Cell Division

Question 1

How many chromosomes do we have in our somatic cells

Answer 1

46 chromosomes in 23 pairs
- one of each pair is inherited from you mother and from your father

Question 2

How many chromosomes do gametes have (sex cells)

Answer 2

23 chromosomes
- they only have one of each pair

Question 3

Describe mitosis

Answer 3

• mitosis is an asexual method of cell division and only occurs in the body’s normal somatic cells
• both daughter cells produced are genetically identical to the parent cell
• mitosis produces additional cells needed for growth and replacement

1) DNA replication:
the parent cell replicates it’s DNA to form 2 copies of each chromosome in the nucleus
- the cell grows and copies it’s other internal structures (mitochondria and ribosomes)

2) Mitosis
One set of chromosomes is pulled to each side of the cell and the nucleus divides

3) Division
The cytoplasm and cell membrane divide to form 2 identical cells

Question 4

What are stem cells

Question 5

What is differentiation

Answer 5

• the cells during early development of animal and plant embryos are in specialised (stem cells)
• the can become any type of cell that’s needed
• stem cells usually become specialised very early in life, when a cell becomes specialised, it has differentiated

Question 6

What is a stem cell

Answer 6

An undifferentiated cell which can give rise to more cells of the same type and differentiate to form other types of cells

Question 7

What are the different stem cells and where are they found

Answer 7

• embryonic stem cells are found in the embryo during its early stages
• they differentiate earlier on in life
• adult stem cells are found in bone marrow and only differentiate to form cells found in our blood
  (Rbc, wbc, platelets)
• plant stem cells are found in meristem tissues of plants
• these stem cells can differentiate into any type of plant tissue at any point of the plant’s life
• we can use meristem tissue to produce clones of plants

Question 8

Explain cloning of plants using stem cells

Answer 8

• plants can be cloned by using just a small piece of meristem tissue
• in the right conditions plant cells will become unspecialised and undergo mitosis many times, forming multiple undifferentiated cells
• given different conditions, these cells can then differentiate into a new tiny plant, identical to the original one

Question 9

Why can’t animals be cloned using stem cells

Answer 9

• animal stem cells differentiate permanently and early during embryo development so they can’t change back

Question 10

Examples of meristem tissues

Answer 10

Shoot and root tips

Question 11

Explain meiosis

Answer 11

• sexual method of cell division
• produces 4 genetically different sex cells
• the female gametes are made in the ovaries and male gametes are made in the testes

In meiosis the chromosome number are reduced by half:

• A cell in the reproductive organs looks just like a normal somatic cell before it divides into gametes (has 46 chromosomes)
  1) all the chromosomes are copied
  2) the cell divides in two, and the new cells immediately divide again to form 4 gametes
• each gamete has only one chromosome from the pair so total of 23

Question 12

Explain fertilisation

Answer 12

• more variation is added when fertilisation takes place
• each gamete has a single set of chromosomes so when 2 gametes join during fertilisation, the single cell formed has all 46 chromosomes which makes it a somatic cell
• the combination of genes for every fertilised ovum is unique

After fertilisation:
- after fertilisation is complete, the unique cell begins to divide by mitosis to form a new individual
- as the organism develops the cells differentiate to form different kinds of cells.
- mitosis will continue long after the fetus is fully developed and the baby is born, as adults also need tn repair and replace worn out cells

Question 13

Problem with taking and using stem cells

Answer 13

• many embryonic stem cells come from aborted embryos, raising ethical issues

Question 14

What are tumours?

Answer 14

• the cells in the body divide on a regular basis in a set sequence, this is the cell cycle
• a tumor forms when control of the sequence is lost and the cell grows in an abnormal, uncontrolled way
• this results in a mass of abnormally growing cells invading surrounding tissues

Question 15

What is the interphase stage of the cell cycle

Answer 15

• after cell division through mitosis, cells enter an interphase stage
• this is a period of non-division. During this time cells get bigger, increase their mass, carry out regular cell activities and replicate their dna for the next division

Question 16

What benign tumours

Answer 16

• grow in one place and do not invade other tissues
• contained within a membrane
• can grow very large very quickly
• can cause pressure and damage to organs in the body

Question 17

What are malignant tumours

Answer 17

• can spread around the body and invade healthy tissues
• known as cancer
• divide more rapidly than normal cells
• live longer than normal cells
• the initial tumour may split up and release malignant cells into the bloodstream
• the cells circulate and are carried around the body where they spread to different tissues and organs, forming secondary tumours

Question 18

What causes cancer
(What causes the cell cycle to become uncontrollable)

Answer 18

Mutations:
- most cancer is the result of mutations, changes in genetic material of the cell causing it to divide more than normal
- chemicals such as abestos and tar found in tobacco smoke can cause these mutations.
- cancer causing chemicals like this are called carcinogens

Ionising radiation:
- radiation like UV light and X-rays can interrupt the normal cell cycle causing tumours

Virus infections are also a cause of cancer