Genetics

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Genetics

DNA

Chromosomes are long molecules of DNA

DNA contains all the instructions to put an organism together and make it work. It is found in the nucleus of animal and plant cells, in really long molecules called chromosomes.

A gene codes for a specific protein

A gene is a section of DNA. It contains the instructions to put make a specific protein. Cells make protein by stringing the amino acids together in a particular order. Only 20 amino acids are used up to make thousands of different proteins. DNA determines what proteins the cell produces which then determines what cell is produced.

Everyone has unique DNA

Almost, everybody's DNA is unique, except identical twins, where two people have identical DNA and clones. DNA fingerprinting is a way of cutting up a persons DNA into small sections and then separating them. Every persons DNA fingerprinting is different which means you can tell people apart by comparing samples of their DNA.

Forensic science - DNA is taken from the crime scene and compared to that from a suspect of a crime.

Paternity testing - To see if a man is the biological father of the child.

Some people think everybody should have their DNA tested in a country so it could be checked against everybody in the country. This could be seen as being an invasion of privacy and might be used for different reasons than necessary. There are also scientific problems that could occur for example false results can lead to the wrong person being conflicted if the data is misinterpreted.

Cell Division - Mitosis

Mitosis makes new cells for growth and repair.

When a body cell divides it needs to make a new cell with new cells identical to the original cell - with the same number of chromosomes so there are 23 pairs of chromosomes per human cell. Mitosis occurs when the plants and animals want to grow or replace cells that have been damaged.

Mitosis is when a cell reproduces itself by splitting to form two identical offspring

In a cell that is not dividing, the DNA is all spread out in long strings.

If the cell gets a signal to divide it needs to duplicate its DNA - so there's one copy for each new cell.

The chromosomes then line up at the centre of the cell and the cell fibres pull them apart. The two arms of chromosomes go to the opposite side of the cell.

Membranes form around each set of chromosomes. These become the nuclei of the two cells.

Now the cytoplasm divides. You now have exactly two cells containing exactly the same DNA.

Asexual reproduction

Some organisms reproduce by mitosis which becomes new plants. This is an example of asexual reproduction. The offspring has exactly the same genes as the parent - so there's no variation.

Cell division - Meiosis

Gametes have half the usual number of chromosomes

During sexual reproduction two sex cells called gametes combine to form a new individual. Gametes only have one copy of each chromosome. One the egg, from the mother and the sperm from the father. Despite this, humans still end up with the right number human body cells of 46 chromosomes. The gametes have 23 chromosomes each.

Meiosis involves two divisions

Meiosis produces cells which half the number of normal chromosomes

As with Mitosis before the cell starts to divide so it duplicates it DNA.

In the first divisions in meiosis the chromosomes pairs line up in the centre of the cell.

The pairs are pulled apart so each new cell has only one chromosome copy. Some from the father and some from the mother go into the new cell.

In the second division the chromosomes line up in the center of the cell. It's a bit like mitosis. The arms of the chromosomes are pulled apart.

You get four gametes with only a single set of chromosomes in it. After both cells join at fertilization they grow by mitosis.

Stem Cells

Embryonic cells can turn into any type of cell

Some cells are undifferentiated. They can different types of cell depending on what instructions they are given. These are called stem cells. These are exciting because they have the potential to change into any kind of cell that needs to be treated. Adults also have stem cells but they are only found in places like the bone marrow so are not as versatile as embryonic stem cells.

Stem Cells may be able to cure disease

Medicine already uses adult stem cells to cure disease. For example some people with blood disease can be treated by bone marrow transplants. Bone marrow contains stem cells to replace old faulty ones. Scientists can extract stem cells from embryos to grow them. These embryonic stem cells could could be used to replace faulty cells. To get cultures of one specific type of cell, researchers try to control the differentiation of the stem cells by changing the environment they are growing in.

Some people are against stem cell research

People believe embryos should not be used because they are potential humans beings from embryos; however, others think that curing patients who already exist and who are suffering is more important than the right of the embryo. One argument for this is that these embryos have been used for research and are usually unwanted ones from fertility clinics, which if they were not used for research would be destroyed. But some still disagree with this and feel scientists should concentrate more of finding and developing sources of stem cells without using embryos. In some countries it is disallowed but is allowed in the UK under strict guidelines.

X and Y chromosomes

Chromosomes control whether you are male or female

There are 22 matched pairs of chromosomes in every body cell. The 23rd pair are labelled XX or XY. They are the two chromosomes that decide whether you are male or female. All men have an X and Y chromosome: XY. The Y chromosome causes male characteristics.

All women have two X chromosomes: XX. The XX combination allows female characteristics to develop. When making sperm, the X and Y chromosome is split in the first meiosis. There is a 50% chance each sperm cell is an X-chromosome and 50% chance that it gets a Y chromosome. A similar thing happens when making eggs. But the original cell has two X chromosomes, so all eggs have at least one X-chromosome.

The work of Mendel

Mendel reached three important conclusions

Mendel had shown that the height in characteristics in pea plants is characteristic in pea plants was determined by separately inherited "hereditary units" passed on by each parent.

1) Characteristics in plants are determined by the "hereditary units".

2) Hereditary units are passed on from both parents , one unit from each parent.

3) Hereditary units can be dominant or recessive - if the individual has both dominant and recessive unit fora characteristic, the dominant characteristic will be expressed.

Genetic diagrams

Genetic diagrams show the possible genes off an offspring

Alleles are different versions of the same gene. In genetic diagrams the alleles are usually represented by the letters. If the offspring has a particular gene that is the same then it is a homozygous. If the offspring has genes that are different for each gene then it is a hetrozygous. The genotype means the allele you have. The phenotype means the actual characteristic.

Genetic disorders

Cystic fibrosis is caused by a recessive Allele

The allele which causes cystic fibrosis is a recessive allele carried by one in twenty five people. Because it is recessive people with only one copy of the disorder are known as carriers. For a child to have the disorder both children must have either carriers or suffers.

Polydactyly is caused by dominant alleles

Polydactyly is a genetic disorder that involve a baby being born with extra fingers and toes. The allele is dominant and so can be inherited if only on parent has the disorder. The parent that has the defective allele will be a sufferer since the allele is dominant.

Embryos can be screened for genetic disorders

During IVF embryos are fertilized in a laboratory and then implanted in the mothers womb. More than one egg is fertilized to make it a better chance of the IVF being successful. Before being implanted it is easy to remove a cell from each embryo and analyse the genes. Many genetic disorders are identifies this way. Embryos with good alleles would be implanted into the mother.