PSEB SS Biology key terms

The biological process by which organisms produce new individuals of their own kind, ensuring continuation of the species. It may be asexual, involving a single parent, or sexual, involving fusion of gametes from two parents.

Formation of new individuals from a single parent without fusion of gametes, so offspring are genetically identical to the parent. Examples include binary fission in bacteria, budding in yeast and vegetative propagation in plants.

Asexual reproduction in plants in which new plants arise from vegetative parts such as roots, stems or leaves rather than from seeds. Examples are runners in grass, tubers in potato and bulbs in onion.

A specialised haploid reproductive cell, such as a sperm or egg, that fuses with another gamete during sexual reproduction. Gametes are produced by meiosis, so each carries half the normal chromosome number.

The fusion of a male gamete with a female gamete to form a diploid zygote. In flowering plants it involves pollen tube growth and double fertilisation; in humans it normally occurs in the fallopian tube.

The process unique to angiosperms in which one male gamete fuses with the egg to form the zygote, while a second male gamete fuses with two polar nuclei to form triploid endosperm.

The diploid cell formed when two haploid gametes fuse at fertilisation. It divides repeatedly by mitosis to develop into an embryo and eventually a new individual.

The female gametophyte of flowering plants, usually containing eight nuclei in seven cells, including the egg cell and two polar nuclei. It develops within the ovule and is the site of double fertilisation.

The transfer of pollen grains from the anther to the stigma of a flower. It may be self-pollination within the same flower or plant, or cross-pollination between different plants, often aided by wind, water or animals.

The approximately 28-day cycle of hormonal and structural changes in the human female reproductive system, including follicular development, ovulation and preparation of the uterine lining for possible implantation.

The release of a mature secondary oocyte from a ruptured ovarian follicle, usually around the middle of the menstrual cycle. It is triggered by a surge in luteinising hormone.

The temporary glandular structure formed from the remains of an ovarian follicle after ovulation. It secretes progesterone, which maintains the uterine lining during the early stages of pregnancy.

The attachment and embedding of the blastocyst in the endometrium of the uterus, usually about six to seven days after fertilisation. It establishes the connection needed for placental development.

The temporary organ formed from maternal and foetal tissues that allows exchange of nutrients, oxygen and waste between mother and foetus. It also secretes hormones such as human chorionic gonadotropin and progesterone.

The deliberate prevention of pregnancy through methods such as barrier devices, hormonal pills, intrauterine devices or surgical sterilisation. Methods differ in effectiveness, reversibility and mode of action.

Patterns of inheritance described by Gregor Mendel, based on segregation of alleles and independent assortment of genes. These principles explain dominant and recessive traits in organisms such as pea plants.

One of two or more alternative forms of a gene occupying the same locus on homologous chromosomes. Alleles may be dominant or recessive and determine the variations of a particular trait.

The genetic constitution of an organism at one or more loci, described by the alleles it carries, such as homozygous dominant, heterozygous or homozygous recessive. It interacts with the environment to produce the phenotype.

The observable characteristics of an organism, such as flower colour, blood group or height, resulting from the expression of its genotype in a given environment.

Describing an organism that carries two identical alleles for a particular gene, either both dominant or both recessive. Homozygous individuals breed true for that trait when self-crossed.

Describing an organism that carries two different alleles for a particular gene. A heterozygous individual may show the dominant phenotype while still carrying a hidden recessive allele.

A type of cell division that produces four haploid daughter cells from one diploid parent cell through two successive divisions. It generates genetic variation by crossing over and independent assortment and forms gametes.

A type of nuclear division producing two genetically identical diploid daughter cells. It is responsible for growth, repair and asexual reproduction in body cells.

The exchange of segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis. It creates new combinations of alleles and contributes to genetic variation in offspring.

The tendency of genes located close together on the same chromosome to be inherited together rather than assorting independently. The closer the genes, the lower the chance of separation by crossing over.

Inheritance of genes carried on sex chromosomes, most commonly the X chromosome. Examples include haemophilia and red-green colour blindness, which appear more often in males.

Deoxyribonucleic acid, the double-stranded molecule that stores genetic information in the sequence of its bases adenine, thymine, guanine and cytosine. It is the hereditary material of almost all organisms.

The basic building block of nucleic acids, consisting of a nitrogenous base, a pentose sugar and a phosphate group. Nucleotides join in chains to form DNA and RNA.

The semi-conservative process by which a DNA molecule makes an identical copy of itself before cell division. Each new double helix contains one original strand and one newly synthesised strand.

The synthesis of a messenger RNA molecule from a DNA template, catalysed by RNA polymerase. It occurs in the nucleus of eukaryotic cells and is the first step of gene expression.

The process in which ribosomes read the sequence of codons on messenger RNA and assemble the corresponding sequence of amino acids into a polypeptide chain, using transfer RNA molecules.

A sequence of three consecutive nucleotides in messenger RNA that specifies a particular amino acid or a stop signal during translation. The genetic code is degenerate, meaning several codons may code for one amino acid.

The set of rules by which nucleotide triplets in DNA or RNA are translated into amino acids in proteins. It is nearly universal, degenerate and read in non-overlapping triplets from a fixed start point.

A permanent change in the nucleotide sequence of DNA, which may be a substitution, insertion or deletion. Mutations can be harmful, neutral or occasionally beneficial and are a source of genetic variation.

The change in the inherited characteristics of populations over successive generations. It is driven by mechanisms including natural selection, mutation, genetic drift and gene flow.

The process by which individuals with traits better suited to their environment tend to survive and reproduce more successfully, so those traits become more common in later generations.

The formation of new and distinct species from an ancestral population, usually after reproductive isolation. Isolation may result from geographic barriers, as in allopatric speciation, or other factors.

The ability of the body to resist and defend against disease-causing pathogens. It includes innate immunity, which is non-specific, and acquired immunity, which is specific and involves memory.

A protein produced by plasma cells in response to a specific antigen. Antibodies bind to antigens and help neutralise or destroy pathogens, forming part of the acquired immune response.

The administration of a weakened, inactivated or subunit antigen preparation to stimulate an immune response and produce memory cells, giving protection against future infection by that pathogen.