extra r Flashcards
Random assortment
During the first stage of
meiosis the arrangement of the pairs of
homologous chromosomes is at random. When
these chromosomes are drawn to the poles of the
cell during anaphase, this leads to many
different combinations of maternal and
paternal chromosomes. Random assortment is
an important source of genetic variation
Repressor molecule
Molecule involved in gene
regulation by binding to DNA and preventing
gene transcription.
Reptile
A member of the class of animals that
includes crocodiles, lizards and snakes. Reptiles
have dry skin covered in scales, demonstrate
internal fertilisation, and lay eggs. The eggs are
sometimes retained inside the body of the female
until they hatch.
Resistant
An organism is described as resistant
to a substance such as a pesticide or an
antibiotic when the substance no longer affects
the organism. Genes that give resistance to
antibiotics are often used as markers in genetic
engineering.
Resting potential
The difference in electrical
charge across the membrane of a resting
neurone. When the inside of the axon is more
negative than the outside, the membrane is said
to be polarised. The potential difference across
the membrane is about –70 mV; this is known as
the resting potential. This is due to the difference
in permeability of the cell surface membrane to
different ions which results in their uneven
distribution. A nerve impulse involves the
temporary reversal of this negative potential
difference, changing it to a positive value.
Reinforcement
When an animal gains a reward
for displaying a particular behaviour pattern, this
reward acts as a reinforcement. Operant
conditioning is a form of learning in which the
consequences of an animal’s behaviour affect
whether that behaviour will happen again. A
reward reinforces the particular pattern of
behaviour, so it is likely to be repeated.
Replication
A general term that means repetition.
In DNA replication, an exact copy of the DNA is
made prior to cell division.
Rigor mortis
The stiffening of muscles that
occurs after death. After death, oxygen is no
longer supplied to muscles. For a short time
respiration continues anaerobically and lactic
acid accumulates as a result. This causes the pH
in muscle cells to fall and inhibits respiratory
enzymes. ATP is no longer formed and, as a
result, the bonds between muscle proteins
become fixed.
Rhizoid
A small root-like structure found on
organisms such as algae and mosses. The main
function of a rhizoid is to anchor the organism,
and in this way it is very like a root. However, it
does not contain vascular tissue.
Root hair
A thin hair-like structure which grows
out from a cell in the epidermis of a root. Root
hairs are usually found on young roots in the
region just behind the tip. They provide a large
surface area through which mineral ions and
water are absorbed.
Sebaceous gland
A gland found in the skin of a
mammal which opens at the base of a hair. It
produces an oily substance called sebum, which,
in addition to its waterproofing properties, has
an antimicrobial effect.
Saprobiont, Saprobiotic, Saprophyte
Bacteria
and fungi which feed on dead organic matter.
Saprobiotic organisms secrete enzymes onto
their food. These enzymes digest substances in
the dead material on which the saprobionts are
feeding, and break them down into smaller
soluble molecules. The smaller molecules then
diffuse into cells of the saprobiont.
Secondary consumer, tertiary consumer
An
organism which feeds on animals or other
heterotrophic organisms. In food chains and
food webs a secondary or tertiary consumer is a
carnivore. In the food chain: microscopic algae
brine shrimp flamingo eagle, flamingos
are secondary consumers, and the eagle is the
tertiary consumer.
Seed
A structure produced by a flowering plant
containing an embryo and a store of food
material. A seed develops from the embryo sac
once fertilisation has taken place. The food
material may be in the cotyledons, which form
part of the embryo, or it may be in another tissue, the endosperm. A protective coat, the
testa, surrounds the embryo and its food store.
Seedbank
All the seeds produced by a particular
species of plant which are present in the soil and
able to germinate when conditions are
favourable. A seedbank may contain seeds of
different ages.
