Lunularia cruciata is a primitive plant. Its body consists of a flattened sheet of photosynthetic tissue called a thallus.
Fig. 5.1 shows L. cruciata with two different types of reproductive structure, labelled A and B, on its surface.
(a) Structures A and B and the thallus of L. cruciata are haploid.
Explain what is meant by haploid.
(b) Structure A contains pale discs of tissue, C, that can germinate into new L. cruciata. The new plants that develop from C are genetically identical to the parent plant in Fig. 5.1.
Structure B contains male sperm that are chemically attracted to swim to female eggs on a neighbouring parent plant. When the egg and sperm fuse, they form structure D. Structure D develops to produce spores that grow into new plants that are genetically different from the two parent plants.
Identify which of the structures A, B, C and D are:
- associated with sexual reproduction
- produced by mitosis
- the site of meiosis
Each letter may be used once, more than once, or not at all.
(c) Fig. 5.2 shows a horse, Equus caballus. Horses are diploid animals that reproduce sexually. Male and female horses produce gametes, which fuse to form genetically different offspring.
Explain the need for a reduction division during meiosis in the production of gametes in animals such as horses.
▶️ Answer/Explanation
(a)
Haploid means having a single set of chromosomes (n). In haploid cells:
- There is only one copy of each chromosome (not in homologous pairs)
- The chromosome number is half of the diploid number
- Each chromosome is different in size, shape, and gene content
- This is typical of gametes and some simple organisms like L. cruciata
(b)
- Associated with sexual reproduction: B and D
Explanation: Structure B produces male gametes (sperm) and D is formed by fertilization (fusion of gametes), both of which are sexual reproduction processes.
- Produced by mitosis: Any two from A, B, C, D
Explanation: All these structures can be produced by mitosis in different contexts. A and C are produced vegetatively by mitosis, B produces sperm via mitosis (in haploid organisms), and D initially grows by mitosis after fertilization.
- The site of meiosis: D
Explanation: Structure D produces spores through meiosis, which is the only reduction division in this life cycle. The spores are haploid and genetically different from the parent.
(c)
Reduction division during meiosis is essential for several reasons:
- Maintenance of chromosome number: Without reduction division, the chromosome number would double with each generation when gametes fuse during fertilization.
- Genetic variation: Meiosis introduces genetic variation through independent assortment and crossing over, which is important for evolution and adaptation.
- Production of haploid gametes: Diploid organisms must produce haploid gametes (n) so that when fertilization occurs, the zygote has the correct diploid number (2n).
- Prevention of polyploidy: If gametes were diploid, the offspring would be tetraploid (4n), which could cause developmental problems.
In horses specifically, this ensures that each generation maintains the same chromosome number (64 in horses) and that there is genetic diversity among offspring, which is important for the survival of the species.