Cell division and growth:
New cells are needed for a part of an organism to grow and to replace cells that have been worn out and repair damaged tissue.
The new cells have to have the same genetic material in them as the originals. Each of the cells has a nucleus containing information to make new cells. The ‘instructions’ are carried in the form of genes.
A gene – small packet of information which controls a characteristic (or part of one). Genes are grouped on chromosomes, which can carry hundreds/thousands of genes.
Human has 46 chromosomes in the nucleus of cells (except the gametes, sperm and ova cells). Chromosomes are in 23 pairs, and each pair contains one inherited from the mother and one from the father.
Body cells divide to make new cells. Mitosis – creates identical daughter cells and takes place in normal body cells. So all these cells have the same genetic information. This is used in asexual production to form identical offspring (no variation).
Before a cell divides it makes copies of the chromosomes in the nucleus. This goes from two pairs of chromosomes to four pairs of chromosomes. The cell then divides in two, forming two daughter cells. Each daughter cell is identical to the parent, each containing a nucleus with two pairs of chromosomes.
In some areas in animal or plant bodies, mitosis happens rapidly all the time. Mitosis constantly happens in human skin.
Early plant and animal embryos have very unspecialised cells. These can become any type of cell, called stem cells.
The cells in an animal become specialised very early in life. By the time a human is born, most of its cells have become specialised (e.g. liver cells, skin cells, or muscle cells). These cells have differentiated, some genes have been switched off and some have been switched on.
Differentiated = specialised for a particular function.
A muscle cell will divide by mitosis and can only form muscle cells. Specialised cells divide by mitosis, so can only replace damaged tissue and replace worn out cells of the same kind.
Most plant cells can differentiate throughout their life. Undifferentiated cells are made at active regions at the stems and roots of plants, which divide by mitosis almost constantly.
Plants keep growing all their lives at these ‘growing points’. These cells don’t differentiate until they are in their final position in the plant. This differentiation isn’t permanent. If a plant cell moves to a different part of the plant, it can re-differentiate any other type of cell.
Huge amounts of plant clones can be produced from a tiny piece of leaf tissue. This is because plant cells can become unspecialised and divide by mitosis or the undifferentiated can produce more by mitosis. These will differentiate to make a new plant identical to the parent.
Animal clones are harder to make because animal cell differentiate permanently early in the embryo’s development. Animal clones have to be made by cloning embryos.
Stem cells: Human stem cells are found in human embryos and some adult tissue like bone marrow. There may be some stem cells in most of the different tissues of an adult body, in the brain, blood, muscles and the liver.
The stem cells can stay here for many years until the tissues are injured or diseases, when they will divide to replace the different types of damaged cells.
The function of stem cells:
Stem cells divide and then become specialised cells to make up tissue and organs. When a sperm and egg fuse to form an embryo, they form one new cell. This cell divides and the embryo becomes a hollow ball of cells. The inner cells of this ball are stem cells which eventually form every type of cell in the body.
Using stem cells: People can die when parts of their bodies stop working properly, e.g. spinal injuries can cause paralysis. The spinal nerves don’t repair themselves.
In 1998, two American scientists cultured human embryonic stem cells (grow them in the laboratory) which could form other types of cell. Scientists hope that embryonic stem cells can be encouraged to into almost every type of cell in the body - new nerve cells could be used to reconnect the spinal nerves and cure paralysed people.
Stem cells could be used to grow new organs to use in transplant surgery. The new organs wouldn’t be rejected by the body, treating infertility and dementia. At the moment scientists don’t know how the cells in an embryo are switched on and off, so we don’t know how to form particular types of tissue.
Problems with stem cells:
Stem cells come from aborted embryos or spare embryos in fertility treatments. These are potential human lives and religious groups, etc, think this is ethically wrong.
The embryo can’t give its permission, so this may be a violation of its human rights. Progress with stem cells is slow and some people think they could cause cancer if used to treat sick people. This has been seen in mice.
Making stem cells is slow, difficult, hard to control and expensive.
The future of stem cell research:
Overcoming ethical issues:
- Embryonic stem cells have been found in the umbilical cord blood of newborn babies.
- Scientists can grow adult stem cells, but those found so far can only develop into a small range of cell types.
Therapeutic cloning is being researched but is difficult. This involves using cells from an adult to produce a cloned early embryo of themselves. The embryo will have identical embryonic stem cells to the person. These could be used to heal the donor and others.
Cell division in sexual reproduction:
Meiosis only happens in reproductive organs of animals and plants. Meiosis makes sex calls (gametes) with half the original number of chromosomes.
Reproductive organs in people = the ovaries and the testes
Female gametes (ova) = made in the ovaries
Male gametes (sperm) = made in the testes
In meiosis, the chromosomes are first copied so there are four pairs of chromosomes, then the cell divides twice, one division straight after the other. This forms four sex cells (gametes) with one set of chromosomes each, instead of the original two pairs.
Why is meiosis so important?
Normal body cells have 46 chromosomes in 23 pairs (one from mother and one from father). If two ‘normal’ cells joined together, the new cell would have 92 chromosomes.
Gametes contain half of the normal chromosome number, so when gametes fuse together in fertilisation, the new cell has the normal 46 chromosomes.
In girls – the ovaries of a baby girl contain all the ova she will ever have.
In boys – in puberty the testes start to produce sperm and this continues for their whole lives.
Every gamete produced is different, so the combination of chromosomes will be different. Some genes are exchanged between the chromosomes in meiosis, so no two eggs or sperm are the same, creating genetic variety.
Each sex cell has a single set of chromosomes, so that two sex cells form two pairs as is normal.. They each have 23 chromosomes and the new cell has 46.
The combination of genes on the chromosomes of every different fertilised ovum is different. The unique cell divides by mitosis after fertilisation and this continues even after the fetus is fully developed.
Asexual reproduction – mitosis from parent cells. No variation, same chromosomes and genes as parents.
Sexual reproduction – meiosis in sex organs of parents. Each gamete is different. When gametes fuse one of each pair of genes and chromosomes are from each parent.
The combination of genes in the new pair will contain alleles from each parent. This produces the characteristics of the offspring.
Allele = a version of a particular gene.
From Mendel to DNA:
Gregor Mendel was born in 1822 in Brunn, Czechoslovakia. He became a monk for education.
He carried out breeding experiment with peas. He used pure strains of round peas, wrinkled peas, green peas and yellow peas.
He cross-bred the peas and counted the different offspring. He found inherited characteristics in clear patterns.
He though there were separate units of inherited genetic material, and that some were dominant over others.
People didn’t know about genes or chromosomes, people didn’t understand his ideas. By 1900, people saw chromosomes through a microscope. Three scientists found Mendel’s notes and repeated his experiments, giving Mendel full credit although he had been dead 16 years.
People thought that his idea of units of inheritance might be carried on chromosomes.
DNA – The molecule of inheritance: Characteristics are inherited on genes carried on chromosomes. Chromosomes are made up of long molecules of a chemical called DNA (deoxyribose nucleic acid). Genes are small sections of DNA.
DNA carried the instructions to make proteins which form most of the cell structures in the body. These proteins include enzymes which control cell chemistry.
The long strands of DNA are made up of combinations of four different chemical bases. These are grouped into threes and each group of three codes for one amino acid.
Each gene is made up of hundreds/thousands of bases. The order of the bases controls the order that the amino acids are put together, making a particular protein to be used in the cell. Each gene codes for a particular combination of amino acids which make a specific protein.
A mutation (change) in a single group of bases can disrupt and change the whole protein structure and the way it works.
DNA is unique except to identical twins because they developed from the same original cell. Members of the same family have strong similarities in their DNA.
‘DNA fingerprinting’ can be used to identify a person, producing varied patterns under a microscope. DNA fingerprints can be produced from tiny samples of body fluids (blood, saliva and semen).
The chance of two people who aren’t identical twins having the same DNA is virtually impossible. DNA fingerprinting can be used to solve crimes and to see who is the biological father of a child.
Inheritance in action:
Humans have 23 pairs of chromosomes, and 22 of these are pairs with similar shaped chromosomes and have genes carrying information about the same things. One pair of chromosomes may be different from each other, called sex chromosomes. These determine the sex of a human.
XX chromosomes = female
XY chromosomes = male
The Y chromosome is much smaller than the X chromosome.
Chromosomes carry genetic information in the form of genes. An allele is the particular form of information on a gene on an individual chromosome. E.g. the gene for dimples may have the dimple or no-dimple allele in place on the chromosome. The allele for no dimples is recessive, so is a child has no dimples the parents must have had no dimples.
Most characteristics, like eye colour, are controlled by many genes. Some characteristics, like dimples or attached earlobes, are controlled by only one gene. There are normally two possible alleles but there can be more possibilities.
Dominant alleles can control a characteristic even though they are only present on one chromosome. Recessive alleles can only control a characteristic if they are present on both alleles (no dominant is present).
If one parent has dimples and one doesn’t, the child could have two dimple alleles, two no dimple alleles or one of each.
Inherited conditions in humans:
Some diseases are from problems in genes and can be inherited.
Huntington’s disease – dominant allele, so can be inherited from one parent who has or carries it. It is a disorder of the nervous system. If one parent has it, the baby has a 50% chance of inheriting it (half of the child’s gametes with hold the faulty allele).
The condition is fatal but the symptoms don’t appear until a person is 30 – 50 years old, so people may have already had children and passed on the faulty allele without knowing.
Cystic fibrosis – recessive allele. Affects body organs, particularly the lungs and pancreas. The organs become clogged up by very thick and sticky mucus, stopping them working properly.
This affects the reproductive system, so most people with cystic fibrosis are infertile. There is no cure but it can be treated by physiotherapy and antibiotics to help clear the lungs of mucus and infections. Enzymes are used to replace the ones the pancreas would normally produce to thin the mucus.
Cystic fibrosis has to be inherited from both parents, who may just be carriers with a dominant healthy allele. They have no symptoms so they don’t know the allele is there.
One person in 25 in the UK carries the cystic fibrosis allele, and the baby won’t have it unless their partner is also a carrier. There will then be a 25% chance that the child will be affected by cystic fibrosis.
A genetic cross = when the genes from two parents are combined. A genetic diagram can show the alleles carried by the parents, the possible gametes that can be formed and how these could combine to form the characteristics of their offspring.
The chance of a child inheriting an allele is independent each time a sperm and egg cell fuse. It’s all down to chance.
Curing genetic diseases: We can’t cure them yet but scientists think genetic engineering can lead to being able to cut out faulty alleles and replace with healthy ones. This hasn’t yet cured anyone.
People can take genetic tests to see if they carry a faulty allele, so they can make the choice to have a family or not. Embryos can be screened for alleles which cause genetic disorders and faulty alleles could be removed in IVF treatments.