Cell division

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Apuntes de la lección

Chromosomes

The nucleus of a eukaryotic cell contains thread-like structures called chromosomes.
Chromosomes are made from long, coiled strands of DNA; each chromosome contains one DNA molecule.
A chromosome carries many genes, and different versions of the same gene are called alleles.
In body cells of diploid organisms, chromosomes are found in pairs — one inherited from the mother, one from the father.
Different species have different numbers of chromosomes: humans have 46 chromosomes (23 pairs); horses have 64 (32 pairs).
Red blood cells are an exception — they lose their nucleus, so they have no chromosomes.
Before a cell divides, its DNA is doubled, giving the characteristic 'X' shape seen in micrographs.

Zooming from cell to gene: chromosomes in the nucleus, made of DNA wrapped around proteins.

The Cell Cycle

The cell cycle is the series of stages in the life of a cell, made up of three overall parts.
Interphase (growth phase): just before mitosis, the DNA copies itself exactly and the number of sub-cellular structures such as ribosomes and mitochondria increases.
Mitosis: the nucleus divides — chromosomes line up along the centre of the cell and cell fibres pull them apart.
Cytokinesis: the cytoplasm and cell membrane divide, producing two daughter cells, each with a copy of every chromosome.
During interphase, two copies of each chromosome are produced; these copies are called chromatids and initially remain attached to each other.

Mitosis

Mitosis is a nuclear division that produces two genetically identical diploid daughter cells.
In humans, the diploid number is 23 pairs of chromosomes (46 in total).
All body cells (not gametes) are produced by mitosis of the zygote.
Mitosis is needed for growth (producing new cells), repair (replacing damaged or dead cells) and asexual reproduction (producing offspring genetically identical to the parent).
During mitosis, chromosomes line up along the equator of the cell, the chromatids are separated and pulled to opposite ends, and a new nucleus forms around each group of chromosomes.
After mitosis, cytokinesis divides the cytoplasm and cell membrane, forming two genetically identical daughter cells.
If you start with one cell and it divides n times, you end up with 2ⁿ cells (e.g. 7 divisions give 2⁷ = 128 cells).

Mitosis

Uncontrolled Cell Division and Cancer

Cells divide by mitosis to produce more cells — essential for growth and to repair damaged cells.
For example, cells of the intestines constantly divide to replace those shed as food moves through the digestive system.
The cell cycle is regulated by many different genes so that cells divide only when needed and stop when required.
Cancer is caused by mutations in DNA that lead to uncontrolled cell growth and division, forming a tumour (a mass of cells).
Benign tumours are growths of abnormal cells contained in one area, usually within a membrane; they do not invade other parts of the body and are not cancerous.
Malignant tumours are cancerous — their cells invade neighbouring tissues and spread via the blood and lymphatic system to form secondary tumours.
Malignant tumours are dangerous because they disrupt the functioning of the organ they originate in and the organs they spread to.

Stem Cells

A stem cell is an undifferentiated cell capable of dividing to produce many more cells of the same type.
Stem cells can give rise to other cell types through the process of differentiation.
Embryonic stem cells are found in the very early embryo and can differentiate into any type of body cell.
Adult stem cells are partially specialised and can only differentiate into a few cell types (mainly blood cells); they are used to replace damaged cells and for growth.
Adult stem cells are found in bone marrow, skin, and some other organs such as the liver and brain.
Umbilical cord blood contains stem cells that can differentiate into blood cells, muscle and nerve tissue.
In plants, stem cells are found in the meristem tissue at the tips of roots and shoots; they can differentiate into any type of plant cell throughout the plant's life.

Stem Cells in Medicine

Adult stem cells can be cultured in the lab and made to differentiate into specialised cells (mainly blood cells) but into fewer types than embryonic stem cells.
Stem cells are already used to treat some diseases such as leukaemia, and have potential to treat diabetes and paralysis in the future.
Embryonic stem cells can be stimulated to differentiate into most types of specialised cell, so they are potentially very effective for treating diseases or repairing damaged organs.
Therapeutic cloning produces an embryo with the same genes as the patient, so stem cells from it are not rejected by the patient's body.
Benefits of stem cell therapy include treating a wide variety of diseases, reducing the risk of organ rejection, and avoiding the wait for an organ donation.
Risks include transfer of viral infection from cultured cells, accumulation of mutations that could lead to cancer, and low numbers of stem cell donors.
Ethical issues include whether it is right to use unused IVF embryos, create and destroy embryos through therapeutic cloning, and whether an embryo should be treated as a person.

Plant Stem Cells and Cloning

Plant stem cells can be obtained easily and without ethical objection from meristems.
They can be used to produce clones of plants quickly and economically — clones can be produced in weeks at relatively low cost.
Rare species at risk of extinction can be cloned to protect them and maintain biodiversity.
The plants produced are clones, so although numbers increase, genetic diversity within the clones is low.
Crop plants with special features such as disease or pest resistance can be cloned to produce large numbers of identical plants for farmers.
This reduces the need to use chemicals such as pesticides, which has environmental benefits.

Growth and Cell Differentiation

Growth is a permanent increase in size or mass.
Both plants and animals grow via cell division (mitosis) and cell differentiation.
Plants also grow through cell elongation, where hormones such as auxin cause cells to grow longer in response to stimuli like sunlight.
As a multicellular organism develops, its cells differentiate to form specialised cells with structural adaptations for specific functions.
Almost all cells in a multicellular organism contain the same genetic information, but each cell has a different set of active genes.
In animals, most cells differentiate early in development and lose the ability to differentiate further; cell division is mainly for repair and replacement in mature animals.
In plants, many types of cell retain the ability to fully differentiate throughout the life of the plant.

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Preguntas de práctica

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1.Which of the following statements indicates that a cell is preparing to divide?
Easy
- AThe nucleus has visible pores
- BThe nucleus is visible and 'X' shaped
- CThe chromosomes are visible and 'X' shaped
- DThere is a haploid number of chromosomes
2.A diploid nucleus from the body cell of a dog contains 78 chromosomes. How many chromosomes would be found in the nucleus of a dog's gamete?
Medium
- A156
- B78
- C39
- D23
3.Which of the following statements about stem cells is not true?
Medium
- AMeristem cells can differentiate into any type of plant cell throughout the life of the plant
- BAdult stem cells are partially specialised
- CUmbilical cord stem cells can develop into any of the different cell types in the human body
- DEmbryonic stem cells could be used to cure type 1 diabetes
4.Which of the following is a true statement about mitosis?
Medium
- AMitosis produces genetically identical cells
- BA cell divides twice in the process of mitosis
- CDaughter cells from mitosis are haploid
- DDNA is replicated after mitosis
5.A cell is observed under a microscope as it prepares for mitosis. Which of the following observations indicates that the cell is preparing to divide?
Medium
- AThe nucleus has visible pores
- BThe nucleus is visible and 'X' shaped
- CThe chromosomes are visible and 'X' shaped
- DThere is a haploid number of chromosomes
6.Which of the following statements about mitosis are true? (select all that apply)
Medium
- AMitosis produces genetically identical cells
- BA cell divides twice in the process of mitosis
- CDaughter cells from mitosis are haploid
- DDNA is replicated before a cell can divide
- EMitosis is important for growth and repair
7.In mitosis, the chromosome number is halved in the daughter cells.
Easy
True or false?
8.DNA replication occurs during interphase, before mitosis begins.
Easy
True or false?

Más preguntas

9.Draw a line to link each stage of the cell cycle with the correct description.
Medium
- Interphase
- Mitosis
- Cytokinesis
- DNA copies itself exactly, forming X-shaped chromosomes
- Chromosomes line up along the centre and cell fibres pull them apart
- The cytoplasm and cell membrane divide to produce two daughter cells
10.Place the statements about mitosis in the correct order, from first to last.
Medium
- Cell growth and DNA replication occurs
- Chromosomes are pulled to either end of the cell
- Two genetically identical daughter cells are produced
- Cytoplasm splits
11.A cell cycle lasts 10 hours in total. Stage 2 takes up 30% of the cycle. How long does Stage 2 last?
Medium
- A3 hours
- B30 minutes
- C0.3 hours
- D7 hours
12.Stem cells from adult bone marrow can form many types of cell including blood cells.
Easy
True or false?
13.Which of the following are advantages of using stem cells from plants? (select all that apply)
Medium
- AClones can be produced quickly
- BPopulations of species under threat can be cultivated
- CThere will be low genetic diversity in the cloned plants
- DCrops with resistance to pests can be cloned
- EThey can differentiate into any type of plant cell
14.Link each stem cell type with the issue that might be involved in its use.
Medium
- Embryonic stem cells
- Adult stem cells
- Plant meristem cells
- Ethical objections to destruction of embryos
- Painful operation to collect cells
- No ethical objections and easy to obtain
15.A cell divides by mitosis 4 times. How many cells are produced in total from one original cell?
Hard
- A8
- B16
- C32
- D64
16.A student views a photomicrograph of an onion root tip. Most cells show uncoiled, thread-like chromosomes that are not distinct. A few cells show chromosomes that are visible and 'X' shaped. Which statement best explains this observation?
Medium
- AThe 'X' shaped chromosomes are in cells that have replicated their DNA and are preparing to divide
- BThe 'X' shaped chromosomes are in cells that have just finished dividing and are growing
- CThe thread-like chromosomes are in cells that have lost their DNA
- DThe 'X' shaped chromosomes are in cells that have halved their chromosome number
17.A student counts cells in a root tip. The results are: Non-dividing cells 27, Stage 1 14, Stage 2 11, Stage 3 2. Total 54 cells. A complete cell cycle takes 15 hours. Calculate the length of time Stage 1 lasts. Give your answer in hours to 2 significant figures.
Medium
- A3.9 hours
- B4.2 hours
- C3.5 hours
- D2.8 hours
18.Which of the following statements about benign and malignant tumours are true? (select all that apply)
Hard
- ABenign tumours are contained in one area, usually within a membrane
- BMalignant tumours invade neighbouring tissues and spread via the blood and lymphatic system
- CBenign tumours are cancerous
- DMalignant tumours can form secondary tumours in other parts of the body
- EBenign tumours often spread to other organs
19.In therapeutic cloning, an embryo is produced with the same genes as the patient.
Easy
True or false?
20.Match each stem cell source with its potential to differentiate.
Medium
- Embryonic stem cells
- Adult stem cells
- Meristem cells
- Can differentiate into all types of specialised cells in the body
- Can differentiate into a limited range of cell types, mainly blood cells
- Can differentiate into any type of plant cell throughout the plant's life
21.Place the stages of the cell cycle in the correct order, starting with the first.
Medium
- Growth phase (DNA replication and increase in sub-cellular structures)
- Mitosis (nucleus divides)
- Cytokinesis (cytoplasm and cell membrane divide)
22.Which stage of the cell cycle is the slowest?
Medium
- ANon-dividing
- BStage 1
- CStage 2
- DStage 3
23.A cell in the root tip of a plant has 24 chromosomes. It divides by mitosis. How many chromosomes will each daughter cell have?
Hard
- A24
- B12
- C48
- D6
24.Which of the following are potential risks or issues associated with using stem cells in medicine? (select all that apply)
Medium
- AStem cells cultured in the lab could become infected with a virus
- BCultured stem cells may accumulate mutations and become cancerous
- CStem cells from a patient's own body are always rejected
- DThere is a lack of peer-reviewed clinical evidence for some stem cell treatments
- EStem cell treatments are always completely free of ethical concerns
25.In plants, meristem cells can differentiate into any type of plant cell throughout the life of the plant.
Medium
True or false?
26.Match each disease with how stem cells could be used to treat it.
Medium
- Type 1 diabetes
- Paralysis
- Stem cells could be differentiated into insulin-producing pancreatic cells
- Stem cells could be differentiated into nerve cells (neurones)
27.A student prepares a slide of an onion root tip to observe dividing cells. Why is the root tip used rather than a leaf from the same plant?
Medium
- ARoot tip cells contain meristem tissue where many cells are actively dividing
- BLeaf cells contain no chromosomes so division cannot be seen
- CRoot tip cells are haploid so mitosis is easier to observe
- DLeaf cells divide more slowly so fewer stages are visible
28.In a root tip squash, a student counts 68 cells in one field of view. 36 cells are in the growth phase, 15 in Stage 1, 9 in Stage 2, 6 in Stage 3 and 2 in Stage 4. Which conclusion is best supported?
Hard
- AThe growth phase is the longest part of the cell cycle
- BStage 4 is the longest part of the cell cycle
- CAll stages last the same length of time
- DMitosis takes longer than the growth phase
29.A student views a photomicrograph of an onion root tip. Most cells show uncoiled, thread-like chromosomes that are not distinct. A few cells show chromosomes that are short, thick and clearly visible. What is the best explanation?
Medium
- AMost cells are not dividing, so their chromosomes are uncoiled; the few visible ones are preparing to divide
- BMost cells are dead, so their chromosomes have broken down
- CThe uncoiled chromosomes are found only in dividing cells
- DThe distinct chromosomes show cells that have finished dividing and lost their DNA
30.A student wants to make the chromosomes in an onion root tip slide more visible under a light microscope. Which step is most useful?
Medium
- AStaining the root tip before squashing it on the slide
- BAdding more water to the slide to dilute the cells
- CFreezing the root tip before viewing it
- DUsing a lower power objective lens
31.A cell in the root tip of a plant has 24 chromosomes. It completes one cell cycle. Which statement about the resulting daughter cells is correct?
Hard
- AEach daughter cell has 24 chromosomes because the chromosome number is maintained
- BEach daughter cell has 12 chromosomes because the number is halved
- CEach daughter cell has 48 chromosomes because the DNA was doubled
- DEach daughter cell has 24 pairs of chromosomes because the DNA was copied
32.Which of the following are important when preparing a slide of an onion root tip to observe mitosis? (select all that apply)
Medium
- ACutting the very tip of the root where cells are dividing
- BStaining the tissue to make chromosomes visible
- CSquashing the tissue to spread the cells into a single layer
- DUsing a root tip that has been left in the dark for several days
- EViewing the slide under a high power objective lens
33.A student counts cells in a root tip. The results are: Non-dividing cells 27, Stage 1 14, Stage 2 11, Stage 3 2. Total 54 cells. A complete cell cycle takes 15 hours. Which statements are correct? (select all that apply)
Hard
- AStage 1 lasts about 3.9 hours
- BStage 3 lasts about 0.56 hours
- CStage 3 lasts about 33 minutes
- DStage 2 lasts about 3.1 hours
- EThe non-dividing cells are in the growth phase
34.In a root tip squash, the number of cells seen in a stage of the cell cycle is proportional to the length of time that stage lasts.
Medium
True or false?
35.In a root tip squash, cells whose chromosomes appear as long, uncoiled threads are actively dividing.
Medium
True or false?
36.A student describes the practical steps used to make a slide of an onion root tip to observe mitosis. Place the steps in the correct order, from first to last.
Medium
- Cut the very tip of the root where cells are actively dividing
- Place the root tip on a slide and add a stain
- Squash the tissue to spread the cells into a single layer
- View the slide under a light microscope on high power
37.Match each practical observation from a root tip squash with the correct interpretation.
Hard
- Most cells show uncoiled, thread-like chromosomes
- A few cells show short, thick, X-shaped chromosomes
- A cell shows chromatids being pulled to opposite ends
- The cytoplasm and membrane are dividing to form two cells
- The cell is not actively dividing
- The cell is preparing to divide
- The cell is in anaphase of mitosis
- The cell is in cytokinesis
38.Non-dividing cells 30, Stage 1 12, Stage 2 8, Stage 3 2, Total 52. Which stage of the cell cycle is the slowest?
Medium
- ANon-dividing cells
- BStage 1
- CStage 2
- DStage 3
39.Place the events of the cell cycle in the correct order, starting with the earliest.
Hard
- The DNA replicates and the number of ribosomes and mitochondria increases
- Chromosomes line up along the equator of the cell
- Spindle fibres pull chromatids to opposite ends of the cell
- New membranes form around each group of chromosomes
- The cytoplasm and cell membrane divide to form two daughter cells
40.A scientist wants to grow large numbers of identical plants that are resistant to a disease. Why are meristem cells particularly useful for this?
Hard
- AMeristem cells are fully undifferentiated and can be cloned to produce whole new plants
- BMeristem cells are already differentiated into disease-resistant cells
- CMeristem cells are haploid so they produce new gene combinations
- DMeristem cells cannot divide, so they stay genetically identical
41.Name the process by which a cell becomes specialised.
Medium
42.A cell divides by mitosis 3 times. How many cells are produced in total?
Medium
43.Complete the sentence about chromosomes using the correct words.
Easy
A chromosome is a thread-like structure found in the nucleus of a ____ cell. In a human body cell there are ____ pairs of chromosomes.
44.It takes 18 hours for one complete cell cycle in a typical onion root tip cell. Stage 3 lasts 6/68 of the cell cycle. Calculate the length of time Stage 3 lasts, in minutes, to 2 significant figures.
Hard
45.Name the location of the stem cells in plants.
Medium
46.A cell divides by mitosis 5 times. How many cells are produced in total from one original cell?
Medium
47.Complete the sentence about the cell cycle using the correct words.
Medium
During the growth phase of the cell cycle, the genetic material is ____ and the number of sub-cellular structures such as ____ and mitochondria increases.
48.Calculate the length of time Stage 3 lasts in a typical onion root tip cell. Give your answer in minutes to 2 significant figures.
Medium
49.Complete the sentence about tumours using the correct words.
Medium
Benign tumours are contained in one area, usually within a ____, and do not ____ other parts of the body.
50.What is the name of the process by which the cytoplasm and cell membrane divide at the end of the cell cycle?
Easy
51.A body cell of a human contains 46 chromosomes. How many pairs of chromosomes are there in a human body cell?
Easy
52.A single cell divides by mitosis 7 times. How many cells are produced in total?
Medium
53.Explain how an embryonic stem cell differs from an adult stem cell.
Hard
54.Explain why the production of stem cells from embryos is opposed by many people.
Hard
55.Evaluate the use of embryonic and adult stem cells to treat human diseases.
Hard
56.A student counts 54 cells in a root tip squash and records the number of cells in each stage of the cell cycle. Explain how the student can use these counts to estimate how long each stage lasts, and state one limitation of this method.
Hard
57.Explain why only a few cells in the field of view are seen in the later stages of mitosis, and suggest one way the student could make the results more reliable.
Hard
58.A cell divides by mitosis 3 times. How many cells are produced in total?
Medium
- A10 cells
- B8 cells
- C9 cells
- D7 cells
59.It takes 18 hours for one complete cell cycle in a typical onion root tip cell. Stage 3 lasts 6/68 of the cell cycle. Calculate the length of time Stage 3 lasts, in minutes, to 2 significant figures.
Hard
- A95 minutes
- B115 minutes
- C105 minutes
- D85 minutes
60.A cell divides by mitosis 5 times. How many cells are produced in total from one original cell?
Medium
- A38 cells
- B35 cells
- C29 cells
- D32 cells
61.Calculate the length of time Stage 3 lasts in a typical onion root tip cell. Give your answer in minutes to 2 significant figures.
Medium
- A33 minutes
- B36 minutes
- C30 minutes
- D39 minutes
62.A body cell of a human contains 46 chromosomes. How many pairs of chromosomes are there in a human body cell?
Easy
- A21 pairs
- B23 pairs
- C25 pairs
- D27 pairs
63.A single cell divides by mitosis 7 times. How many cells are produced in total?
Medium
- A128 cells
- B115 cells
- C141 cells
- D154 cells

Tarjetas

What is mitosis?

A nuclear division that produces two genetically identical diploid daughter cells.

How many chromosomes are found in a human body cell?

46 chromosomes, arranged in 23 pairs.

How many chromosomes does a human gamete contain?

23 chromosomes — half the diploid number, since gametes are haploid.

Name the three stages of the cell cycle in order.

Interphase (growth and DNA replication), mitosis, then cytokinesis.

What happens during interphase?

The cell grows, increases its number of sub-cellular structures such as ribosomes and mitochondria, and the DNA replicates to form two copies of each chromosome.

What happens during mitosis?

The chromosomes line up along the centre of the cell and cell fibres pull them apart, so one set of chromosomes moves to each end and the nucleus divides.

What happens during cytokinesis?

The cytoplasm and cell membrane divide, producing two daughter cells, each with a copy of every chromosome.

Why must DNA replicate before a cell divides?

So that each of the two daughter cells receives a complete copy of the genetic material.

What do chromosomes look like in a cell preparing to divide, and why?

They appear 'X' shaped because the DNA has been doubled, giving two identical strands called chromatids joined together.

Give three reasons why mitosis is important in multicellular organisms.

Growth (producing new cells), repair (replacing damaged or dead cells) and asexual reproduction (producing genetically identical offspring).

How do you calculate the number of cells produced after a given number of divisions?

Calculate 2 to the power of the number of divisions (2ⁿ). For example, 1 cell dividing 7 times gives 2⁷ = 128 cells.

How can you estimate how long a stage of the cell cycle lasts?

Divide the number of cells observed in that stage by the total number of cells counted, then multiply by the total length of the cell cycle.

What is a stem cell?

An undifferentiated cell that is capable of dividing to give rise to many more cells of the same type, from which other cells can arise by differentiation.

What can embryonic stem cells differentiate into?

All the different types of specialised cell found in the body.

Where are adult stem cells found and what can they produce?

In places such as bone marrow, skin, liver and brain; they are partially specialised and can only form a limited range of cells, mainly blood cells from bone marrow.

Where are stem cells found in plants?

In the meristem tissue at the tips of roots and shoots — the growing regions of the plant.

What is special about plant meristem cells?

They are fully undifferentiated and retain the ability to differentiate into any type of plant cell throughout the life of the plant.

Give two advantages of using plant stem cells to clone plants.

Clones can be produced quickly and economically, and rare species at risk of extinction can be cloned to protect them and maintain biodiversity.

Give one disadvantage of cloning plants from stem cells.

The plants produced are genetically identical, so genetic diversity within the clones is low.

What is therapeutic cloning?

An embryo is produced with the same genes as the patient, so the stem cells taken from it are not rejected by the patient's body.

Why is rejection not a problem with therapeutic cloning?

The embryo is produced using the patient's own genetic information, so the stem cells are genetically matched to the patient and are not rejected.

Give two risks of using stem cells in medicine.

Cultured stem cells could become infected with a virus that is passed to the patient, and they could accumulate mutations that lead to cancer cells.

Give two ethical concerns about using embryonic stem cells.

Some people object to using unused embryos from IVF or to creating and destroying embryos through therapeutic cloning; there are also questions about who gives permission and whether an embryo should be treated as a person.

What causes cancer?

Mutations in the DNA of cells that lead to uncontrolled cell growth and division, which can form a tumour.

What is the difference between a benign and a malignant tumour?

Benign tumours are abnormal cell growths contained in one area, usually within a membrane, and do not invade other parts of the body. Malignant tumours are cancerous: their cells invade neighbouring tissues and spread via the blood and lymphatic system to form secondary tumours.

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