OCR Biology 21st Century (new) B5

DNA is a double helix of paired bases:

• a DNA molecule has two strands coiled together in the shape of a double helix.
• Each strand is made up of lots of small units called nucleotides.
• Each nucleotide contains a small molecule called a base. DNA has just four different bases -- adenine (A), cytosine (C), guanine (G) and thymine (T).
• The two strands are held togetherby the bases, which always pair up in the same way -- it's always A-T and C-G. This is called base-pairing.

DNA controls the production of proteins in a cell

• A gene is a section of DNA that contains the instructions for one particular protein.
• Cells make proteins by joining amino acids together 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 together.
• Each set of three bases (called a triplet) codes for one amino acid. 

Proteins are made by ribosomes:

Proteins are made in the cell's cytoplasm by organelles called ribosmes. DNA is found in the cell's nucleus and can't move out of it because it's too big. To get the information from the DNA to the ribosme, a copy of the DNA is made using a molecule called messenger RNA. Messenger RNA is very similar to DNA but it's much shorter and only a single strand.

• Two DNA strands unzip. A molecule of mRNA is made using one strand of DNA as a template. Bas pairing insures an exact match.
• The mRNA molecule then moves out of the nucleus and joins with a ribosome in the cytoplasm. 
• The job of the ribosome is to stick the amino acids together in a chain to amke a protein, following the order of bases in the mRNA.

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. Cells grow and divide over and over again -- this is called the cell cycle. Of course, cell division doesn't just happen in humans -- animals and plants do it too. There are two stages...

1) The cell physically grows and duplicates it's contents
The number of organelles increases during cell growth.
The chromosomes are copied, so that the cell has two copies of its DNA. 

2) then it splits into two by MITOSIS:

• The cell has two copies of its DNA all spread out in long strings
• Before the cell divides, the DNA forms X-shaped chromosomes. Each 'arm' of a chromosome is an exact duplicate of the other.
• The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell.
• Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells.
• Lastly, the cytoplasm divides.
• You now have two new cells containing exactly the same DNA-- they're genetically identical to each other and to the parent cell.

Gametes have half the usual number of chromosomes

• During sexual reproduction, an egg and a sperm combine to form a new cell, called a zygote. 
• All human body cells have two copies of the 23 chromosomes (so 46 in total). But gametes only have one copy of each chromosome (23 in total)
• So when the egg and sperm combine in the zygote will contain 46 chromosomes - one set of 23 from each parent.

Gametes are produced by Meiosis

Meiosis involves two divisions. It produces new cells that only have half the original number of chromosomes. In the humans it only happens in the ovaries and testes (reproductive organs)

"Meiosis produces cells which have HALF the number of chromosomes"

• As with mitosis, before the cell starts to divide, it duplicates its DNA -- one arm of each chromosome is an exact copy of the other arm
• FIRST DIVISON:
  The chromosome pairs line up in the centre of the cell
  The pairs are then pulled apart so each new cell only has one copy of each chromosome.  Some of the father's chromosomes and some of the mother's chromsomes go into each new cell.
• SECOND DIVISION
  The chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart.
  YOU GET FOUR GAMETES WITH ONLY A SINGLE SET OF CHROMOSOMES IN IT 

Cells in an Early Embryo can turn into any type of cell

• A fertilised egg (zygote) divides by mitosis to produce a bundle of cells -- the embryo of the new organisms.
• To start with, the cells in the embryo are all the same. They're called embryonic stem cells.
• Embryonic stem cells are unspecialised. This means they are able to divide to produce any type of specialised cell (e.g. blood cells, nerve cells).
• In humans, all the cells in the embryo are unspecialised up to the eight cell stage.
• The process of stem cells becoming specialised is called differentiation. After the eight cell stage, most of the stem cells in a human embryo start to differentiate. The embryo then begins to develop tissues (groups of specialised cells) and organs (groups of tissues).
• Adult humans only have stem cells in certain places like bone marrow. Adult cells can become specialised but they aren't as versitle as embryonic stem cells --- they can only become certain types of cell.
• All body cells contain the same genes, but in specialised cells miost of the genes are not active -- they only produce the proteins they need.  

Adult stem cells:
Are already used to cure disease. For example, people with some blood diseases (e.g. sickle-cell anaemia) can be treated by bone marrow transplants. Bone marrow contains stem cells that can turn into new blood cells to replace old faulty ones.

Embryonic stem cells:
These can be extracted from very early human embryos. These could then be made to differentiate into specific cells to replace faulty cells in ill people -- e.g. heart muscle cells for people with heart disease. To get one specific type of cell, scientists try to control differentiation of the stem cells by altering the conditions to activiate certain genes. However, it is very difficult and more research is needed. But some people think it;s unethical to use embryonic stem cells because the embryos used to provide the stem cells are destroyed and they could have become a person. It's such as trickyissue that scientific research using embryonic stem cells is regulated by the government.  

• Basically, you take an egg cell and remove its genetic material.
• A nucleus from a body cell of the adult you're cloning is then inserted into the 'empty' egg cell. 
• Under the right conditions, inactive genes in the nucleus of the body cell can be reactivated (switched on) so that an embryo forms.
• Embryonic stem cells can then be extracted from the embryo -- these stem cells could then be controlled to form any type of specialised cell.

Meristems contain plant stem cells

• In plants, the only cells that are mitotically active are found in plant tissues called meristems.
• Meristem tissue is found in the areas of a plant that are growing -- such as the roots and shoots
• Meristems produce unspecialised cells that are able to divide and form any cell type in the plant -- they act like embryonic stem cells. But unlike human stem cells, these cells can divide to generate any type of cell for as long as the plant lives.
• The unspecialised cells can become specialised and form tissues like xylem and phloem (the water and food transport tissues).
• These tissues can group together to form organs like leaves, roots, stems and flowers.

• A cutting is part of a plant that has been cut off it.
• Cuttings taken from a narea of the plant that's growing will contain unspecialised meristem cells which can differentiate to make any cell.
• This means a whole new plant can grow from the cutting which will be a clone of the parebnt plant . 
• Gardeners often take cuttings from parent plants with desirable characteristics, and then plant them to produce identical copies of the parent plant.

Rooting Powder helps cuttings to grow into complete plants

• If you stick cuttings in the soil they won;t always grow. If you add rooting powder, whcih contains plant hormones (auxins), they'll produce roots rapidly and start growing as new plants. 
• this helps growers to produce lots of clones of a good plant, quickly.

Phototropism is growth towards or away from light

• Some parts of a plant, e.g. roots and shoots, can respond to light by growing in a certain direction -- this is called phototropism.
• Shoots are posaitively phototropic - they grow towards the light
• Roots are negatively phototropic - they grow away from light
• Phototropism helps plants to survive:
  POSITIVE PHOTOTROPISM
  Plants need sunlight for photosynthesis. Photosynthesis occurs mainly in the leaves, so it's important for plant shoots, which will grow in the leaves, to grow towards the light.

  NEGATIVE PHOTOTROPISM
  Plants need nutrients and water from the soil to grow. Negative phototropism means roots grow down towards the soil, away fromm the light where they can absorb the water and nutrients the plant needs for healthy growth. 

Auxins are chemicals that control growht near the tips of shoots and roots. Auxins are produced in the tips and diffuses backwards to stimulate the cell elongation, which occurs in the cells just behind the tips.

If the tip of a shoot is removed, no auxins are available and the shoot may stop growing. Auxins are involved in the responses of plants to light, gravity and water. 

AUXINS MAKE SHOOTS GROW TOWARDS THE LIGHT
When a shoot tip is exposed to light, more ausins accumulate on the side that's in the shade than the side that's in the light.

This make the cells grow faster on the shaded side, so the shoot grows towards the light.