5 Cell division Flashcards
What needs to happen in most parts of the body in terms of cells?
In most parts of the body, cells need to divide so that organisms can grow and
replace worn out or damaged cells.
What should the cells produced in this cell division be like?
The cells that are produced in this type of cell division should be exactly the same as the cells they are replacing.
Does this kind of cell division happen frequently?
This is the most common form of cell division.
Where is cell division different?
In the sex organs.
How is cell division different in the sex organs?
Here, some cells divide to produce gametes (sex cells), which contain only half the original number of chromosomes.
Why do they only have half the number of chromosomes?
This is so that when male and female gametes fuse together (fertilisation) the resulting cell (called a zygote) will contain the full set of chromosomes and can then divide and grow into a new individual.
What is a zygote?
It is a single cell resulting from fusion of a male and female gamete.
What are human body cells?
They are diploid.
What does diploid mean?
It is the number of chromosomes found in body cells. Diploid cells contain both chromosomes of
each homologous pair.
What kind of cells are gametes?
Haploid cells.
What does haploid mean?
It is the number of chromosomes found in gametes. Haploid cells contain one chromosome from each homologous pair.
What are the two types of cell division?
- Mitosis.
- Meiosis.
What is mitosis?
It is a type of cell division that produces diploid body cells for growth and repair of tissues.
What are the characteristics of mitosis?
When cells divide by mitosis, two cells are formed. These have the same number and type of chromosomes as the original cell.
When is mitosis used as a cell division?
Mitosis forms all the cells in our bodies
except the gametes.
What is meiosis?
It is a type of cell division that produces haploid cells (gametes).
What are the characteristics of meiosis?
When cells divide by meiosis, four cells are formed. These have only half the
number of chromosomes of the original cell.
When is meiosis used as a cell division?
Meiosis forms gametes.
What happens during mitosis to produce ‘daugher’ cells?
When a ‘parent’ cell divides it produces ‘daughter’ cells.
What kind of daughter cells do mitosis produce?
Mitosis produces two daughter cells that are genetically identical to the parent cell.
What do we mean by the daughter cells being genetically identical?
Both daughter cells have the same number and type of chromosomes as the parent cell.
What must be done during mitosis for the daughter cell to be genetically identical to the parent cells?
- It must copy each chromosome before it divides. This involves the DNA
replicating and more proteins being added to the structure. Each daughter
cell will then be able to receive a copy of each chromosome (and each
molecule of DNA) when the cell divides. - It must divide in such a way that each daughter cell receives one copy
of every chromosome. If it does not do this, both daughter cells will not
contain all the genes
What are the four different stages of mitosis?
- Prophase.
- Metaphase.
- Anaphase.
- Telophase.
What is a diagram of what happens in the prophase?
What happens in the prophase?
Before mitosis the DNA replicates and
the chromosomes form two exact copies called chromatids. During the first stage of mitosis (prophase) the chromatids become visible, joined at a centromere. The nuclear membrane breaks down.
What is a diagram of what happens in the metaphase?
What happens in the metaphase?
During metaphase a structure called the spindle forms. The chromosomes line up at the ‘equator’ of the spindle, attached to it by their centromeres.
What is a diagram of what happens in the anaphase?
What happens in the anaphase?
During anaphase, the spindle fibres shorten and pull the chromatids to the opposite ends (‘poles’) of the cell. The chromatids separate to become the chromosomes of the two daughter cells.
What is a diagram of what happens in the telophase?
What happens in the telophase?
In the last stage (telophase) two new nuclei form at the poles of the cell. The cytoplasm starts to divide to produce two daughter cells. Both daughter cells have a copy of each chromosome from the parent cell.
What does each daughter cell formed by mitosis receive?
A copy of every chromosome, and therefore every gene, from the parent cell.
What is each daughter cell identical to?
To the others.
What are all the cells in our body (except gametes) formed by?
By mitosis from the zygote.
What does this mean then about the cells in our bodies?
They all contain copies of all the chromosomes and genes of that zygote. They are all genetically identical.
When does mitosis usually happen?
Whenever cells need to be replaced in our bodies, cells divide by mitosis to
make them.
Where are the main regions where mitosis happens usually?
- The skin loses thousands of cells every time we touch something. This adds
up to millions every day that need replacing. A layer of cells beneath the
surface is constantly dividing to produce replacements.
-Cells are scraped off the lining of the gut as food passes along. Again,
a layer of cells beneath the gut lining is constantly dividing to produce
replacement cells. - Cells in our spleen destroy worn out red blood cells at the rate of
100 000 000 000 per day! These are replaced by cells in the bone marrow
dividing by mitosis. In addition, the bone marrow forms all our new white
blood cells and platelets. - Cancer cells also divide by mitosis. The cells formed are exact copies of the
parent cell, including the mutation in the genes that makes the cells divide
uncontrollably.
What does meiosis form?
Meiosis forms gametes.
Which is more complex, mitosis or meiosis?
It is a more complex process than mitosis and takes place
in two stages called meiosis I and meiosis II.
What is the outcome of meiosis?
Four haploid cells.
How many daughter cells are formed in meiosis?
4, the four haploid cells.
What is the genetic make-up of each daughter cell?
Each daughter cell is genetically different from the other three and from the parent cell.
What must the parent cell do during meiosis?
- It must copy each chromosome so that there is enough genetic material to
be shared between the four daughter cells. - It must divide twice, in such a way that each daughter cell receives just one
chromosome from each homologous pair.
What is a diagram which shows the stages of meiosis?
What are the two main events of meiosis?
- During the first division, one chromosome from each homologous pair goes
into each daughter cell - During the second division, the chromosome separates into two parts. One part goes into each daughter cell.
What is different about the gametes formed by meiosis?
The gametes formed by meiosis don’t all have the same combinations of
alleles – there is genetic variation in the cells.
What happens during the two cell divisions of meiosis?
The chromosomes of each homologous pair are shared between the two daughter cells independently of each of the other homologous pairs.
Why are they shared independently?
This allows for much possible genetic variation in the daughter cells.
What is a diagram which shows how meiosis produces variation?
What is a table showing the comparison of meiosis and mitosis?
What does sexual reproduction in any multicellular organism involve?
Sexual reproduction in any multicellular organism involves the fusion of two gametes to form a zygote.
What are some of the reasons that the offspring from sexual reproduction vary genetically?
One reason is because of the huge
variation in the gametes. Another reason is because of the random way in
which fertilisation takes place.
Why is fertilisation random in humans?
In humans, any one of the billions of sperm formed by a male during his life could, potentially, fertilise any one of the thousands of ova formed by a female.
To which gametes does this variation apply?
This variation applies to both male and female gametes.
What can we deduce from the fact that there is a low estimate of about 8.5 million different types of human gametes in terms of sperm and ova?
It means that there can be 8.5 million different types of sperm and 8.5 million different types of ova.
What happens when fertilisation takes place?
Any sperm could fertilise any ovum.
What is the number of possible combination of chromosomes and genes in the zygote?
It is 8.5 million × 8.5 million = 7.2 x 1013, or 72 trillion! And remember,
this is using our ‘low’ number.
What does this random aspect of fertilisation mean for individuals?
This means that every individual is likely to be genetically unique.
What are the only exception of every individual being genetically unique?
The only exceptions are identical twins.
How are identical twins formed?
Identical twins are formed from the same zygote – they are sometimes called monozygotic twins.
What happens when the zygote divides by mitosis?
The two genetically identical cells formed do not ‘stay together’.
What do these cells do instead?
Instead, they separate and each cell behaves as though it were an individual
zygote, dividing and developing into an embryo.
What happens to the embryos (children) because they have genetically identical cells?
Because they have developed from genetically identical cells (and, originally, from the same zygote), the embryos (and, later, the children and the adults they become) will
be genetically identical.
Are all types of twins genetically identical?
No, non-identical twins develop from different zygotes and so are not genetically identical.
How are seeds made?
Seeds are made by sexual reproduction in plants.
What does each seed contain?
Each seed contains an embryo.
What is this embryo a result of?
Which results from a pollen grain nucleus fusing with an egg cell nucleus.
Why will embryos from the same plant vary genetically?
Embryos from the same plant will vary genetically because they are
formed by different pollen grains fertilising different egg cells and so contain different combinations of genes.
What have plant breeders known for a long time about sexual production?
Plant breeders have known for a long time that sexual reproduction produces
variation.
What did they realise about plants with desirable features and how to gain more of it?
They realised that if a plant had some desirable feature, the best way to get more of that plant was not to collect and plant its seeds, but to clone it in some way.
What does clone mean?
Groups of cells, or organisms, that are genetically identical.
What do modern plant-breeding techniques allow in terms of cloning?
Modern plant-breeding techniques allow the production of many
thousands of identical plants from just a few cells of the original
What happens in terms of asexual reproduction and gametes?
When organisms reproduce asexually, there is no fusion of gametes.
What happens to the organism in asexual reproduction?
A part of the organism grows and somehow breaks away from the parent organism
What kind of cells do the new organism contain?
The cells it contains were formed by mitosis, so contain exactly the same genes as the parent.
What does asexual reproduction produce in terms of genetics?
Asexual reproduction produces offspring that are genetically
identical to the parent, and genetically identical to each other.
Where is asexual reproduction most commonly seen?
- Asexual reproduction is common in plants.
- Also in some animals.
What factors produce variation?
Genes and environment both produce variation.
What are the two varieties of pea plants?
They are either tall or short.
What is this difference in height due to?
This difference in height is due to the genes they inherit. There are no ‘intermediate height’ pea plants.
What is important to note about the heights in different species of pear plants?
However, all the tall pea plants are not exactly the same height and neither are all the short pea plants exactly the same height.
What is a bar chart showing variation in height of pea plants?
What are several environmental factors that can influence the height of the plants?
- They may not all receive the same amount of light and so some will not
photosynthesise as well as others. - They may not all receive the same amount of water and mineral ions from
the soil – this could affect the manufacture of a range of substances in the plant. - They may not all receive the same amount of carbon dioxide. Again, some
plants will not photosynthesise as well as others
Do these principles also apply to humans?
Similar principles apply in humans.
How do we know that these principles also apply in humans?
Identical twins have the same genes, and often grow up to look very alike (although not quite identical). Also, they often develop similar talents.
Why do identical twins never look exactly the same?
However, identical twins never look exactly the same. This is especially true if, for some reason, they grow up apart.
What would the different environments do to them?
The different environments affect their physical, social and intellectual development in different ways.
Understand that variation within a species can be
genetic, environmental or a combination of both?
- Genetic Variation: Genetic variation refers to the differences in the genes or DNA sequences of individuals within a species. These variations can be inherited from parents or occur due to mutations. For example, in humans, genetic variations can lead to differences in traits such as eye color, height, or blood type.
- Environmental Variation: Environmental variation, on the other hand, is caused by differences in the environment that individuals are exposed to. This can include factors like temperature, humidity, availability of food, or exposure to pollutants. For instance, plants growing in different climates may have different growth patterns or flower colors due to the environmental conditions they experience.
- Combination of Genetic and Environmental Factors: In many cases, the variation within a species is a result of both genetic and environmental factors working together. For example, the height of a person can be influenced by both their genetic predisposition and their nutrition during childhood. Similarly, the color of a butterfly’s wings can be influenced by both its genetic makeup and the temperature it experiences during development.
Overall, variation within a species can be genetic, environmental, or a combination of both. Genetic variation is inherited and can lead to differences in traits, while environmental variation is caused by differences in the environment. Often, both genetic and environmental factors play a role in shaping the variation we observe in individuals within a species.
