3.1 In the beginning

Question 1

Eukaryote, Eukaryotic

Answer 1

An organism that has cells containing a nucleus. Eukaryotic cells also contain organelles such as chloroplasts and mitochondria, which are surrounded by membranes. Animals and plants are eukaryotes; bacteria are prokaryotes. Prokaryotic cells are very small and do not have nuclei or other membrane-bound organelles.

Question 2

Prokaryote, Prokaryotic

Answer 2

An organism which has cells that do not contain a nucleus or other organelles surrounded by membranes. Bacteria are prokaryotes. Animals and plants are eukaryotes; their cells have nuclei and membrane-bound organelles such as mitochondria and chloroplasts.

Question 3

Mitochondrion, Mitochondria, Mitochondrial

Answer 3

An organelle found in plant and animal cells. It is the site of the biochemical reactions involved in aerobic respiration. Although mitochondria vary in size and shape, they usually appear as small, elongated structures about a micrometre in length. Each mitochondrion is surrounded by a double membrane, the inner one of which is folded to form cristae.

Question 4

Cristae

Answer 4

A feature of mitochondria. Mitochondria are organelles that have two cell membrane layers, an outer membrane and an inner membrane. The outer membrane is smooth and surrounds the mitochondrion. The inner membrane, however, is folded inwards to form a series of cristae. The cristae increase the membrane surface area. This is important because it is on this membrane that reactions involved in respiration take place.

Question 5

Nucleus, Nuclei, Nuclear

Answer 5

A large organelle that contains a cell’s genetic material. It is enclosed by a nuclear envelope. This is composed of a double membrane perforated by tiny pores. The nucleus contains DNA, which is packaged into chromosomes. This DNA contains genes, which code for the proteins made by the cell.

Question 6

Nucleolus, Nucleoli

Answer 6

A dark staining body found in the nucleus of a cell. The nucleolus is involved in making ribosomes.

Question 7

Rough endoplasmic reticulum, Rough ER

Answer 7

A network of interconnected sacs in the cytoplasm of a cell. These sacs are surrounded by membranes. There are ribosomes attached to the outer surfaces of the membranes. Proteins made by the ribosomes are transported through the endoplasmic reticulum to other parts of the cell.

Question 8

Ribosome

Answer 8

A small organelle made of RNA and protein found either free in the cytoplasm or attached to the membranes of the rough endoplasmic reticulum. Protein synthesis takes place on ribosomes.

Question 9

Cell membrane

Answer 9

A membrane found either at the surface or inside a cell. Every cell in an animal or a plant is surrounded by a membrane, referred to as the cell surface membrane or plasma membrane. Cell cytoplasm also contains membranes, and organelles are surrounded by membranes. Cell membranes are very thin and consist mainly of a double layer of phospholipid molecules in which there are proteins.

Question 10

Smooth endoplasmic reticulum, Smooth ER

Answer 10

A network of interconnected sacs in the cytoplasm of a cell. These sacs are surrounded by membranes. Unlike rough endoplasmic reticulum, smooth endoplasmic reticulum does not have ribosomes attached to the outer surfaces of these membranes. Smooth endoplasmic reticulum makes lipids.

Question 11

Golgi apparatus

Answer 11

An organelle consisting of a series of flattened sacs, each one enclosed by a membrane. These membranes are continually being formed on one side and pinched off into vesicles on the other. Functions of Golgi apparatus include processing, packaging and secretion of proteins, secretion of the carbohydrates, which form cell walls in plants, and formation of lysosomes.

Question 12

Lysosome

Answer 12

An organelle containing digestive enzymes surrounded by a membrane. This membrane prevents the enzymes digesting the proteins and lipids in the cell. Lysosomes are involved in the breakdown of unwanted structures and in the destruction of old cells when they are replaced during development.

Question 13

Centriole

Answer 13

An organelle found in animal cells which is associated with the separation of chromosomes during mitosis. Biologists are not certain about the exact function of the centrioles, but they are associated with the protein fibres forming the spindle.

Question 14

Sperm

Answer 14

A male gamete from a mammal or other animal. A mature human sperm consists of a head which contains a large nucleus with a single set of chromosomes. The head also has an acrosome, an enzyme-filled sac which plays an important part in fertilisation. The middle piece of a sperm is packed with mitochondria. These provide the energy for movement of the long tail which a sperm uses to swim up the female reproductive tract.

Question 15

Ovum, Ova

Answer 15

A female gamete or egg cell from an animal. The ovum is a large cell. It has a nucleus containing a single set of chromosomes. Its cytoplasm contains protein and lipid food reserves for the early development of the embryo. The plural of ovum is ova.

Question 16

Gamete

Answer 16

A sex cell, e.g. ova and sperm. In animals and plants, gametes are haploid and each contains a single set of chromosomes. A special form of cell division, meiosis, takes place in their formation. Meiosis results in the production of gametes with half the number of chromosomes found in a body cell. Reproduction that involves the fusion of gametes is called sexual reproduction.

Question 17

Acrosome, Acrosome reaction

Answer 17

An organelle in the head of a sperm that contains digestive enzymes. When fertilisation takes place, the membrane surrounding these enzymes bursts. The enzymes digest the follicle cells; they also digest the jelly-like layer surrounding the egg cell allowing the sperm to fuse with the egg membrane. This process is known as the acrosome reaction.

Question 18

cortical reaction

Answer 18

One a sperm fuses with and penetrates the membrane surrounding the egg, chemicals released by the ovum cause the zona pellucida, to thicken, preventing any further sperm entering the egg

Question 19

Mitosis, Mitotic

Answer 19

The process by which the cell nucleus divides to give two new nuclei. Each of these new nuclei has the same number of chromosomes as the original nucleus. Mitosis is a continuous process but it is conveniently divided into four stages: prophase, metaphase, anaphase and telophase.

Question 20

Diploid

Answer 20

Cells or organisms in which the nuclei contain two copies of each chromosome. The diploid number of chromosomes differs from one species to another. In humans, the diploid number of chromosomes is 46. This is sometimes written as 2n = 46.

Question 21

Meiosis

Answer 21

A type of nuclear division important in
the formation of the gametes or sex cells in
animals and plants. Meiosis results in the
production of haploid cells each of which
contains half the number of chromosomes found
in a body cell, one chromosome from each
homologous pair. The processes of independent
assortment and crossing over which occur
during meiosis contribute to genetic variation.

Question 22

Haploid

Answer 22

Cells or organisms in which the nuclei contain one copy of each chromosome. The gametes produced by animals and plants are haploid. In humans, 46 chromosomes are found in each body cell. This is the diploid number. Gametes are formed by a type of cell division called meiosis. This results in gametes which have 23 chromosomes so 23 is the human haploid number. This may be written as n = 23.

Question 23

Independent assortment, Random assortment

Answer 23

During the first stage of meiosis the arrangement of the pairs of homologous chromosomes on the spindle is at random with each chromosome acting independently. When these chromosomes are drawn to the poles of the cell during anaphase, this leads to many different combinations of paternal and maternal chromosomes. Independent assortment is an important source of genetic variation.

Question 24

Crossing over

Answer 24

The process in which chromatids break and rejoin during meiosis. At the start of meiosis, pairs of homologous chromosomes come together, each chromosome consisting of two chromatids. Crossing over occurs between these chromatids. It involves rearranging genetic material and is an important source of genetic variation.

Question 25

Chromatid, Chromatids

Answer 25

One of the two strands of genetic material that make up a chromosome. When chromosomes become apparent at the beginning of mitosis, each can be seen to consist of two strands of genetic material. Each of these strands is called a chromatid. During mitosis these chromatids are pulled apart and go to the opposite poles of the cell

Question 26

Chiasma, Chiasmata

Answer 26

The place where chromosomes can be seen to cross over each other in a cell undergoing meiosis. At the start of meiosis, pairs of homologous chromosomes come together, each chromosome consisting of two daughter chromatids. Chiasmata occur where these chromatids break and rejoin in the process known as crossing over. This involves rearranging genetic material and is an important source of genetic variation.

Question 27

Linkage

Answer 27

When any two genes with a locus on the same chromosome are liked together and will tend to be passed as a pair to the same gamete

Question 28

Sex chromosome

Answer 28

One of the chromosomes which determines the sex of an organism. In humans and other mammals, the female has two X chromosomes in each of her body cells. A male has one X chromosome and one Y chromosome. The X and Y chromosomes are the sex chromosomes.

Question 29

Sex-linked, Sex-linkage

Answer 29

When a gene is carried on a sex chromosome, it is said to be sex- linked. In most animals, the Y chromosome contains very few genes. Sex-linked genes, like those that produce tortoiseshell cats, are therefore most likely to be found on the X chromosome.

Question 30

Fertilisation, Fertilise

Answer 30

The process in which a male gamete fuses with a female gamete to form a zygote.

Question 31

Zygote

Answer 31

In sexual reproduction, a male gamete fuses with a female gamete to form a zygote. This process is called fertilisation. The zygote contains two sets of chromosomes. One set comes from the mother. These are the maternal chromosomes. The other set comes from the father. These are the paternal chromosomes.

Question 32

Flagellum

Answer 32

hollow cylindrical thread like structure rotates to move the cell

Question 33

F1

Answer 33

A term used in genetics to refer to the

offspring of a cross between two organisms.

Question 34

F1 hybrid

Answer 34

Offspring produced by crossing two
different pure-bred lines. Pure-bred plant lines
have been inbred for many generations. These
inbred lines are usually not very productive
because they are homozygous for a number of
unfavourable recessive alleles. When plants
from these inbred lines are crossed, the F1
hybrids produced will be heterozygous for
most of these recessive alleles. The F1 plants
will produce much higher yields than either of
the parent strains.