B5

DNA has 2 strands coiled together. Each strand is made up of lots of small units called nucleotides. Each nucleotide contains one of 4 bases:

• G (guanine)
• C (cytosine)
• A (adenine)
• T (thymine)

Genes contain the instruction to make proteins. Proteins are made by joining amino acids together. Each set of 3 bases codes for one amino acid, this is the triplet code.

These proteins are made by organelles called ribosomes in the cyptoplasm of the cell. DNA is found in the nucleus of a cell and is too big to get out so a copy is made called an RNA messenger by taking one strand of the DNA as a template.

The RNA messenger moves out of the nucleus to join the ribosome of the cell. The job of the ribosome is to stick amino acids together to make a protein following the order of the bases from the RNA messenger.

Chromosomes are long lengths of coiled DNA, Genes are short sections of DNA on chromosomes. Cell division is needed for growth and repair. There are 2 stages in this process:

1) Development

The cell physically grows and duplicates its contents by splitting the DNA  into 2 long chains and using free floating bases to pair up with each strand.The cell now has 2 copies of DNA  spread out in long strings

2)Mitosis

Before the cell divides the DNA forms an X shape (each 'arm' is a duplicate of the other). The chromosomes line up in the centre of the cell and cell fibres pull them apart into 2 arms and go to opposite ends of the cell. Membranes form around the sets of chromosomes and make the nuclei. Lastly, the Cell divides into 2.

All the cells in your body divide by mitosis apart from the reproductive organs which divide to form gametes (the sprem and egg cells). Gametes only have one copy of each chromosome so that when they combine there are 46.

During meiosis there are 2 divisions. Here are the stages:

1) As with mitosis before the cell divides it duplicates the DNA

2) FIRST DIVISION:

The Chromosomes pairs line up in the centre of the cell. The pairs are then pulled apart into two cells so that each cell has one copy of each chromosome. A mixture of the father's and mother's chromosomes go into each cell.

3) SECOND DIVISION:

The chromosomes then line up again in the centre of the cell. They are pulled apart and left with two arms on each.

4) You end up with 4 gametes each with a single set of chromosomes. After two gametes join at fertilisation, the zygote grows by mitosis.

Embryo Stages and  their cells:

1) A fertilised egg divides by mitosis to produce a bundle of cells- the embryo of the new organism

2) To start with all cells in the embryo are the same (embyonic stem cells). These cells are undifferentiated, this mean they can divide to make any type of specialised cell. This is the same until the 8 cell stage in humans.

3) The process of cells becoming specialised is called differentiation. After the 8 cell stage most of the tem cells start to differentiate, this is when the embryo starts to develop tissues and organs. 

Adult stem cells are also undifferentiated however they arent as versatile and can only be found in certain places like bone marrow. All body cells contain genes but in body cells some arent active. Stem cells switch on any gene during their development. This determines the type of cell it will specialise to become.

Stem cells can cure diseases.

Adult stem cells can cure some blood diseases by bone marrow transplants. Bone marrow contains stem cells that can turn into new blood cells to replace faulty old ones.

Embryonic stem cells can be extracted from early embryos. These could then be made into specific cells to replace the faulty cells in sick people e.g. heart muscle cells for people with heart disease. To do this scientists have to control which genes become active by altering the conditions. More research is needed.

Its thought of as unethically wrong as the embryo used to provide stem cells are destroyed and they could become a person. Its so tricky that its regulated by the government.

Cloning can be used to make stem cells by taking an egg cell and removing its genetic material. A nucleus from the adult being cloned is inserted into the 'empty' egg cell. Under the right conditions inactive genes can be reactivated to form an embryo. Embryonic cells can then be extraced from the embryo and controlled to form any type of specialised cell.

Meristems are the only cells in plants the divide by mitosis and are found in areas of the plant which grow such as the roots and shoots. Meristems are a plants version of embryonic cells but these can divide and form any type of cell for as long as the plant lives.

These can produce xylem and phloem (tissues) which group together to form organs like leaves, roots, stems and flowers.

Clones of plants can be produced from cuttings as they will contain meristems.Gardeners will take cuttings from desirable characteristics and plant them to produce clones of the parents.

Rooting powder is often used as it helps cuttings grow to complete plants.

Rooting powder contains auxins (plant hormones) that encourage the plant to produce roots rapidily and allow it to grow into a full plant. This allows gardeners to produce lots of clones of a really good plant quickly.

Phototropism is growth towards or away from light. Shoots are positively phototropic- they grow towards the light. Roots are negatively phototropic- they grow away from the light.

Phototropism helps plants to survive:

Positive phototropism: Plants need sunlight for photosynthesis. Without sunlight, plants cant photosynthesise and dont produce the food they need for energy and growth.

Negative Phototropism: Plants need nutrients and water from the soil to grow. Phototropism makes roots grow down into the soil where they can absorb the water and nutrients the plant needs for healthy growth.

Auxins are chemicals that control growth near the tips of roots and shoots. They are produced at the tips and are diffused backwards to stimulate the cell elongation process whcih occurs in the cells just behind the tips. Without tips the plant cant grow.

Auxins are involved in the responses of plants to light, gravity and water. When a shoot tip is exposed to light the auxins move to the shaded side of the plant to stimulate a faster growth so that the shoot grows towards the light.