Pack 2 – Stem Cells and Cell Division

Question 1

Stem cell

Answer 1

Undifferentiated (unspecialised) cells that can give rise to other specialised types of cell

Question 2

Differentiation

Answer 2

When an unspecialised cell gives rise to a more specialised cell type

Question 3

Undifferentiated

Answer 3

A cell which has not yet become specialised

Question 4

A fertillised egg

Answer 4

Zygote

Question 5

Embryonic stem cells

Answer 5

Cells at the early stages in the development of the embryo

Question 6

Totipotent

Answer 6

If cells are removed from the embryo they will differentiate into any of the 216 cell types

Question 7

Embryonic stem cell process:

Answer 7

• As the embryo develops into a multicellular body, the cells from which it is made become increasingly differentiated
• Most of them lose the capacity to develop into a wide range of cells
• Instead, they become increasingly specialised into different types of cells

Question 8

Blastocyst

Answer 8

A hollow ball of cells with an inner cell mass of about 50 cells

Question 9

When is a blastocyst formed?

Answer 9

About 5 days after fertilisation

Question 10

What are cells in the inner cell mass?

Answer 10

Pluripotent stem cells, because they can give rise to most specialised cell types but not totipotent stem cells / embryonic cells

Question 11

Adult/somatic stem cells

Answer 11

Stem cells that remain in the body of adults

Question 12

Where are adult stem cells found?

Answer 12

Bone marrow, brain, liver and skin

Question 13

What are adult stem cells

Answer 13

Multipotent

Question 14

Multipotent

Answer 14

They can differentiate into related cell types only

Question 15

Example of multipotent:

Answer 15

Bone marrow cells can differentiate into blood cells and cells of the immune system but not other cell types

Question 16

Mitosis

Answer 16

• This produces new somatic (body) cells as an organism grows and develops
• This retains the full number of chromosomes, called the diploid number (2n) (46 in humans).

Question 17

Meiosis

Answer 17

• This produces gametes (sperm and egg cells – known as germ cells or the germ line)
• Gametes have only half the number of chromosomes, called the haploid number (n) (23 in humans)

Question 18

Diploid cell

Answer 18

• (Normal body/somatic cell) has 23 pairs of chromosomes (2n) in humans
• In each pair one chromosome is from our father and the other is from our mother

Question 19

Somatic cell

Answer 19

• Contains 22 pairs of homologous chromosomes
• Homologous chromosomes are the same length, have their centromeres in the same position, have their genes in the same position

Question 20

What are the two parts of the cell cycle?

Answer 20

1. During interphase the contents of the cell double
2. During division the cell divides into two: nucleus followed by cytoplasm

Question 21

What are the 3 phases of the interphase?

Answer 21

1. G1 phase (Gap 1)
2. S (DNA synthesis) phase
3. G2 phase (Gap 2)

Question 22

G1 phase (Gap 1)

Answer 22

• The cell grows bigger
• Individual chromosomes are unravelled allowing access to genetic material ready for protein synthesis (transcription and translation) and synthesis of new organelles.

Question 23

S (DNA synthesis) phase

Answer 23

The the cell replicates its DNA ready to divide by mitosis.

Question 24

G2 phase (Gap 2)

Answer 24

The cell keeps growing. Proteins needed for cell division are made. The cell prepares genetic material for mitosis – by supercoiling DNA to form chromosomes.

Question 25

Length of interphase:

Answer 25

- It varies depending on the role of the cell - S and G2 phases of most cells remain relatively constant in duration, but the length of G1 varies among cells

Question 26

What happens after the interphase?

Answer 26

Mitosis

Question 27

Mitosis

Answer 27

- A nuclear division (division of the nucleus), so two nuclei are produced (called daughter nuclei) - The daughter nuclei are genetically identical to both the parent cell and each other - Mitosis results in two cells with diploid number of chromosomes

Question 28

4 stages of mitosis:

Answer 28

1. Prophase 2. Metaphase 3. Anaphase 4. Telophase

Question 29

Prophase

Answer 29

- Chromosomes condense - Microtubules from the cytoplasm from 3D structure called the spindle - Centrioles move around the nuclear envelope to opposite ends of the cell (forming two pieces of the spindle) - Nuclear envelope and nucleolus break down signalling the end of prophase

Question 30

Anaphase

Answer 30

- Centromeres split, separating the two sister chromatids - Spindle fibres shorten, pulling half a centromere and an attached chromatid to opposite poles - Anaphase ends with the separated chromatids at each pole

Question 31

Telophase

Answer 31

- Chromosomes decondense - A nuclear envelope forms around each group of chromosomes (enclosing two sets of genetic information in separate nuclei) - The spindle fibres also break it down

Question 32

Cytokinesis (not a stage of mitosis)

Answer 32

- The cell surface membrane constricts around the centre of the cell - A ring of protein filaments on the inside of the cell surface membrane are thought to contract and divide the cells

Question 33

Meiosis

Answer 33

- The type of nuclear division that produces gamete cells - It produces gamete cells which contain half the number of chromosomes of any other body cell

Question 34

Where does meiosis occur?

Answer 34

Ovaries and testes

Question 35

What are the two important functions of meiosis in biology?

Answer 35

1. It results in haploid nuclei necessary to maintain the diploid number of chromosomes after fertilisation 2. Creates genetic variation among offspring

Question 36

Meiosis overview:

Answer 36

- During meiosis 2 nuclear divisions give rise to 4 daughter cells with a haploid nucleus, each with its own unique combination of genetic material - The daughter cells are completely unique due to the shuffling of genetic material during meiosis known as crossing over and independent assortment

Question 37

What is an important role of meiosis?

Answer 37

It leads to genetic variation in gametes, and therefore in the offspring

Question 38

How is genetic variation created in meiosis?

Answer 38

By the shuffling of existing genetic material during independent assortment and crossing over

Question 39

Independent assortment of chromosomes in meiosis

Answer 39

- Only one chromosome from each homologous pair ends up in each gamete - The daughter cells of the 1st meiotic division contain different assortments of chromosomes due to the random alignment of the maternal and paternal chromosomes during Meiosis I (metaphase I)

Question 40

With 23 pairs of chromosomes how many genetic combinations within the sperm or ovum are possible?

Answer 40

Over 8 million

Question 41

Crossing over in meosis:

Answer 41

- During prophase I, homologous chromosomes come together in pairs - All 4 chromatids come into contact. - At the contact points (chiasmata) the chromatids break & re-join, exchanging sections of DNA - Crossing over produces chromosomes that contain new combinations of alleles from both parents (recombinants)

Question 42

What can errors in crossing over lead to?

Answer 42

Mutations which further increases variation within a species