism, which are the expression of its genotype. It is the result of the interaction between an individual's genetic makeup and environmental influences that can modify gene expression.

What is codominance?

Codominance is a genetic phenomenon where both alleles for a gene in a heterozygote are fully and equally expressed in the phenotype, resulting in characteristics of both alleles being visible. A classic example is the AB blood type in humans, where both A and B antigens are present on red blood cells.

Define incomplete dominance.

Incomplete dominance is a type of inheritance where the heterozygous phenotype is intermediate between the two homozygous phenotypes. Neither allele is completely dominant over the other, resulting in a blended trait. For example, if a red flower (homozygous dominant) is crossed with a white flower (homozygous recessive), the offspring may have pink flowers (heterozygous).

What is a multiple-allele system?

A multiple-allele system describes a gene that has more than two possible alleles (variants) available in a population for a given trait, even though any individual organism can typically only possess two alleles for that gene. The ABO blood group system in humans, with A, B, and O alleles, is a prime example.

Differentiate autosomal and sex-linked inheritance.

The key difference is chromosome location: Autosomal inheritance describes genes located on any of the non-sex chromosomes (autosomes), where traits are typically expressed equally in males and females. Sex-linked inheritance specifically refers to genes located on the sex chromosomes (X or Y), often resulting in different patterns of inheritance and expression between the sexes, as seen with X-linked traits like colour blindness, more common in males.

What tool predicts offspring genotypes from parental alleles?

A Punnett square is a simple diagrammatic tool used in genetics to predict the possible genotypes and phenotypes of offspring resulting from a genetic cross between two parents, based on their known parental alleles. It illustrates all possible combinations of alleles from each parent.

What diagram traces traits through generations?

A pedigree chart is a visual diagram that traces the inheritance pattern of a specific genetic trait or disease through multiple generations within a family. It uses standardised symbols to represent individuals' sex, relationships, and phenotypic expression of the trait, aiding in genetic counselling and disease risk assessment.

Define SNP (single nucleotide polymorphism).

A SNP (Single Nucleotide Polymorphism) is a variation that occurs at a single nucleotide position in the DNA sequence among individuals. It is considered a SNP if it is present in at least 1% of the population, serving as a common type of genetic variation often used as a marker in genetic studies and disease association.

What is the founder effect?

The founder effect is a special type of genetic drift that occurs when a new population is established by a very small number of individuals (founders) who migrate from a larger source population. This small founding group may not carry the full genetic diversity of the original population, leading to a significant loss of genetic variation and an altered allele frequency in the new population, often with an increased prevalence of rare alleles.

Name one environmental factor that can influence phenotypic expression.

Environmental factors that can influence phenotypic expression include:

• Temperature: Affects fur colour in some animals (e.g., Himalayan rabbits).
• Nutrition/Diet: Impacts height, weight, and overall health.
• Light: Influences plant growth and pigment production.
• Chemicals/Toxins: Can induce mutations or alter gene expression.
• Exercise: Affects muscle development and bone density.
• Stress: Can influence behavioural and physiological traits.

Why can identical genotypes produce different phenotypes?

Identical genotypes (e.g., in identical twins) can produce different phenotypes because environmental influences (such as diet, lifestyle, exposure to toxins, and social interactions) can interact with genes. These environmental factors can alter gene expression (epigenetics) and protein function without changing the underlying DNA sequence, leading to observable differences in traits.

What are granulosa cells?

Granulosa cells are a type of somatic cell that surround the oocyte within an ovarian follicle. They play a crucial role in follicle development, providing nutrients to the oocyte, and producing hormones, particularly oestrogens, vital for the menstrual cycle and pregnancy.

Describe the zona pellucida.

The zona pellucida is a thick, transparent, extracellular glycoprotein layer that completely surrounds the plasma membrane of the mammalian oocyte. It plays a critical role in species-specific sperm binding, prevention of polyspermy, and protecting the early embryo before implantation.

What is the acrosome reaction?

The acrosome reaction is an essential event in fertilisation where, upon contact with the zona pellucida, the sperm's acrosome (a cap-like organelle in the sperm head) releases hydrolytic enzymes (e.g., hyaluronidase, acrosin). These enzymes digest components of the zona pellucida, allowing the sperm to penetrate and reach the egg's plasma membrane.

Which cell division process maintains chromosome number across genera

 ­tions?

Meiosis is the cell division process that maintains chromosome number across generations. It produces haploid gametes (sperm and egg cells) containing half the chromosome number of somatic cells. When two haploid gametes fuse during fertilisation, the original diploid chromosome number is restored in the zygote, ensuring continuity of species-specific chromosome count.

Give one role of mitosis in multicellular organisms.

Mitosis plays several vital roles in multicellular organisms, including:

• Growth: Increases the number of cells in an organism, allowing for development from a zygote.
• Repair: Replaces damaged or dead cells, such as in wound healing.
• Tissue Regeneration: Produces new cells to maintain and rejuvenate tissues (e.g., skin, blood cells).

How does mitosis contribute to species continuity?

Mitosis contributes to species continuity by producing genetically identical daughter cells that are essential for the growth, development, and repair of multicellular organisms. By accurately replicating and distributing genetic information, mitosis ensures the stability of the genetic blueprint within an individual, sustaining its life and ability to reproduce.

How does meiosis contribute to species continuity?

Meiosis contributes to species continuity by generating genetically diverse haploid gametes (sperm and egg). This diversity provides the raw material for evolution and adaptability, while the precise reduction of chromosome number ensures that fertilisation restores the stable diploid chromosome complement in the next generation, preventing polyploidy.

What determines a cell’s progression through the cell cycle?

A cell's progression through the cell cycle is tightly controlled by a complex regulatory system involving internal and external signals. Key regulators include cyclin-dependent kinases (Cdks), whose activity is regulated by cyclins, and various checkpoint controls (G1, G2, M) that monitor cell size, DNA integrity, and chromosome alignment before allowing progression to the next phase.

Why might a cell pause the cycle after DNA replication?

A cell might pause the cell cycle in the G2 phase (after DNA replication in S phase and before mitosis) in order to repair any detected DNA damage. This checkpoint ensures that only cells with intact and accurately replicated DNA proceed to division, preventing the transmission of mutations to daughter cells.