Cell Division and Growth
Cell division is necessary for the growth of an organism, or for repair if tissues are damaged. New cells must have the same genetic information in them as the originals. Each of the cells has a nucleus containing instructions carried in the form of genes.
Mitosis results in two identical cells being produced from the original cell. A copy of each chromosome is made before the cell divides and one of each chromosome goes to each new cell.
A gene carries information which controls a characteristic or part of a characteristic of the body. The genes are grouped together on chromosomes. A chromosome may carry several hundred or even thousands of genes.
In the early development of animal and plant embryos, the cells are very unspecialized - known as stem cells. In many animals the cells become specialized very early in life. By the time a human baby is born most of the cells have differentiated (specialized for a particular function). In adult animals, cell division is restricted because differentiation has occured. Plant cells can differentiate all through their life.
Diploid cells - 2 sets of chromosomes (46 chromosomes)
Haploid cells - 1 set of chromosomes (23 chromosomes)
Stem cells are unspecialised, they can develop (differentiate) into many different types of specialised cells. Stem cells are found in the embryo and in adult bone marrow.
When an egg and sperm fuse to form an embryo, they form a single cell. The cell divides and the embryo is soon a hollow ball of cells which are the stem cells. Many embryonic stem cells that we carry research out on are from aborted embryos, or are ‘spare’ embryos from fertility treatment. This results in in ethical issues and much debate, as it is seen as destroying life. There is some concern that embryonic stem cells might cause cancer if they are used to treat sick people as it has been seen in mice.
The use of stem cells from adult bone marrow is still limited by the number of different types of specialized cell they can develop into. Making stem cells is slow, difficult expensive and hard to control. It is also known as therapeutic cloning.
Cell Division in Sexual Reproduction
Meiosis results in sex cells with only half the original number of chromosomes. Cells in reproductive organs, e.g. testes and ovaries, divide to form sex cells (gametes). Before division, a copy of each chromosome is made. The cell divides twice to form four gametes – this type of cell division is called meiosis.
Each gamete has only one chromosome from the original pair. All of the cells are different from each other and the parent cell.
Sexual reproduction results in variation as the sex cells (gametes) from each parent fuse.
In girls, the first stage of meiosis is completed before they are even born. The tiny ovaries of a baby girl contain all the ova she will ever have. In boys, meiosis doesn't start until puberty when testes start to produce sperm. It then carries on for the rest of their lives.
Cell Division in Sexual Reproduction (cont.)
In asexual reproduction, the offspring are produced as a result of mitosis from the parent cells so they contain exactly the same chromosomes and the same genes as their parents. There is no variation in the genetic material.
In sexual reproduction, gametes are produced by meiosis in the sex organs of the parents. This introduces variety as each gamete is different. When gametes fuse, the combination of genes in the new pair will contain alleles (different forms of genes) from each parent, which also helps to produce different characteristics in the offspring.
From Mendel to DNA
Gregor Mendel (1822-1884) - austrian monk who worked out how characteristics were inherited, characteristics were dominant and they were never mixed together. Chromosomes are made up of long molecules of a chemical called DNA (deoxyribonucleic acid). DNA molecules are large and complex, they carry the genetic code that determines the characteristics of a living thing.
A gene is a short section of DNA. Genes code for combinations of specific amino acids, which make up proteins.
Genes controlling the development of characteristics are called alleles.
- The characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
- The characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair
The cells of offspring produced by asexual reproduction are produced by mitosis from the parental cells. They contain the same alleles as the parent. However, sexual reproduction causes variation because when two gametes join together one of each pair of alleles comes from each parent.
Most characteristics - e.g. eye color and nose shape - are controlled by a number of genes. However some characterics - e.g. dimples and attached earlobes - are controlled by a single gene.
For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.
Human beings have 23 pairs of chromosomes, one pair are the sex chromosomes; Females - XX, Males - XY
Chromosomes from a male Chromosomes from a female
The X chromosome carries genes controlling lots of different features.
The Y chromosome is much smaller than the X chromosome and carries information mainly about "maleness".
Inherited Conditions in Humans
Huntington's disease (very serious but rare disease) is a disorder of the nervous system caused by a dominant allele, so even if only one parent has the disease it can be inherited by the child. Half their gametes will contain the faulty allele. It is a fatal disease, it doesn't appear until middle-age.
Cystic fibrosis is a disorder of cell membranes cause by a recessive allele so parents may be carriers (Cc). Only if both parents are either carriers of have the disorder does a child inherit it. It affects organs, particularly the lungs and pancreas, the organs become clogged up by very thick mucus. The reproductive system is affected so most people who have the disorder are infertile. Treatment includes physiotherapy and antibiotics to help the lungs clear of mucus and infections. Enzymes are used to replace the ones the pancreas cannot produce and to thin the mucus.
Embryos can be screened to see if they carry alles for one of these or other genetic disorders.
Polydactyly is an inherited condition in which a person has extra fingers or toes. It is caused by a dominant allele of a gene which means it can be passed on by just one parent if they have the disorder. Offspring need to carry just one dominant allele from their parents to inherit the polydactyly condition.