Module 2: Section 6 - Cell Division and Cellular Organisation

Question 1

The cell cycle is the process that all body cells in multicellular organisms use to grow and divide. Please explain the cell cycle

Answer 1

1) the cell cycle starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells
2) the cell cycle consists of a period of cell growth and DNA replication, called interphase, and a period of cell division, called M phase. M phases involves mitosis (nuclear division) and cytokinesis
3) interphase (cell growth) is subdivided into three separate growth stages. These are called G1, S and G2
4) the cell cycle is regulated by checkpoints. Checkpoints occur at key points during the cycle to make sure its OK to continue.

Question 2

What is mitosis and why is it needed?

Answer 2

1) mitosis is needed for the growth of multicellular organisms and for repairing damaged tissues. It is also a method of asexual reproduction for some plants, animals and fungi
2) mitosis is really one continuous process, but it’s described as a series of division stages - prophase, metaphase, anaphase and telophase
3) interphase comes before mitosis in the cell cycle. It’s when cells grow and replicated their DNA ready for division

Question 3

Explain interphase

Answer 3

Interphase - the cell carries out normal functions, but also prepares to divide. The cell’s DNA is unravelled and replicated, to double its genetic content. The organelles are also replicated so it has spare ones, and its ATP content is increased. (ATP provides the energy needed for cell division).

Question 4

Take a break :)

Answer 4

Go make a cup of tea and just relax for a bit

Question 5

What are the 4 steps of mitosis

Answer 5

1) prophase
2) metaphase
3) anaphase
4) telophase

Question 6

Explain prophase

Answer 6

1) Prophase - the chromosomes condense, getting shorter and fatter. Tiny bundles of protein called centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle. The nuclear envelope breaks down and chromosomes lie free in the cytoplasm

Question 7

Explain metaphase

Answer 7

2) Metaphase - the chromosomes (each with two chromatids) line up along the middle of the cell and become attached to the spindle by their centromere. At the metaphase checkpoint, the cell checks that all the chromosomes are attached to the spindle before mitosis can continue

Question 8

Explain anaphase

Answer 8

3) Anaphase - the centromeres divide, separating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite ends of the cell, centromere first

Question 9

Explain telophase

Answer 9

4) Telophase - the chromatids reach the opposite poles on the spindle. They uncoil and become long and thin again. They’re now called chromosomes again. A nuclear envelope forms around each group of chromosomes, so there are now two nuclei

Question 10

Explain cytokinesis

Answer 10

Cytokinesis - the cytoplasm divides. In animal cells, a cleavage furrow forms to divide the cell membranes. There are now two daughter cells that are genetically identical to the original cell and to each other. Cytokinesis usually begins in anaphase and ends in telophase. It’s a separate process to mitosis

Question 11

Meiosis produces gametes for sexual reproduction. Explain how meiosis is involved in sexual reproduction

Answer 11

1) in sexual reproduction two gametes (an egg and a sperm) join together at fertilisation to form a zygote. The zygote then divides and develops into a new organism
2) meiosis is a type of cell division that happens in the reproductive organs to produce gametes

Question 12

Meiosis involves a reduction division - please explain this

Answer 12

1) meiosis involves a reduction division. Cells that divide by meiosis have the full number of chromosomes to start with, but the cells that are formed from meiosis have half the number. Cells with half the normal number of chromosomes are called haploid cells
2) cells formed by meiosis are all genetically different because each new cell ends up with a different combination of chromosomes

Question 13

What two divisions does meiosis involve?

Answer 13

1) meiosis involves two divisions: meiosis I and meiosis II. Meiosis I is the reduction division (it halves the chromosome number)
2) like mitosis meiosis I and meiosis II are each split into prophase, metaphase, anaphase and telophase stages

The whole of meiosis begins with interphase. During interphase, DNA unravels and replicates to produce double-armed chromosomes called sister chromatids

Question 14

How many homologous pairs do we have? Explain what homologous pairs are and where they come from please

Answer 14

Humans have 46 chromosomes in total - 23 pairs. One chromosome in each pair came from mum and one from dad, e.g. there are two number 1’s (one from mum and one from dad) and two number 2’s etc. The chromosomes that make up each pair are the same size and have the same genes, although they could have different versions of those genes (called alleles). These pairs of chromosomes are called homologous pairs.

Question 15

Explain the stages in Meiosis I (first division)

Answer 15

Prophase I: the chromosomes condense, getting shorter and fatter. The chromosomes then arrange themselves into homologous pairs and crossing-over occurs, Just like in mitosis, centrioles start moving to opposite ends of the cell, forming the spindle fibres. The nuclear envelope starts to break down.

Metaphase I: The homologous pairs line up across the centre of the cell and attach to the spindle fibres by their centromeres

Anaphase I: The spindles contract, separating the homologous pairs - one chromosome goes to each end of the cell

Telophase I: a nuclear envelope forms around each group of chromosomes

Cytokinesis (division of the cytoplasm) occurs and two haploid daughter cells are produced

Question 16

Explain what occurs in meiosis II (second division)

Answer 16

The two daughter cells undergo prophase II, metaphase II, anaphase II, telophase II and cytokinesis - which are a lot like the stages in mitosis.

In anaphase II, the pairs of sister chromatids are separated - each new daughter cell inherits one chromatid from each chromosome. Four (genetically different) haploid daughter cells are produced - these are gametes.

Question 17

Why do chromatids cross over in prophase I?

Answer 17

During prophase I of meiosis I, homologous pairs of chromosomes come together and pair up. The chromatids twist around each other and bits of chromatids swap over. The chromatids still contain the same genes but now have a different combination of alleles.

Question 18

Meiosis produces cells that are genetically different. What two main events during meiosis lead to genetic variation?

Answer 18

1) Crossing over of chromatids

2) independent assortment of chromosomes

Question 19

Explain how the crossing over of chromatids during meiosis I leads to genetic variation

Answer 19

The crossing over of chromatids in meiosis I means that each of the four daughter cells formed from meiosis contains chromatids with different alleles:

1) the chromosomes of homologous pairs come together
2) prophase I - chromatids cross over
3) one chromosome from each homologous pair ends up in each cell
4) each cell has a different chromatid and therefore a different set of alleles, which increases genetic variation in potential offspring

Question 20

How does the independent assortment of chromosomes during meiosis lead to genetic variation?

Answer 20

1) each homologous pair of chromosomes in your cells is made up of one chromosome from you mum (maternal) and one chromosome from your dad (paternal)
2) when the homologous pairs line up in metaphase I and are separated in anaphase I, it’s completely random which chromosome from each pair ends up in which daughter cell
3) so the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes
4) this is called independent assortment (separation) of the chromosomes
5) this shuffling of chromosomes leads to genetic variation in any potential offspring

Question 21

What are stems cells / what can they form and where are they found?

Answer 21

1) multicellular organisms are made up from many different cell types that are specialised for their function, e.g. liver cells, muscle cells, white blood cells
2) all these specialised cell types originally came from stem cells
3) stem cells are unspecialised cells - they can develop into different types of cell
4) all multicellular organisms have some form of stem cell
5) in humans, stem cells are found in early embryos and in a few places in adults. Stem cells in early embryos can develop into any type of human cell. Stem cells in adults can only develop into a limited range of cells

Question 22

Stem cells differentiate into specialised cells.

Stem cells divide to become what?

Answer 22

Stem cells divide to become new cells, which then become specialised

Question 23

Differentiation is?

Answer 23

The process by which a cell becomes specialised for its job is called differentiation

Question 24

In animals, adult stem cells are used to…?

Answer 24

In animals, adult stem cells are used to replace damaged cells, e.g. to make new skin or blood cells

Question 25

Plants are always growing, so why are stem cells needed?

Answer 25

Plants are always growing, so stem cells are needed to make new shoots and roots throughout their lives. Stem cells in plants can differentiate into various plant tissues including xylem and phloem

Question 26

Stem cells can renew themselves - how?

Answer 26

Stem cells are able to divide to produce more undifferentiated stem cells, i.e. they can renew themselves

Question 27

Where do adult stem cells divide and what do they differentiate to?

Answer 27

1) the main bones of the body have marrow in the centres
2) here, adult stem cells divide and differentiate to replace worn out blood cells - erythrocytes (red blood cells) and neutrophils (white blood cells that help to fight infection)

Question 28

Where are stem cells found in plants?

What do stem cells in the root and stem do?

Answer 28

1) in plants, stem cells are found in the meristems (parts of the plant where growth can take place)
2) in the root and stem, stem cells of the vascular cambium divide and differentiate to become xylem vessels and phloem sieve tubes

Question 29

Stem cells can develop into different specialised cell types, so scientists think they could be used to replace damaged tissues in a range of diseases. For example, explain why it might be possible to use stem cells to treat neurological disorders like Alzheimer’s and Parkinson’s

Answer 29

1) with Alzheimer’s, nerve cells in the brain die in increasing numbers. This results in severe memory loss. Researchers are hoping to use stem cells to regrow healthy nerve cells in people with Alzheimer’s
2) patients with Parkinson’s suffer from tremors that they can’t control. The disease causes the loss of a particular type of nerve cell found in the brain. These cells release a chemical called dopamine, which is needed to control movement. Transplanted stem cells may help to regenerate the dopamine-producing cells.

Question 30

How are neutrophils (a type of white blood cell) specialised for their functions?

Answer 30

1) neutrophils defend the body against disease. Their flexible shape allows them to engulf foreign particles or pathogens. The many lysosomes in their cytoplasm contain digestive enzymes to break down the engulfed particles

Question 31

How are erythrocytes (red blood cells) specialised for their function?

Answer 31

Erythrocytes (red blood cells) carry oxygen in the blood. The biconcave disc shape provide a large surface area for gas exchange. They have no nucleus so there’s more room for haemoglobin.

Question 32

How are epithelial cells specialised for their function?

Answer 32

Epithelial cells cover the surfaces of organs. The cells are joined by interlinking cell membranes and a membrane at their base. Ciliated epithelia (e.g. in the airways) have cilia that beat to move particles away. Squamous epithelia (e.g. in the lungs) are very thin to allow efficient diffusion of gases

Question 33

How are sperm cells specialised to their function?

Answer 33

Sperm cells (male sex cells) have a flagellum (tail) so they can swim to the egg (female sex cell). They also have lots of mitochondria to provide the energy needed to swim. The acrosome contains digestive enzymes to enable the sperm to penetrate the surface of the egg

Question 34

How are palisade mesophyll cells specialised to their function?

Answer 34

Palisade mesophyll cells in leaves do most of the photosynthesis. They contain many chloroplasts, so they can absorb a lot of sunlight. The walls are thin, so carbon dioxide can easily diffuse into the cell.

Question 35

How are root hair cells specialised to their function?

Answer 35

Root hair cells absorb water and mineral ions from the soil. They have a large surface area for absorption and a thin, permeable cell wall, for entry of water and ions. The cytoplasm contains extra mitochondria to provide the energy needed for active transport

Question 36

How are guard cells specialised to their function?

Answer 36

Guard cells are found in pairs, with a gap between them to form a stoma. This is one of the tiny pores in the surface of the leaf used for gas exchange. In the light, guard cells take up water and become turgid. Their thin outer walls and thickened inner walls force them to bend outwards, opening the stomata. This allows the leaf to exchange gases for photosynthesis

Question 37

What is a tissue?

Answer 37

A tissue is a group of cells (plus any extracellular material secreted by them) that are specialised to work together to carry out a particular function. A tissue can contain more than one cell type

Question 38

What are the four example of animal tissues you need to know?

Answer 38

1) squamous epithelium
2) ciliated epithelium
3) muscle tissue
4) cartilage

Question 39

What are two examples of plant tissues you need to know?

Answer 39

1) xylem tissue

2) phloem tissue

Question 40

What is squamous epithelium and where is it found?

Answer 40

Squamous epithelium is a single layer of flat cells lining a surface. It’s found in many places, including the alveoli in the lungs

Question 41

What is ciliated epithelium and where is it found?

Answer 41

Ciliated epithelium is a layer of cells covered in cilia. It’s found on surfaces where things need to be moved - in the trachea for instance, where the cilia waft mucus along

Question 42

What is muscle tissue and where is it found?

Answer 42

Muscle tissue is made up of bundles of elongated cells called muscle fibres. There are three different types of muscle tissue: smooth (e.g. found in the lining of the stomach wall), cardiac (found in the heart) and skeletal (which you use to move). They’re all slightly different in structure

Question 43

What is cartilage and where is it found? How is it formed?

Answer 43

Cartilage is a type of connective tissue found in the joints. It also shapes and supports the ears, nose and windpipe. It’s formed when cells called chondroblasts secrete an extracellular matrix (a jelly like substance containing protein fibres) which they become trapped inside

Question 44

What is xylem tissue / what does it do / what does it contain?

Answer 44

Xylem tissue is a plant tissue with two jobs - it transports water around the plant, and it supports the plant. It contains hollow xylem vessel cells, which are dead, and living parenchyma cells

Question 45

What is phloem tissue / what does it do / how is it arranged?

Answer 45

Phloem tissue transports sugars around the plant. It’s arranged in tubes and is made up of sieve cells, companion cells, and some ordinary plant cells. Each sieve cell has end walls with holes in them, so that sap can move easily through them. These end walls are called sieve plates

Question 46

An organ is a group of different tissues that work together to perform a particular function. Explain the two examples of the lungs and a leaf

Answer 46

The lungs - they contain squamous epithelial tissue (in the alveoli) and ciliated epithelial tissue (in the bronchi, etc.). They also have elastic connective tissue and vascular tissue (in the blood vessels).

Leaves - they contain palisade tissue for photosynthesis, as well as epidermal tissue (to prevent water loss from the leaf), and xylem and phloem tissues in the veins

Question 47

Different organs make up an organ system. Explain the two examples of the respiratory system and the circulatory system?

Answer 47

The respiratory system is made up of all the organs, tissues and cells involved in breathing. The lungs, trachea, larynx, nose, mouth and diaphragm are all part of the respiratory system

The circulatory system is made up of organs involved in blood supply. The heart, arteries, veins and capillaries are all parts of this system

Question 48

Take a break

Answer 48

You’re doing a really good job, just relax