Cell division, cell diversity and cellular organisation

Question 1

Define mitosis.

Answer 1

During mitosis, the parent cell divides to produce two daughter cells, each containing an exact copy of the DNA of the parent cell.

Question 2

Why is mitosis important? (4)

Answer 2

This is important for growth
replacement of old cells to repair body tissue
asexual reproduction
To make 2 new genetically identical daughter cells (with the same DNA as the parent cell).

Question 3

What is the first phase of mitosis? What happens in it?What does the cell look like (see summary sheet 2.2 for this)

Answer 3

Replication of organelles
Protein synthesis
DNA replicates
DNA decondensed

Question 4

What is the second phase of mitosis? What happens in it?What does the cell look like (see summary sheet 2.2 for this)

Answer 4

DNA coils up (condenses) – becomes more visible
Nuclear envelope breaks down
Centrioles migrate to opposite poles of the cell and produce spindle fibres which attach to the centromeres of the chromsomes

Question 5

What is the third phase of mitosis? What happens in it?What does the cell look like (see summary sheet 2.2 for this)

Answer 5

Spindle fibres move the chromosomes so that they line up along the equator of the cell

Question 6

What is the fourth phase of mitosis? What happens in it?What does the cell look like (see summary sheet 2.2 for this)

Answer 6

Spindle fibres shorten
This separates sister chromatids so that they are pulled to opposite poles of the cell

Question 7

What is the fifth phase of mitosis? What happens in it?What does the cell look like (see summary sheet 2.2 for this)

Answer 7

Nuclear envelopes reform
Chromosomes decondense

Question 8

What is cytokinesis and when does it occur?

Answer 8

Cytokinesis takes place after telophase. This is when the cytoplasm divides so two separate cells are formed

Question 9

A sample of epithelial tissue from the small intestine of an animal was analysed.
Some of the cells had 8.4 units of DNA, others had only 4.2 units. Use your knowledge of the cell cycle to explain why some cells had 8.4 units of DNA and others had only 4.2 units?

Answer 9

The cells with 8.4 units are cells with twice DNA content because it had replicated its DNA
The cells are in late interphase/prophase / metaphase / anaphase
The cells with 4.2 are cells where DNA was not replicated
The cells are in early interphase / telophase OR the cell has just divided / finished mitosis

Question 10

Draw a chromosome at the beginning of mitosis. Describe and explain the appearance of the chromosome.(4)

Answer 10

DNA is replicated.
DNA is coiled up (condensed) (to make visible)
It is made of two identical sister chromatids
Chromatids are held together at the centromere
see SS2.2 for drawing

Question 11

Why do we take cells from the root tip?

Answer 11

The root tip is the region where mitosis/cell division occurs

Question 12

Why do we squash the root tip? And why is important?

Answer 12

We squash the root tip to make the tissue layer thin. This allows light through.

Question 13

Why is a stain used?

Answer 13

To distinguish chromosomes / chromosomes not visible without stain

Question 14

How many fields of view should you observe and what’s important about the selection of the field of view?

Answer 14

At least three fields of view should be chosen and they should not overlap

Question 15

What equation do we use to calculate the mitotic index?

Answer 15

Mitotic index = number of cells in mitosis/total number of cells

Question 16

Define meiosis.

Answer 16

Meiosis produces daughter cells that are genetically different from each other.

Question 17

What is the definition of a haploid cell? How many chromosomes does a human haploid cell? Give an example of a haploid cell?

Answer 17

Haploid cells contain just one copy of each chromosome.
In humans haploid cells have 23 chromosomes.
The gametes are examples (egg and sperm cell).

Question 18

Why is it so important that gametes are haploid?

Answer 18

TO MAINTAIN CHROMOSOME NUMBER AT FERTILISATION

Question 19

What is a the definition of a diploid cell? A human diploid cell has how many chromosomes? Draw an example

Answer 19

Compared with diploid cells which contain 2 copies of each chromosome.
In humans there are 46 chromosomes (23 pairs) in a diploid cell

Question 20

What is an allele?

Answer 20

different forms of the gene

Question 21

What are homologous chromosomes and explain why they are not genetically identical?

Answer 21

Homologous chromosomes have the same genes at the same loci
They are not genetically identical as they different forms of the gene (alleles).

Question 22

At what division does crossing over occur? What is crossing over? Why does crossing over occur?

Answer 22

Crossing over happens during the first division of meiosis.
Crossing over is when the ‘arms’ of homologous pairs twisted up.
The point at which the cross over is called a chiasma
This leads to the exchange of alleles between homologous chromosomes
THIS IS A RARE PROCESS

Question 23

At what division does independent assortment occur? What is it? And why does it occur?

Answer 23

Independent assortment happens during the first division of meiosis.
It is when the homologous pairs separate and go to opposite poles of the cell.
Independent assortment means that the combination of chromosomes that go into each gamete is random.

Question 24

Give 2 reasons that it is vital for meiosis to take place

Answer 24

To make gametes which are haploid – so that when fertilisation takes place the chromosome number is maintained from one generation to the next
To create genetic variation in gametes and offspring

Question 25

Describe what happens to chromosomes in meiosis.

Answer 25

DNA replication takes place so that each chromosomes replicates
The chromosomes become condensed

Homologous chromosomes pair up (forming bivalents)
Crossing-over can occur where there is an exchange of alleles between homologous pairs
spindle fibres attach to the chromosomes at their centromere 

so the bivalents line up at the equator of the cell
The spindle fibres contract causing the homologous chromosomes to separate and be pulled to opposite poles of the cell
In the 2nd division of meiosis the sister chromatids are separated

Question 26

HOW does meiosis cause variation?

Answer 26

1. Crossing-over
2. Independent segregation of homologous chromosomes in meiosis I (1st division)
3. Independent segregation of chromatids in meiosis II (2nd division)

Question 27

Explain the advantage of variation?

Answer 27

Different adaptations / some better adapted;

Some survive
To reproduce;

Pass on gene / allele;

Allows for survival in changing environment / different environment

Question 28

What are some differences between mitosis and meiosis?

Answer 28

mitosis: Maintains the same chromosome number as in the parent nucleus
meiosis:Reduces the chromosome number
mitosis:Homologous chromosomes do not pair up
meiosis:Homologous chromosomes associate in pairs
mitosis:No crossing-over
meiosis:Crossing-over / chiasmata formation
mitosis:One division / → 2 offspring cells
meiosis:Two divisions / → 4 offspring cells
mitosis:Genetically identical cells produced
meiosis:Genetically different cells produced

Question 29

What is the first stage of meiosis? Is the cell diploid or haploid? What happens in this stage? In this stage, what are:
1. No. of chomosomes
2. No. of homologous pairs
3. Amount of DNA

Answer 29

The cell starts
Number of chromosomes = 4
Number of homologous pairs = 2
Amount of DNA = 60 (in arbitrary units)
DIPLOID

Question 30

What is the second stage of meiosis? Is the cell diploid or haploid? What happens in this stage? In this stage, what are:
1. No. of chomosomes
2. No. of homologous pairs
3. Amount of DNA

Answer 30

After DNA replication
DNA replication happens, so each chromosome is made of 2 identical sister chromatids held together by a centromere.

The homologous pairs then line up next to each other along the equator of the cell
Number of chromosomes = 4
Number of homologous pairs = 2
Amount of DNA = 120 (in arbitrary units)
Diploid

Question 31

What is the third stage of meiosis? Is the cell diploid or haploid? What happens in this stage? In this stage, what are:
1. No. of chomosomes
2. No. of homologous pairs
3. Amount of DNA

Answer 31

After the first meiotic division
The homologous pairs are separated by the spindle fibres which pull them to opposite poles of the original cell.

Each daughter cell that forms is haploid.

(NOTE: crossing over and independent segregation occur during the first meiotic division)
Number of chromosomes = 2
Number of homologous pairs = 0
Amount of DNA = 60 (in arbitrary units)
Haploid

Question 32

What is the fourth stage of meiosis? Is the cell diploid or haploid? What happens in this stage? In this stage, what are:
1. No. of chomosomes
2. No. of homologous pairs
3. Amount of DNA

Answer 32

After the second meiotic division
The sister chromatids are separated by the spindle fibres which pull them to opposite poles of the original cell.Number of chromosomes = 2
Number of homologous pairs = 0
Amount of DNA = 30 (in arbitrary units)
Haploid

Question 33

What is the cell cycle?

Answer 33

G1, S, G2, mitosis, cytokinesis
G = growth, S= synthesis

Question 34

In what stage of the cell cycle is DNA replicated?

Answer 34

S phase

Question 35

How is the cell cycle regulated in:
1. g1 phase
2. s phase
3. g2 phase
4. metaphase

Answer 35

During G1 phase - chromosomes are checked for damage. If damage is detected then the cell does not advance into the S phase until repairs have been made
During S phase - chromosomes are checked to ensure they have been replicated. If all the chromosomes haven’t been successfully replicated then the cell cycle stops
During G2 phase - an additional check for DNA damage occurs after the DNA has been replicated. The cell cycle will be delayed until any necessary repairs are made
During metaphase - the final check determines whether the chromosomes are correctly attached to the spindle fibres prior to anaphase

Question 36

Describe the role of specialised cells in multicellular organisms

Answer 36

Every cell is specialised to perform its function as best as possible

Question 37

Describe the specialisations of the following cell types in animals: erythrocytes (red blood cells)

Answer 37

Carry oxygen from lungs to respiring tissues
Contain large amounts of haemoglobin –> hold oxygen
Many organelles including nucleus are not present –> more space for haemoglobin
Biconcave shape –> maximises surface area and so maximises rate of diffusion

Question 38

Describe the specialisations of the following cell types in animals: neutrophils (phagocyte)

Answer 38

A type of White Blood Cell that ingests and destroys invading microorganisms
Have large amounts of Lysosomes –> contain enzymes that kill ingested cells –> makes their Cytoplasm appear Granular
Have a Lobed Nucleus, which allows for greater flexibility

Question 39

Describe the specialisations of the following cell types in animals: sperm cells

Answer 39

Male sex cell involved in sexual reproduction. They travel toward the female’s Egg and attempt to fertilise it
Have an Undulipodium (tail) which moves by energy generated by many mitochondria and propels the cell
Head of the cell contains an Acrosome, which is a specialised Lysosome –> releases enzymes so that the Sperm Cell can penetrate the Ovum Coat of the Egg
Very small and thin –> aids movement

Question 40

Describe the specialisations of the following cell types in plants: Palisade cells

Answer 40

In the palisade layer of the leaf
Specialised for carrying out Photosynthesis since they contain large amounts of Chlorophyll, and their long shape maximises light absorption.

Question 41

Describe the specialisations of the following cell types in plants: Root hair cells

Answer 41

Found in the roots of plants
Absorb water and minerals in the soil
Have a large surface area, due to their hair-like projections, –> eases uptake
have a large amount of Mitochondria, –> provide more energy for Active Transport.

Question 42

Describe the specialisations of the following cell types in plants: Guard cells (forming stomata)

Answer 42

In lower epidermis
A Stoma is made from two Guard Cells
These have spiral thickenings of Cellulose which mean that when the Cells are Turgid, the Stoma opens, and when they are Flaccid, the Stoma closes

Question 43

Describe the specialisations of the following animal tissues: squamous epithelium (alveoli)

Answer 43

Flat cells that form a single thin, smooth layer that lines tubes where diffusion occurs
Held in place by a Basement Membrane
Composed of Collagen and Glycoproteins, secreted by Epithelial Cells, that binds them to Connective Tissue.

Question 44

Describe the specialisations of the following animal tissues: ciliated epithelium (lung airways)

Answer 44

Column shaped cells, that cover many surfaces
Have tiny projections on their exposed surface, called Cilia, –> beat in a synchronised pattern to move Mucus, produced by Goblet Cells, along the surface.

Question 45

What are the four main groups of tissue in animal cells? What do they do?

Answer 45

Epithelial Tissue - Linings and layers
Connective Tissue - Holding structures together
Muscle Tissue - Actuation of movement
Nervous Tissue - Communication via electrical signals.

Question 46

Describe the specialisations of the following plant tissues: Xylem

Answer 46

Xylem transports water and minerals up the plant
found in Vascular Bundles
It’s a tissue
Xylem and phloem in vascular bundle separated by Cambium - undifferentiated meristemic tissues –> differentiate into xylem/phloem cells
Xylem Tissue consists of Xylem Vessels and Parenchyma Cells
Xylem Vessels are made of dead cells that have become elongated and reinforced and waterproofed with deposits of Lignin
The dead cells also do no have any ends, so that successive cells form a tubes with wide Lumen. Xylem transports water up the plant can helps to support it

Question 47

Describe the specialisations of the following plant tissues: Phloem

Answer 47

Phloem carry sugars up and down the plant
Phloem Tissue is made up of Sieve Tubes and Companion Cells
Sieve tubes line up and their ends form Sieve Plates through which substances can move
Companion Cells lie next to Sieve Tube Cells and allow them to stay alive

Question 48

Summarise the role of stem cells in organisms

Answer 48

Due to their ability to differentiate into multiple cell types, stem cells have huge potential in the therapeutic treatment of disease

Question 49

Discuss (with examples) the different types of stem cell potency

Answer 49

Only 2 embryonic :
Totipotent if taken in the first 3-4 days after fertilisation
Pluripotent if taken on day 5
Multipotent adult stem cells

Question 50

State the sources of human stem cells

Answer 50

Embryonic Stem Cells - from waste (fertilised) embryos from in vitro fertilisation treatment
Adult Stem Cells - from bone marrow (blood cells)
and brain (different types of neural and glial cells)

Question 51

Suggest how stem cells can be used to repair damaged or degenerated tissue

Answer 51

Stem cells can be grown and differentiated into the necessary cells.