Reproduction

Meiosis leads to non-identical cells being formed, while mitosis leads to identical cells being formed. Everything in this section follows from that one difference.

Sexual reproduction involves the joining (fusion) of male and female gametes: sperm and egg cells in animals, pollen and egg cells in flowering plants. There is mixing of genetic information, which leads to variety in the offspring. The formation of gametes involves meiosis.

Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information, so the offspring are genetically identical — clones. Only mitosis is involved.

Cells in reproductive organs divide by meiosis to form gametes. When a cell divides to form gametes:

• copies of the genetic information are made • the cell divides twice to form four gametes, each with a single set of chromosomes • all gametes are genetically different from each other.

Gametes join at fertilisation to restore the normal number of chromosomes. The new cell then divides by mitosis, the number of cells increases, and as the embryo develops the cells differentiate.

Knowledge of the stages of meiosis is not required — only what it achieves.

The halving is the point. A human body cell has 46 chromosomes, in 23 pairs. Meiosis produces gametes with 23 single chromosomes each, so when a sperm and an egg fuse the new cell has 46 again. Without the halving, the number would double every generation.

Some organisms reproduce by both methods, depending on the circumstances, and the specification names three:

• Malarial parasites reproduce asexually in the human host, but sexually in the mosquito. • Many fungi reproduce asexually by spores, but also reproduce sexually to give variation. • Many plants produce seeds sexually, but also reproduce asexually — by runners, such as strawberry plants, or by bulb division, such as daffodils.

Knowledge of reproduction in organisms is restricted to those mentioned. You are, however, expected to explain the advantages and disadvantages for any organism if you are given the information — so learn the reasoning, not the list.

The reasoning is a trade-off between speed and insurance. Asexual reproduction is fast and cheap, and it works while conditions stay the same. But every offspring is identical, so a change in the environment — or a disease — affects all of them equally. Sexual reproduction is slower and needs a mate, but the variation it produces means some offspring may survive a change that kills the rest.

The malaria example carries a piece of history worth knowing. Understanding that the parasite reproduces sexually in the mosquito is what identified the mosquito as the vector rather than as a coincidence — and that historical development in our understanding of the cause is exactly what made prevention possible, by targeting mosquito breeding sites and using nets rather than treating the illness alone.

The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer made up of two strands forming a double helix, and it is contained in structures called chromosomes.

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.

The genome of an organism is the entire genetic material of that organism. The whole human genome has now been studied, and this will have great importance for medicine in the future.

Why understanding the human genome matters is limited to three things, and they make a tidy answer:

• the search for genes linked to different types of disease • understanding and treatment of inherited disorders • use in tracing human migration patterns from the past.

DNA is a polymer made from four different nucleotides. Each nucleotide consists of a common sugar and phosphate group, with one of four different bases attached to the sugar. The long strands consist of alternating sugar and phosphate sections, with a base attached to each sugar.

DNA contains four bases: A, C, G and T.

A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.

You may be asked to interpret a diagram of DNA structure, but you will not be required to reproduce it.

Some characteristics are controlled by a single gene — fur colour in mice, and red-green colour blindness in humans. Each gene may have different forms called alleles. The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.

Most characteristics, though, are the result of multiple genes interacting rather than a single gene. That is worth remembering when a question asks whether a single-gene cross is the whole story: it is not, and saying so is often the last mark.

Express the outcome however the question asks, using direct proportion and simple ratios: as a ratio (3 : 1), as a fraction (¾), as a percentage (75%), or as a probability. All four say the same thing, and being able to move between them is examined directly. You should also be able to extract and interpret information from genetic crosses and family trees — in a family tree, work backwards from anyone showing a recessive phenotype, because you know their genotype for certain.

Family trees are read with two conventions and one deduction. The conventions: squares are male, circles female, and a shaded shape means affected. The deduction: start from anyone affected whose parents are not.

A condition that skips a generation like that must be recessive, because a dominant allele shows itself whenever it is present — so an affected child would need an affected parent. Once you know it is recessive, every affected person is homozygous recessive, and both of their parents must carry a copy. That single step usually unlocks the rest of the tree.

Some disorders are inherited, caused by the inheritance of certain alleles. Two are named:

• Polydactyly — having extra fingers or toes — is caused by a dominant allele. • Cystic fibrosis — a disorder of cell membranes — is caused by a recessive allele.

The difference matters for the genetics. Polydactyly appears if a child inherits just one copy, so an affected child usually has an affected parent. Cystic fibrosis needs two copies, so two unaffected carriers can have an affected child — which is what makes it hard to predict.

Embryo screening tests embryos for these alleles, and you should be able to make informed judgements about the economic, social and ethical issues when you are given information.

In favour: it can prevent a child being born with a painful, life-shortening disorder; it reduces the lifelong cost of treatment and care; and parents who know they are carriers can have a child without that fear.

Against: screening implies that a life with a disorder is worth less, which many disabled people reject; it involves discarding embryos, which some people object to; it is expensive; and there is a concern about where the line is drawn once selection becomes possible at all.

Gene therapy raises the same tension — it may alleviate suffering, and it raises the same ethical questions. A good answer gives both sides and then reaches a judgement.

Ordinary human body cells contain 23 pairs of chromosomes. Twenty-two pairs control characteristics only, but one pair carries the genes that determine sex.

• In females the sex chromosomes are the same: XX. • In males they are different: XY.