Life Sciences Grade 12 definitions

Abscisic acid

The plant growth hormone that promotes seed dormancy.

Accommodation

The series of changes that take place in the shape of the lens and the eye- ball in response to the distance of an object from the eye.

Aldosterone

A hormone that regulates the salt concentration in human blood.

Allantois

The structure in the amniotic egg that stores wastes.

Altricial development

The reproductive strategy in which hatchlings of birds are not able to move and feed themselves.

Aqueous humour

The watery fluid that supports the cornea and the front chamber of the eye.

Autonomic nervous system

The part of the peripheral nervous system that controls involuntary action.

Biodiversity

The variety of plant and animal species on earth.

Carbon footprint

Measurement of the total amount of carbon dioxide emissions of an individu- al per year.

Centrosome

Organelle found only in animal cells that forms the spindle during cell divi- sion.

Centriole

Structures formed when the centrosome divides into two, and move to oppo- site poles of the cell during cell division.

Chiasma

The point where two chromatids overlap during crossing over.

Choroid

A layer inside the eye that absorbs light, thus reducing reflection.

Chorion

The outermost membrane found around the embryo/foetus.

Cristae

The receptors in the ear that detect changes in the direction and speed of any movement of the body.

Cytokinesis

The division of the cytoplasm through the constriction of the cell membrane at the end of cell division.

Deforestation

Removal of large numbers of trees from an area.

Endometrium

The inner lining of the uterus where implantation of the embryo occurs.

Eustachian tube

The part of the ear that equalises the pressure on either side of the tympanic membrane.

Eutrophication

The accumulation of nutrients in water bodies from over-use of fertilisers on land, which stimulates excessive plant growth.

External fertilisation

A type of fertilisation in which the nucleus of a sperm fuses with the nucleus of an ovum outside the body of the female.

Fallopian tube

Part of the female reproductive system where fertilisation occurs.

Food security

Access by all people, at all times, to adequate, safe and nutritious food.

Geotropism

The growth of part of a plant in response to gravity.

Greenhouse effect/En- hanced greenhouse effect

Heat being trapped by a layer of greenhouse gases (e.g. carbon dioxide, methane) surrounding the earth. The enhanced greenhouse effect is a result of large-scale release of greenhouse gases surrounding the earth, thereby causing global warming.

Grommets

Small tubes placed in the tympanic membrane to drain liquid from the middle ear.

Homeostasis

The maintenance of a constant internal environment in the body within cer- tain limits.

Internal fertilisation

A type of fertilisation in which the nucleus of a sperm fuses with the nucleus of an ovum inside the reproductive system of the female.

Interphase

The phase in the cell cycle when DNA replication occurs.

Long-sightedness

A defective condition of the eye that means that a person can see distant objects clearly while nearby objects are blurred.

Luteinising hormone/LH

The hormone responsible for ovulation and the formation of the corpus lute- um.

Medulla oblongata

The part of the brain that controls heart rate.

Meninges

A collective name for the membranes that protect the brain.

Oestrogen

"The hormone that causes thickening of the endometrium and is produced by

the Graafian follicle."

Oval window

The membrane that transmits sound vibrations to the inner ear.

Ovipary

The reproductive strategy that involves laying eggs.

Ozone/O3

A layer in the atmosphere that is damaged by chlorofluorocarbons (CFCs).

Phototropism

The growth of part of a plant in response to light.

Pinna

The part of the human ear that directs sound waves into the auditory canal.

Precocial development

The reproductive strategy in which hatchlings of birds are able to move and feed themselves.

Puberty

The stage in humans when sexual maturity is reached in males and females.

Retina

The innermost membrane in the eye that contains photoreceptors.

Short-sightedness

A defective condition of the eye which means that a person can see nearby objects clearly while distant objects are blurred.

Stimulus

A detectable change (e.g. pain, heat, light, sound) that will be received by a receptor and converted into an impulse.

Testosterone

A hormone that stimulates the maturation of sperm and stimulates puberty in males.

Thermal pollution

The type of pollution caused when water is released into a river after being heated in power stations or other industries.

TSH/Thyroid-stimulating hormone

A hormone that stimulates the secretion of thyroxin.

Vas deferens

The male reproductive tube that connects the testis with the urethra.

Vasoconstriction

Narrowing of the blood vessels in the skin in humans, which decreases the amount of blood flowing to the skin when the environmental temperature is low.

Vasodilation

Widening of the blood vessels in the skin in humans, which increases the amount of blood flowing to the skin when the environmental temperature is high.

Vivipary

A type of reproduction, including in humans, in which the foetus develops inside the uterus.

Albinism

The condition that results from the absence of skin pigmentation.

Alleles

Two alternative forms of a gene at the same locus.

Binocular

Using two eyes with overlapping fields of view to create a single visual im- age.

Biodiversity

The variety of life forms that exist on Earth.

Biogeography

The distribution of species in different parts of the world.

Bipedalism

The ability of an organism to walk on two limbs.

Clone

A copy of an organism that is genetically identical to the original organism.

Cloning

The process by which genetically identical organisms are formed using biotechnology.

Co-dominance

The type of inheritance where both alleles are equally dominant and both express themselves equally in the phenotype. E.g. a white cow crossed with a black bull will produce a calf with black and white patches.

Complete dominance

The type of inheritance in which the dominant allele masks the expression of the recessive allele in the heterozygous condition.

Dihybrid

A genetic cross involving two different characteristics, e.g. shape and colour of seeds.

DNA profile

A bar code pattern formed from DNA that is unique to each person/organ- ism.

Down syndrome/trisomy 21

A human disorder caused by non-disjunction of chromosome pair number 21.

Extinction

The permanent disappearance of a species from earth.

Foramen magnum

The opening at the base of the skull through which the spinal cord enters.

Gene

A segment of DNA/a chromosome that contains the code for a particular characteristic.

Genome

The complete set of chromosomes in the cell of an organism.

Haemophilia

A sex-linked genetic disorder characterised by the absence of a blood-clot- ting factor.

Hominidae/Hominids

The family to which humans belong.

Homologous structures

Similar structures on different organisms that suggest they have a common ancestor.

Homologous chromosomes

Two chromosomes that carry the same set of genes.

Hydrogen bond

The weak bond between nitrogenous bases in a DNA molecule.

Hypothesis

A tentative explanation of a phenomenon that can be tested and which may be accepted or rejected.

Incomplete dominance

The type of inheritance in which both alleles express themselves in such a way that an intermediate phenotype is formed. E.g. a white flowering plant crossed with a red flowering plant will produce a pink flowering plant.

Karyotype/Karyogram

A representation of the number, shape and arrangement of all the chromo- somes in the nucleus of a somatic cell.

Locus

The position of a gene on a chromosome.

Monohybrid cross

A genetic cross involving one characteristic, e.g. the colour of seeds.

Natural selection

The process by which organisms best suited to survival in the environment achieve greater reproductive success, thereby passing advantageous char- acteristics onto future generations.

Non-disjunction

The process in which homologous chromosome pairs fail to separate during meiosis.

Nuclear pores

Openings in the nuclear membrane that allow mRNA to leave the nucleus.

Out of Africa hypothesis

The hypothesis that states that modern humans/Homo sapiens originated in Africa and migrated to other parts of the world.

Peptide bond

The name of the bond that forms between amino acids in a protein mole- cule.

Phylogenetic tree/cladogram

A diagrammatic representation showing possible evolutionary relationships among different species.

Population

A group of organisms of the same species living in the same habitat at the same time.

Quadrupedal

The ability of an organism to walk on four limbs.

Replication

The process by which a DNA molecule makes identical copies of itself.

Ribosome

The organelle in the cytoplasm that is the site of protein synthesis.

Speciation

A process whereby new species are formed from the original population.

Species

A group of organisms that can interbreed to produce fertile offspring.

Spindle fibres

The structure responsible for pulling chromosomes to the poles of an cell during cell division in animals.

Stereoscopic vision

The type of vision that allows for depth perception and 3-dimensional vision.

Transcription

The stage of protein synthesis during which mRNA forms from DNA.

Translation

"The stage of protein synthesis during which tRNA interprets the message on

the mRNA to form a specific protein."

tRNA/transfer RNA

The type of nucleic acid that carries a specific amino acid.