B5 Growth and Development

DNA is a double helix of paired bases.

Each strand is made up of small groups called "nucleotides". Each nucleotide contains a small molecule called a "base".  

DNA has 4 different bases- Adenine A, Cytosine C, Guanine G, Thymine T. The 2 strands are held together by these bases. A and T, C and G always go together, this is called Base Pairing.

DNA controls the production of proteins in a cell. A gene is a section of DNA that codes for one particular protein. Cells make proteins by joining amino acids in a particular order.

It's the order of the bases in a gene that tells the cell in what order to put the amino acids. 3 bases e.g. TCG (a triplet) codes for one amino acid.

DNA also determines which genes are switched on or off- and so which proteins the cell produces e.g. Keratin or Haemoglobin. This then determines what type of cell it is.

Proteins are made in the cell cytoplasm by organelles (parts of cells) called ribosomes.

To get information from the DNA to the ribosome, a copy of DNA is made using a molecule called RNA- single strand. RNA is like a messenger between DNA in the nucleus and the ribosome in the cytoplasm.

1. The 2 DNA strands unzip. A molecule of RNA is made usin one strand of DNA as a template.

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2. RNA molecule then moves out of the nucleus and joins with a ribosome in the cytoplasm.

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3. The job of the ribosome is to stick amino acids together in a chain to make a protein, following the order of the bases in the RNA.

New cells are needed for growth and repair. The cells of your body divide to produce more cells, so your body can grow and replace damaged cells (happens in animals, humans and plants).

There are 2 stages to Mitosis:

First the cell physically grows and duplicates its contents..... The cell copies everything so that when it splits in half, the 2 new cells will contain the right amount of material.

Then the cell splits into 2 by mitosis:

• The cell has 2 copies of DNA and spreads out into long strings.
• Before the cell divides the DNA forms x-shaped chromosomes. Each 'arm' is an exact duplicate.
• The chromosomes then line up at the centre of the cell and cell fibres pull them apart. 2 arms go to opposite ends of the cell.
• Membranes form around each of the sets of chromosomes. These become the nuclei of the 2 new cells.
• Lastly the cytoplasm's divide. You now have 2 new cells containing exactly the same DNA- they are identical to each other and to the parent cell.

Gametes have half the usual number of chromosomes and are produced by meiosis.

During sexual reproduction, the egg and the sperm combine to form a zygote. Gametes (the egg and the sperm) only have one copy of each chromosome (23), so when they combine, they will have a full set of chromosomes (23 from each parent). 

Meiosis involves 2 divisions and produces half the normal number of chromosomes. In humans, meiosis only happens in the reproductive organs.

• 1. Before the cell divides it duplicates its DNA to make double the 46 chromosomes.
• 2. First division:
• The chromosome pairs line up in the centre of the cell.
• The pairs are then pulled apart so each cell only has one copy of each chromosome.
• 3. Second division:
• The chromosomes line up in the centre of the cell. The arms of the chromosomes are pulled apart.
• 4. You get 4 gametes each with only a single set of chromosomes in it.
• 5. After 2 gametes then join at fertilisation, the zygote grows by repeatedly dividing by mitosis.

Cells in an early embryo can turn into any type of cell.

A zygote divides by mitosis to produce a bundle of cells- they are all the same and so therefore embryonic stem cells- they are undifferentiated. This means that they can divide to make any type of specialised cell.

Becoming specialised means that differentiated cells start to differentiate- they develop tissue and organs.

All cells contain the same genes but not all genes are active (switched on). The type of cell a stem cell differentiates into depends on what genes are active- and what proteins the cell produces.

Stem cells may be able to cure many diseases. Adult stem cells that are found in the bone marrow can cure blood disease by having a bone marrow transplant.

Embryonic stem cells- replace faulty cells e.g. heart muscle for people with heart disease.

Adult stem cells are found in the bone marrow and in the umbilical chord.

Cloning could also be used to make stem cells.

Human adult cell cloning can be used so that it wouldn't be rejected by the sufferers immune system.

Cloned embryo- genetically identical to the sufferer.

• Take an egg cell and remove the nucleus.
• Add a nucleus from the adult cell to the embryo.
• If the conditions are right, inactive genes are reactivated and an embryo grows.

Animals stop growing, but plants grow contiuously. In animals, growth happens by cells division. In plants, growth in height is mainly due to cell enlargement- cell division usually just happens in the tips of the roots and shoots.

Meristems (plant tissue) contain unspecialised cells which can divide into any cell type. They act like embryonic stem cells, but they can divide to generate any type of of cell for as long as the plant lives- unlike human cells.

If hormone conditions are right, the unspecialised cells can differentiate into:

• Plant tissue- xylem and phloem (water and food transport tissue).
• Plant organs- flowers, leaves and roots.

As with human cells all the cells contain exactly the same DNA- differentiation is a matter of switching genes on and off.

Meristems are found in parts of the plant that are growing- the roots and the shoot. The dividing meristems produce cells that increase the plants height and girth of the shoot.

Clones of plants can be produced from cuttings. Cuttings will contain unspecialised meristem cells which can differentiate to make any cell. This produces an identical copy of the parent. Reasons for this: it produces a good plant each time and is cheaper.

Phototropism is movement in response to light. Shoots are positively phototropic- they grow towards the light. Roots are negatively phototropic- they grow away from the light. The movement of the roots and shoots helps the plant survive.

Positive phototropism- plants need sunlight for photosynthesis, without light, they cant produce food for energy and growth. Photosynthesis is mainly in leaves- so the plant needs to grow towards the light. Negative phototropism- plants need nutrients and water from the soil to grow. Therefore, roots grow away from the light, down into the soil where they can absorb the water and nutrients for plants to grow healthily.

Auxin's are plant growth hormones near the tips of the shoots and roots. Auxin is produced in the tips and diffuses backwards to stimulate the cell elongation process, which occurs just behind the tips. If the tip is removed- no auxin is available- shoot may stop growing. Auxin's are involved in the responses of plants to light, gravity and water.

If the shoot tip is exposed to light, more auxin accumulates on the side in the shade. shade. This makes cells grow faster on the shaded side so shoots bend towards the light.

Auxin's help cuttings grow into a complete plant. Rooting powder contains hormone auxin  which helps cuttings produce shoots rapidly- this helps produce clones rapidly.