BioExplorers

Fertilisation in Mammals

100

How many structures in a sperm cell contain DNA? List them.

                               (2 marks)

Ans: Two structures (1 mark)--> the "Nucleus" and the "mitochondria". (1 mark)

200

Explain how the cortical reaction prevents more than one sperm from fertilising an egg.

                                (2 marks) 

Ans: Cortical granules release contents (enzymes) into the zona pellucida (1 mark), causing it to harden/thicken, which prevents other sperm from entering. (1 mark)

300

List four structures of the male haploid gamete in animals.

                               (3 marks)

Any four of the following:

1- Acrosome, 2- Nucleus, 3- Mitochondria, 

4- Microtubules, 5- Tail (flagellum)

400

The Zona Pellucida of some egg cells were damaged. Suggest why the damaged fertilised egg cell may contain three nuclei.

                                 (3 marks)

Ans: [As a sperm entered egg cell, cortical granules have fused to cell surface membrane/cortical reaction resulted in hardening of zona pellucida] (1 mark) + [As zona pellucida (of some egg cells) are damaged there are areas where it is not hardened] (1 mark) + [Resulting in "polyspermy" / {an extra/two} sperm have entered (the egg cell)] (1 mark) 

500

Does the Acrosome produce enzymes? If so, Explain why a higher activity of hydrolytic enzymes can lead to a higher percentage of egg cell fertilisation.

                              (3 marks)

A higher activity of "hydrolytic enzymes" allows "sperm cells" to "digest" the "outer/zona pellucida" layers of many more egg cells. ACCEPT: more digestion of the layers occurs. (1 mark) + sperm cells can bind/fuse to the egg cell OR reach the "egg nucleus." (1 mark) + Fusion of sperm and egg cell nuclei occurs to form a zygote (1 mark)

Fertilisation in Plants

100

Define pollination?

                                (1 mark)

Ans: The transfer of pollen grains from the anther to the stigma. (1 mark)

200

What is the process called when a "triploid endosperm nucleus" and a "diploid zygote" are formed?

                               (1 mark)

Ans: "Double fertilisation" (1 mark) -in plants

300

Name all types of nuclei present in double fertilisation.

                                 (3 marks)

Ans:

1- Tube nucleus

2- Generative nucleus--> two male gametes

3- Egg cell nucleus

4- Two polar nuclei

400

Describe the events that occur after pollination but before fertilisation.

                              (4 marks) 

Ans: Mention the following points:

1. Pollen grain germinates on the stigma.
2. The pollen tube grows down the style.
3. The generative nucleus divides by mitosis.
4. The pollen tube enters the ovule through the micropyle.

500

Flowering plants can contain several ovules. Explain how flowering plants can produce seeds that are genetically different from each other.

                                   (4 marks)

Ans: [Each male nucleus and egg cell nucleus from the flowering plant is genetically different from each other/ovule and may have been fertilised by pollen/gamete from (many) different trees] (1 mark) + [Due to "crossing over" of (alleles/DNA) between chromatids/-->mutation] (1 mark) + [Due to "independent/random" assortment (of chromosomes)] (1 mark) + [In meiosis] (1 mark) 

General 3A-3C Knowledge

100

Does External Fertilisation require fewer gametes?

                                 (1 mark)

No! (1 mark) - "many" gametes are needed. Higher probability for fertlilisation to occur.

200

Which organelle is seen as "several curved membrane-bound sacs of decreasing size?"

                                    (1 mark)

Ans: The Golgi Apparatus (1 mark)

300

State how the ribosomes in prokaryotic cells differ from eukaryotic cells.

                                    (1 mark)

Ribosomes in prokaryotic cells are smaller than in eukaryotic cells (1 mark) -- Or -- Ribosomes in prokaryotic cells are "70S", whereas those in eukaryotic cells are "80S" (1 mark) 

Accept: Ribosomes in prokaryotic cells contain less RNA than those in eukaryotic cells.

400

Explain how plant cells increase in size after cell division occurs.

                             (3 marks)

Because of water uptake that increases volume of cytoplasm or vacuole (1 mark) + Increase in "size" due to the synthesis/production of new organelles   (1 mark) + Therefore, {synthesis/production} of {new/more} cell {membrane / wall} (1 mark) 

ACCEPT: organelles increase in size

500

Describe the process of therapeutic cloning.

                                  (6 marks)

Mention the following points:

1- Nucleus is removed from adult cell, 

2- insert nucleus into "enucleated" ovum, 

3- electric shock,     

4- the cell divides, 

5- stem (unspecialised) cells produced, 6- that are genetically identical

6 MARKS achieved :)

CELL DIVISION (MEIOSIS AND MITOSIS)

100

What is meant by mitosis?

                          (1 mark)

A1: The process by which a cell divides to produce two genetically identical cells.

IGNORE: cell divides (only).

200

Define mitotic index.

                                (1 mark)

It is the number of cells in mitosis divided by the total number of cells.

REJECT: the formula to be given.

300

What occurs in ‘Prophase 1’ and ‘Metaphase 1’ in meiosis?

                            (2 marks)

Prophase 1 --> Crossing over.

Metaphase 1 --> Independent Assortment

REJECT: Any other answer.

400

State the phases of the cell cycle in order.

                         (3 marks)

A4: 1. Interphase (G1-S phase- G2), 2. Mitosis. 3. Cytokinesis.

REJECT: Any other answer

500

What is the difference between mitosis and meiosis?

                               (4 marks)

A5: Mitosis involves one division to produce two genetically identical diploid cells for growth and repair, whereas meiosis involves two divisions to produce four genetically different haploid cells for sexual reproduction and genetic variation.

REJECT: Mitosis produces gametes./Meiosis produces identical

cells./Mitosis halves the chromosome number./Meiosis is used

for growth./Confusion between haploid and diploid.