3.2 From one to many: the cell cycle

Question 1

Cell cycle

Answer 1

The well-organised pattern of events in which a cell that has just resulted from a cell division eventually itself divides to form new cells.

Question 2

Interphase

Answer 2

As an organism grows, its cells make new contents and then divide. The resulting pattern of growth and mitosis is called the cell cycle. Interphase is the stage in the cell cycle between mitotic divisions when new organelles are produced and DNA replication takes place. At the end of interphase, the cell enters mitosis and starts to divide again.

Question 3

S phase

Answer 3

The S phase is the part of the cell cycle between the G1 or first gap phase and the G2 or second gap phase. It is the part of the cycle when the cell is synthesising more DNA. The S in S phase stands for synthesis.

Question 4

G1 phase

Answer 4

As an organism grows, its individual cells make new cell contents and then divide. The resulting pattern of growth and mitosis is called the cell cycle. The G1 or first gap phase is part of the cell cycle between the end of mitosis and start of the S phase. G1 is a period of growth and protein synthesis.

Question 5

G2 phase

Answer 5

As an organism grows, its individual cells make new cell contents and then divide. The resulting pattern of growth and mitosis is called the cell cycle. The G2 or second gap phase is part of the cell cycle between the end of the S phase and the start of mitosis. G2 is a period of growth and protein synthesis.

Question 6

Mitosis, Mitotic (nuclear division)

Answer 6

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 7

Cytoplasmic division

Answer 7

The stage after the end of mitosis in which the cytoplasm divides into two, allowing the two new nuclei to belong to distinct cells.

Question 8

Prophase

Answer 8

A stage in cell division in which the chromosomes become shorter and thicker. Each chromosome is visible as two strands called chromatids. Other events which take place during prophase are the formation of the spindle fibres which will eventually pull the chromatids apart, and the breakdown of the nuclear envelope

Question 9

Metaphase

Answer 9

A stage in cell division in which chromosomes line up across the equator of the dividing cell. During metaphase, the centromeres on the chromosomes attach themselves to spindle fibres.

Question 10

Anaphase

Answer 10

A stage in cell division in which chromosomes are pulled apart by the spindle fibres. One chromosome from each pair goes to each of the ‘poles’ of the cell.

Question 11

Telophase

Answer 11

The last stage in cell division. The chromosomes unravel and a new nuclear envelope forms. At the end of telophase in mitosis, the cell has two sets of identical genetic information. Each set becomes enclosed in a
separate nucleus.

Question 12

Cell plate

Answer 12

After the nucleus of a cell has divided by mitosis, the cytoplasm divides. In a plant cell, a thin layer forms in the cytoplasm between the nuclei. This is the cell plate. Vesicles formed by the Golgi apparatus bring substances to the cell plate and it eventually develops into the new
cross wall between two cells.

Question 13

Why is mitosis so important?

Answer 13

• ensures genetic consistency > daughter cell identical to parent cell
• growth and repair
• asexual reproduction

Question 14

totipotent

Answer 14

Totipotent cells can give rise to any type of specialised cell. We all started life as a single cell or zygote. This zygote divides by mitosis. After it has undergone three mitotic divisions there will be eight cells present. Each of these totipotent embryonic stem cells can give rise to any of the specialised cells which make up the adult human body.

Question 15

Blastocyst

Answer 15

When a zygote develops into an embryo, it first divides to form a hollow ball of cells. This hollow ball of cells is called a blastocyst.

Question 16

Placenta

Answer 16

In a mammal, the placenta is an organ formed partly from the tissue of the fetus and partly from the lining of the uterus. The mother’s blood exchanges with the fetus’ blood without the two actually coming in contact with each other. Oxygen and nutrients enter the fetal blood, and carbon dioxide and waste products are removed into the mother’s blood. The placenta also produces a number of hormones that are important in controlling the events of pregnancy. Once a baby has been born, the placenta forms part of the ‘afterbirth’.

Question 17

Pluripotent

Answer 17

Pluripotent cells are able to give rise to many types of specialised cell. The cells that make up the inner mass of a blastocyst are known as pluripotent embryonic stem cells.

Question 18

Embryonic stem cells

Answer 18

Stem cells obtained from an embryo.

Question 19

Stem cell

Answer 19

A cell that has the ability to develop into different types of cell. Stem cells are found in embryos, but some stem cells are also found in adults. Blood stem cells for example are found in the bone marrow and can develop into the various types of specialised cell found in blood

Question 20

Differentiation, Differentiate

Answer 20

We all started life as an undifferentiated single cell or zygote. As we grew and developed into adults, our cells became specialised for different purposes. The process by which cells become specialised is called differentiation. When plants are produced by micropropagation, unspecialised cells differentiate to form the many types of cell in an
adult plant

Question 21

Undifferentiated

Answer 21

An undifferentiated cell is one that has not differentiated to form one of the many types of specialised cells in the adult organism. We all started life as a single cell or zygote. As we grew and developed into adults, our cells became specialised for different purposes. The process by which cells become specialised is called differentiation.

Question 22

Multipotent

Answer 22

We all started life as a multipotent single cell or zygote. As we grew and developed into adults, our cells became specialised for different purposes. The process by which cells become specialised is called differentiation. In adults, some cells still have the ability to differentiate and give rise to a variety of cell types. These cells are called multipotent stem cells. Multipotent stem cells in bone marrow develop into different sorts of blood cell.

Question 23

UK law only allowed the use of human embryos where HFEA considered their use to be necessary or desirable

Answer 23

• to promote advances in the treatment of infertility
• to increase knowledge about the causes of congenital disease
• to increase knowledge about the causes of miscarriage
• to develop more effective methods of contraception
• to develop methods for detecting gene or chromosome abnormalities in embryos before implantation

Question 24

Chromatid, Chromatids

Answer 24

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 25

Centromere

Answer 25

The region on a chromosome where
two chromatids are held together during the
early stages of cell division. The centromere is
also the region to which the spindle fibres
attach. The spindle fibres pull the chromatids
apart during anaphase.

Question 26

Spindle

Answer 26

A structure formed from protein tubules
in the cytoplasm during cell division. The
spindle fibres attach to the centromeres of the
chromosomes and shorten. As they shorten they
pull the two halves of the centromere apart. In
mitosis the centromeres split and, as a result, one
chromatid of each chromosome is pulled to
each of the poles of the dividing cell. The
spindle plays a similar part during meiosis.

Question 27

Centriole

Answer 27

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 28

Asexual reproduction, Asexually

Answer 28

Reproduction
by any means which does not involve the fusion
of gametes or sex cells. Asexual reproduction
occurs when a bacterial cell divides into two
new cells. The growth of new plants from tubers
and bulbs also involves asexual reproduction.

Question 29

Therapeutic cloning

Answer 29

Research procedure in
which it is hoped that a diploid cell could be
induced to develop so as to form a tissue or
organ needed for a transplant.