3.2 From one to many: the cell cycle

Question 1

What is interphase?

Answer 1

A time of intense and organised activity during which the cell synthesises new cell components, such as organelles and membranes, and new DNA.
Involves 3 phases - Gap 1 and 2, and synthesis

Question 2

What happens during the S phase?

Answer 2

DNA synthesis, growth and replication.

4n

Question 3

What is a chromatid?

Answer 3

The name given to a copied chromasome during mitosis.

Question 4

What happens in the G1 phase?

Answer 4

Growth (2n)

Length of this phase is variable

Question 5

What happens in the G2 phase?

Answer 5

Growth and final preparations for division. (4n)

Question 6

Why is there no interphase in the first few divisions in a developing human cell?

Answer 6

The zygote already contains the materials needed to form the first 16 or so cells

Question 7

How is DNA organised in chromosomes?

Answer 7

1) DNA double helix
2) DNA winds around histone proteins
3) DNA and histone proteins coil to form chromatin fibre
4) Chromatin fibre attaches to a protein scaffold, forming loops
5) Folding the protein scaffolding produces the condensed chromosome structure seen during nuclear division

Question 8

What happens during interphase?

Answer 8

The individual chromosomes are unravelled. This allows access to the genetic material, enabling new proteins to be synthesised. In preparation for cell division, the cell synthesises additional cytoplasmic proteins and organelles.
By the end of interphase, the cell contains enough cytoplasm, organelles and DNA to form two new cells

Question 9

What is cytokinesis?

Answer 9

Cytoplasmic division - 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 10

What is the order of mitosis?

Answer 10

Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytoplasmic division

Question 11

What happens in prophase?

Answer 11

The chromosomes condense, becoming shorter and thicker (they thicken by spiralisation), with each chromosome visible as two strands called chromatids. Apart from the occasional mutation, the two strands are identical copies of one another, produced by replication. They are joined at the centromere.
The nucleoli also decrease in size as their nucleic acid passes to certain pairs of chromatids.
During prophase, microtubules from the cytoplasm form the 3D structure called the spindle. The centrioles move around the nuclear envelope and position themselves at opposite sides of the cell.

Question 12

What signals the end of prophase and the start of the next stage?

Answer 12

The breakdown of the nuclear envelope

Question 13

What is the spindle?

Answer 13

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.

Question 14

What happens during metaphase?

Answer 14

The chromosomes’ centromeres attach to spindle fibres at the equator. When this has been completed the cell has reached the end of metaphase.

Question 15

What happens during anaphase?

Answer 15

The centromeres split. The spindle fibres shorten, pulling the two halves of each centromere in opposite directions. One chromatid of each chromosome is pulled to each of the poles. Anaphase ends when then separated chromatids reach the poles and the spindle breaks down.
A very rapid stage.

Question 16

What happens during telophase?

Answer 16

The chromosomes reach the poles of the cell, uncoil, lengthen and lose the ability to be seen clearly. The spindle fibres disintegrate and the centrioles replicate. A nuclear membrane forms around the chromosomes at each pole so that the two sets of genetic information become enclosed in separate nuclei, and the nucleoli reappear.

Question 17

Why is mitosis so important?

Answer 17

It ensures genetic consistency, with daughter cells
genetically identical to each other and to the parent cell.
It allows for growth and repair - it develops a single cell into a multicellular organism. It allows old and damaged cells to be replaced with identical new copies.
It is also used for asexual reproduction - producing genetically identical offspring.

Question 18

How does each daughter cell in mitosis contain exactly the same number and type of chromosomes as their parent cell?

Answer 18

1) DNA replication prior to nuclear division

2) The arrangement of chromosomes on the spindle and the separation of chromatids to the poles

Question 19

Why do prokaryotes not carry out mitosis or meiosis?

Answer 19

They do not possess chromosomes

Question 20

What are embryonic stem cells derived from?

Answer 20

Embryos generated by in vitro fertilisation

Question 21

When the zygote divides, what is the name of the multicellular ball of cells that is formed?

Answer 21

Blastocyst

Question 22

What does the outer layer of the blastocyst go on to form?

Answer 22

The placenta

Question 23

Describe the two cell types that make up the blastocyst.

Answer 23

Outer layer = trophoblast (eventually forms placenta)

Inner cluster = inner cell mass - goes on to form the tissues of the developing embryo

Question 24

What does pluripotent mean?

Answer 24

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

Question 25

What does totipotent mean?

Answer 25

Totipotent cells can give rise to any type of specialised cell. 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 26

What are the two important properties of embryonic stem cells?

Answer 26

1) Pluripotency | 2) Ability to self-renew / self-divide indefinitely while retaining their pluripotent state.

Question 27

What does multipotent mean?

Answer 27

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 28

What is differentiation?

Answer 28

The process by which cells become specialised

Question 29

How do the properties of plant cells differ from animal cells in terms of cell differentiation?

Answer 29

Many plants remain totipotent throughout their life. Cell differentiation is irreversible in animals, but differentiated plants can de-differentiate and then develop into a complete new plant. Totipotency of plants allows them to be reproduced using plant tissue culture.

Question 30

How does the totipotency of plants allow them to be reproduced?

Answer 30

Using plant tissue culture Small pieces of plant (explants) are surface sterilised and then placed on an agar medium with nutrients and growth regulators. The cells divide to form undifferentiated cells called a callus. The plants are genetically identical. Tissue culture allows production of large numbers of plants rapidly.

Question 31

What is therapeutic cloning?

Answer 31

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. 1) The nucleus is removed from a human egg cell 2) The nucleus from a skin cell of a patient is removed 3) The skin cell nucleus is inserted into the enucleated human egg cell. 4) Cell cleavage occurs as the embryo begins to develop in Vitro 5) The embryo reaches the blastocyst stage 6) Embryonic stem cells are extracted and grown in culture 7) The stem cells are developed into healthy cells needed by the patient 8) The healthy tissue is injected or transplanted into the patient.

Question 32

What is reproductive cloning?

Answer 32

Involves creating an animal that is genetically identical to a donor animal through somatic cell nuclear division. 1) The nucleus is removed from a human egg cell 2) The nucleus from a skin cell of a patient is removed 3) The skin cell nucleus is inserted into the enucleated human egg cell. 4) Cell cleavage occurs as the embryo begins to develop in Vitro 5) The embryo reaches the blastocyst stage 6) The blastocyst is kept intact and is implanted into the uterus of the surrogate mother 7) The resulting baby is a clone of the patient.

Question 33

What are induced pluripotent stem cells and what are their advantages?

Answer 33

They are programmed body cells to make them pluripotent. They overcome the problems of cell rejection and address ethical concerns.