Reproduction, Genetics & Evolution

Reproduction is how a species makes more of itself, so that life continues even though every individual dies. There are two big ways. In asexual reproduction a single parent makes offspring without gametes, so the young are exact copies (clones) of the parent. In sexual reproduction two gametes, usually from two parents, fuse, and the offspring carry a mixture of both parents' genes, which is the source of variation. Variation is what lets a species survive when the environment changes.

Fission divides the whole cell; budding makes an unequal outgrowth. In vegetative propagation a new plant grows from a vegetative part (root, stem or leaf) and not from a seed. It works because the plant has regenerative tissue in the cut part, and the new plant is an exact copy of the parent, so farmers use it to keep a good variety unchanged and to grow seedless fruits like banana.

A flower is the reproductive organ of a flowering plant. The male part is the stamen (anther that makes pollen, and filament). The female part is the pistil or carpel (stigma, style and ovary; the ovary holds ovules, and each ovule holds the egg). Self-pollination is within the same flower or plant; cross-pollination brings pollen from another plant and gives more variation. A fruit that develops without fertilisation is parthenocarpic (banana, seedless grapes). The arrangement of ovules inside the ovary is called placentation: parietal (mustard, cucumber), axile (tomato, lemon), marginal (pea, bean), free central (primrose, Dianthus) and basal (sunflower, marigold).

In animals, fertilisation can be external (outside the body, in water: fish, frog) or internal (inside the female body: reptiles, birds, mammals). Animals that lay eggs are oviparous (birds, most reptiles), those that give birth to live young are viviparous (humans, cows), and some retain eggs inside the body until they hatch and are called ovoviviparous (some sharks and snakes). A frog's life cycle passes through egg, tadpole and adult by metamorphosis, and a butterfly's through egg, larva, pupa and adult. Earthworms, Hydra and tapeworm are hermaphrodite (both sexes in one body), yet the earthworm still reproduces sexually by cross-fertilisation, not by fission.

Human reproduction is sexual, internal and viviparous. At puberty the pituitary gland sends FSH and LH to the gonads, and the gonads begin to make sex hormones: testosterone in the testes and oestrogen and progesterone in the ovaries. These hormones also bring the secondary sexual characters, such as a deeper voice and beard in boys and breast development in girls. This section is kept strictly to the textbook facts needed for the exam.

A sperm is a tiny motile cell with a head that carries the nucleus and an acrosome cap of enzymes, a middle piece full of mitochondria for energy, and a tail for swimming. The egg is the largest human cell and is non-motile. Both gametes are haploid with 23 chromosomes, so the zygote is diploid (46). Ovulation happens about the 14th day of a 28-day menstrual cycle, and if the egg is not fertilised the thickened lining of the uterus breaks down as menstruation over 3 to 5 days. Menstruation first starts at puberty (menarche) and stops permanently at about 45 to 50 years of age (menopause).

The placenta is a temporary organ that joins the fetus to the mother's uterine wall through the umbilical cord. Oxygen and nutrients pass from the mother's blood to the fetus, and carbon dioxide and wastes pass back, but the two bloodstreams do not mix. The placenta also makes hormones such as hCG and progesterone to maintain the pregnancy. The fetus floats in amniotic fluid, which is a shock absorber. Identical twins come from one zygote that splits (same sex, same genes); fraternal twins come from two eggs fertilised by two sperms (can differ in sex).

Why do children look like their parents, and yet are never exact copies? Heredity is the passing of characters from parents to offspring, and variation is the difference between individuals. The scientist who first explained heredity by experiment was Gregor Johann Mendel, an Austrian monk, who worked in his monastery garden between 1856 and 1863 and is called the Father of Genetics. His work was published in 1866 but ignored until it was rediscovered in 1900 by de Vries, Correns and Tschermak.

Mendel chose the garden pea (Pisum sativum) for good reasons: it has many clear contrasting characters, a short life cycle, many seeds, and it is naturally self-pollinating, so he could keep pure lines and also cross-pollinate by hand. He counted offspring in large numbers and treated the result mathematically, which is why heredity needed both science and mathematics to be understood.

Genes sit in a line on thread-like structures called chromosomes, made of DNA wrapped around histone proteins. A human body cell has 46 chromosomes in 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes. A gamete (sperm or egg) carries only 23, one from each pair. When the two gametes fuse, the number comes back to 46 in the zygote. Different species have different numbers: the pea 14, onion 16, Drosophila (fruit fly) 8, chimpanzee 48 and dog 78. The number tells nothing about how advanced an organism is.

Before a cell divides, the DNA is copied in the S-phase of the cell cycle, so each chromosome becomes two sister chromatids joined at the centromere. The number of chromosomes does not change (a cell with 46 chromosomes still has 46 chromosomes) but the number of chromatids doubles to 92. After division, each daughter cell gets back 46 single-chromatid chromosomes. Cell cycle order: G1, S, G2, then M (mitosis).

The sex of a human child is decided by genetic constitution, and the child always gets an X from the mother; whether the child is a boy or a girl depends on the chromosome received from the father. A child who inherits Y from the father is a male. In birds, butterflies and some reptiles the system is reversed (ZW): the female has two different chromosomes (ZW) and the male has two similar ones (ZZ), so the female bird determines the sex. In honey bees, fertilised eggs become females and unfertilised eggs become males (drones). In crocodiles and turtles, the temperature of incubation decides the sex, and some snails and fishes can change sex during life, which shows that sex is not always fixed genetically.

There are two kinds of errors. A gene (point) mutation changes one gene, as in sickle cell anaemia, PKU, haemophilia. A chromosomal disorder changes the number or structure of chromosomes, as in Down (trisomy 21), Klinefelter (XXY) and Turner (XO). Chromosome-number errors arise from non-disjunction, when chromosomes fail to separate during meiosis. X-linked recessive diseases show up mostly in males because a male has only one X chromosome, so a single recessive allele on it is enough.

DNA (deoxyribonucleic acid) is the molecule of heredity. Friedrich Miescher first isolated it from the nuclei of pus cells in 1869 and called it 'nuclein'. In 1952 Alfred Hershey and Martha Chase used viruses (bacteriophages) labelled with radioactive phosphorus (32P, which marks DNA) and sulphur (35S, which marks protein) and showed that DNA, not protein, enters the bacterium and carries the genetic message. In 1953 James Watson and Francis Crick, using the X-ray pictures of Rosalind Franklin and Maurice Wilkins, gave the double-helix model of DNA.

A nucleotide = a sugar + a phosphate + a nitrogenous base. Bases are of two kinds: purines (adenine, guanine; two rings) and pyrimidines (cytosine, thymine, uracil; one ring). A always pairs with T (two hydrogen bonds) and G with C (three), which is Chargaff's rule: A = T and G = C. So if a DNA has 20 percent adenine, it has 20 percent thymine, and the remaining 60 percent is shared equally by G and C: 30 percent guanine and 30 percent cytosine. Phosphorus is part of the sugar-phosphate backbone of nucleic acids, which is why radioactive phosphorus marked the DNA of the virus.

The flow of genetic information is DNA to RNA to protein. This is called the Central Dogma, given by Francis Crick. A codon is a set of three bases that codes for one amino acid. There are 64 codons: 61 code for amino acids, and three (UAA, UAG, UGA) are stop signals; AUG is the start codon and codes for methionine. The genetic code is nearly universal, which is why a gene from a bacterium can work in a plant.

Evolution means the gradual change in living organisms over many generations that gives rise to new species. It is not a ladder in which animals 'try' to improve; it is a branching tree in which variations that happen to help survival become more common in a population. Earth is about 4.6 billion years old, and the first life, simple prokaryotic cells, appeared roughly 3.8 billion years ago in the ocean.

Miller and Urey (1953) tested the Oparin-Haldane idea. They circulated methane, ammonia, hydrogen and water vapour through a flask, passed electric sparks (to mimic lightning), and within days found amino acids in the water. It showed that the building blocks of life can form from non-living chemicals under early-Earth conditions.

Darwin travelled on HMS Beagle (1831 to 1836) and was struck by the Galapagos finches, whose beaks differed from island to island to suit the food available. This is adaptive radiation. The idea was triggered partly by Malthus's essay on population: more young are born than can survive, so there is a struggle for existence; individuals with useful variations survive and reproduce more (survival of the fittest, a phrase from Herbert Spencer); and over many generations the useful variations spread. Famous example: the peppered moth in industrial England, where dark moths became common on soot-darkened trees (industrial melanism). Darwin did not know about genes; later, the modern synthetic theory joined Darwin's natural selection with Mendel's genetics, mutation and genetic drift. August Weismann cut the tails of mice for 22 generations and showed that the offspring were still born with tails, which went against Lamarck.