Cell Biology & Genetics

Why is the mitochondrion called the powerhouse of the cell?

1. Cellular respiration breaks down glucose to release energy, and most of that process occurs in the mitochondrion.
2. The energy released is stored in ATP, the molecule cells use for every energy-requiring process.
3. Since the mitochondrion is where almost all the cell's ATP is generated, it is described as the powerhouse.

Answer: Because cellular respiration and ATP production occur there.

A tall pea plant is crossed with a dwarf one. What appears in the F1 and F2 generations?

1. Tallness is dominant, so the F1 generation, all heterozygous, is entirely tall — the law of dominance.
2. Crossing the F1 plants among themselves, the alleles segregate and recombine.
3. The F2 generation shows the 3:1 phenotypic ratio — three tall to one dwarf — with a genotypic ratio of 1:2:1.

Answer: All tall in F1; three tall to one dwarf in F2.

Why is haemophilia far commoner in men than in women?

1. The gene lies on the X chromosome and the condition is recessive.
2. A male has one X and one Y, so a single recessive allele on his X is expressed, with no second copy to mask it.
3. A female has two X chromosomes and expresses the condition only if both carry the allele, which is much rarer. She may be an unaffected carrier.

Answer: Because it is X-linked recessive and males have only one X chromosome.

Distinguish mitosis from meiosis in one sentence each.

1. Mitosis is one division producing two genetically identical diploid cells, used for growth and repair of body tissue.
2. Meiosis is two successive divisions producing four genetically distinct haploid cells, used for gamete formation.
3. The key contrast is that mitosis is equational and meiosis reductional, and that crossing over occurs only in meiosis.

Answer: Mitosis: two identical diploid cells for growth. Meiosis: four varied haploid cells for reproduction.

Why does the sex of a human child depend on the father?

1. The mother is XX, so every ovum she produces carries an X chromosome.
2. The father is XY, so he produces X-bearing and Y-bearing sperm in roughly equal numbers.
3. An X-bearing sperm gives XX, a girl; a Y-bearing sperm gives XY, a boy. The determining chromosome therefore comes from the father.

Answer: Because the ovum always carries X, so the sperm supplies the determining chromosome.

Why does the sickle cell allele persist in malaria-endemic regions?

1. Sickle cell anaemia is autosomal recessive, and the homozygous condition is severe.
2. But heterozygous carriers, with one normal and one sickle allele, have some resistance to malaria.
3. In malarial regions that advantage outweighs the cost, so selection maintains the allele at a substantial frequency — a classic case of heterozygote advantage.

Answer: Carriers gain protection against malaria, so selection preserves the allele.

Distinguish homologous from analogous organs, with examples.

1. Homologous organs share the same basic structure and embryonic origin but perform different functions — the forelimbs of a human, a whale, a bat and a horse.
2. They indicate descent from a common ancestor, and are evidence of divergent evolution.
3. Analogous organs perform the same function with different structure and origin — the wings of a bird and of an insect — and indicate convergent evolution under similar pressures.

Answer: Homologous means same structure, different function; analogous means same function, different structure.

What did the Miller-Urey experiment demonstrate?

1. The Oparin-Haldane hypothesis held that life arose from simple molecules in an oxygen-free early atmosphere.
2. Miller and Urey, in 1953, passed electric sparks through a sealed mixture of methane, ammonia, hydrogen and water vapour, simulating lightning in that atmosphere.
3. Amino acids formed, showing that the organic building blocks of life can arise from inorganic starting materials without any living thing being present.

Answer: That amino acids can form abiotically under early-earth conditions.