Difficulty: Introductory | Prerequisites: Basic understanding of DNA structure and cell biology fundamentals.
This topic covers how cells reproduce and how genetic information is packaged, passed on, and maintained across generations. It sits at the heart of introductory biology: everything from growth and tissue repair to sexual reproduction depends on the cell cycle and chromosome behaviour. You should already be comfortable with the basics of DNA as the molecule of heredity. If you know that DNA is organised into chromosomes and that cells divide, you are ready for this material.
Your body's cells carry 46 chromosomes arranged in 23 homologous pairs, one set from each parent. Somatic (body) cells are made by mitosis and keep the full diploid set; gametes (sex cells) are made by meiosis and carry only a haploid set of 23. Fertilisation restores the diploid number and combines genetic material from two parents, which is the basis of heredity and variation.
Genome
The full set of DNA present in an individual organism (or species). In simple terms, it is the complete instruction manual for building and running a living thing.
Chromosome
A single molecule of DNA packaged with proteins. One chromosome = one molecule of DNA. Think of each chromosome as one chapter of that instruction manual.
Karyotype
An ordered display of the chromosome pairs from a single cell, arranged by size and banding pattern. In simple terms, it is a photograph of all your chromosomes lined up for inspection.
Homologous chromosomes (homologs)
A matched pair of chromosomes, one inherited from each parent, that share the same gene sequence and structure but carry different allele versions. Think of them as two editions of the same book: same chapter order, but some of the words differ.
Somatic cell
Any body cell that is not a gamete. Somatic cells are diploid (2n) and are produced by mitosis. Examples include muscle cells, nerve cells, and stomach cells.
Gamete
A sex cell (sperm or egg). Gametes are haploid (n) and are produced by meiosis. Think of a gamete as half a deck of cards, waiting to be combined with another half at fertilisation.
Diploid (2n)
A cell containing two complete sets of chromosomes, one from each parent. In humans, 2n = 46.
Haploid (n)
A cell containing only one set of chromosomes. In humans, n = 23.
Heredity
The transmission of traits (genes) from one generation to the next.
Variation
The differences in appearance and traits that offspring show compared to their parents and siblings.
Zygote
The single diploid cell produced when a sperm fertilises an egg. In simple terms, it is the very first cell of a new organism.
Autosomes
The 22 pairs of chromosomes that are not sex chromosomes. They carry genes for most body traits.
Sex chromosomes
The 23rd pair of chromosomes that determines biological sex. XX = female, XY = male.
The genome is the complete set of DNA in an organism. In humans, this DNA is distributed across 46 chromosomes.
Each chromosome is a single, continuous molecule of DNA packaged with histone proteins.
Chromosomes carry genes at specific locations called loci (singular: locus).
Different chromosomes carry different segments of DNA, so losing even one has severe consequences.
Somatic cells are all body cells except gametes. They are diploid (2n = 46 in humans) and are produced by mitosis.
Examples: muscle cells, nerve cells, epithelial cells.
Gametes are the sex cells (sperm and egg). They are haploid (n = 23 in humans) and are produced by meiosis.
Oogenesis produces eggs; spermatogenesis produces sperm.
Gametes are the only human cell type produced by meiosis. Every other cell type comes from mitosis.
Diploid (2n): two copies of each chromosome, one from each parent. Somatic cells are diploid.
Haploid (n): one copy of each chromosome. Gametes are haploid.
Meiosis reduces the chromosome number from diploid to haploid. Fertilisation restores it.
A karyotype is an image of all 46 chromosomes arranged in their 23 homologous pairs, sorted by size and centromere position.
22 pairs are autosomes (non-sex chromosomes).
1 pair is the sex chromosomes: XX in females, XY in males.
Banding patterns, centromere position, gene order, and chromosome size are consistent across all humans.
Each pair of homologs shares the same gene sequence and structure, but the two chromosomes are not genetically identical.
One homolog comes from the mother, the other from the father.
At each locus, an individual carries two alleles, one on each homolog.
Whether a trait is expressed depends on dominance relationships between alleles.
Asexual reproduction: a single parent passes all its genes to offspring without gamete fusion (e.g. binary fission). Offspring are genetically identical clones.
Sexual reproduction: two parents contribute gametes. Offspring inherit unique gene combinations, producing both heredity and variation.
At fertilisation, one haploid egg (always carries an X chromosome) fuses with one haploid sperm (carries either X or Y).
The result is a diploid zygote with a full complement of 46 chromosomes.
The zygote undergoes DNA replication followed by mitosis, building the entire multicellular body.
The cycle repeats: meiosis in the adult produces new haploid gametes, fertilisation restores the diploid number.
Karyotyping is used clinically to diagnose chromosomal disorders such as Down syndrome (trisomy 21) or Turner syndrome (monosomy X) by examining a patient's chromosome set under a microscope. Understanding the diploid-to-haploid cycle also underpins assisted reproductive technologies like IVF, where eggs and sperm are handled outside the body and fertilisation is monitored at the single-cell level.
Students often think homologous chromosomes are genetically identical. They are not. They share the same gene sequence and structure, but they carry different allele versions because one came from each parent.
Students sometimes confuse "haploid" with "half a cell." A haploid cell is a complete, functional cell; it simply has one set of chromosomes instead of two.
Students mix up "chromosome" and "chromatid." Before DNA replication, a chromosome is a single DNA molecule. After replication, it consists of two identical sister chromatids joined at the centromere, but it is still counted as one chromosome.
Students sometimes assume that mitosis and meiosis are interchangeable terms for cell division. Mitosis produces genetically identical diploid cells for growth and repair; meiosis produces genetically unique haploid gametes for reproduction.
⚠️ Know the difference between homologous chromosomes and sister chromatids. Exams test this distinction heavily.
⚠️ Be able to state the chromosome number at each stage: diploid somatic cells (2n = 46), haploid gametes (n = 23), and the diploid zygote formed at fertilisation (2n = 46).
⚠️ Understand that gametes are the only human cells produced by meiosis. All other cells are produced by mitosis.
⚠️ Know that the sex of offspring is determined by the sperm (X or Y), not the egg (always X).
True or false: Homologous chromosomes are genetically identical.
Fill in the blank: Human somatic cells contain ___ chromosomes, arranged in ___ pairs.
True or false: Gametes are produced by mitosis.
Fill in the blank: The single cell produced by fertilisation is called a ___.
True or false: An unfertilised egg always carries an X sex chromosome.
Answers: 1. False (same structure, different alleles). 2. 46; 23. 3. False (meiosis). 4. Zygote. 5. True.
Q: What is the difference between a somatic cell and a gamete in terms of chromosome number and how each is produced?
A: A somatic cell is diploid (2n = 46 in humans) and is produced by mitosis. A gamete is haploid (n = 23) and is produced by meiosis.
Q: A human zygote has 46 chromosomes. Explain where these chromosomes came from.
A: 23 came from the mother's egg (haploid) and 23 came from the father's sperm (haploid). Fertilisation combined the two haploid sets into one diploid cell.
Q: Why are homologous chromosomes described as having the same structure but not being genetically identical?
A: Homologs carry genes at the same loci in the same order, but the specific alleles at those loci can differ because one homolog was inherited from the mother and the other from the father.
Q: If a species has a diploid number of 14, how many chromosomes are in its gametes?
A: 7. Meiosis halves the diploid number, so n = 14 / 2 = 7.
Q: Explain why meiosis, rather than mitosis, is essential for sexual reproduction.
A: Meiosis reduces the chromosome number by half, producing haploid gametes. Without this reduction, fertilisation would double the chromosome number every generation. Meiosis also introduces genetic variation through crossing over and independent assortment.
This material connects directly to the Meiosis study notes (Part 2 continued), where the stages of meiosis I and II, crossing over, and independent assortment are covered in detail. Understanding diploid vs haploid is essential there.
It also feeds into Mendelian genetics: the law of segregation and the law of independent assortment both depend on how homologous chromosomes behave during meiosis.
Later in the course, chromosome disorders (nondisjunction, aneuploidy, structural alterations) build on the concept of normal chromosome number and pairing covered here.
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