B5 revision

DNA - double helix of paired bases. Two strands coiled together.

Nucleotide: a strand of DNA. (Contains a base)

Adenine - Thymine
Cytosine - Guanine

Always A and T and always C and G. (If this is not the case it is a mutation)

Base pairing hold the bases together.

Gene - section of DNA.

DNA controls production of proteins in a cell.

Proteins: made by joining amino acids together in a particular order.

Proteins are made form by ribosomes (organelles)

Messenger RNA (mRNA) gets the information from the DNA to the ribosome by making a copy- in a more detailed explaination of the method:

1. 2 DNA strands 'unzip' using a strand of DNA as a template. The mRNA is made.
2. This moves out of the nucleus and joins a ribosome in the cytoplasm.
3. This sticks the amino acids together in a chain to make the protein.

Each triplet codes for one amino acid.

In mitosis the cells split into two and are needed for growth and repair.

How it works:

1. Cell grows physically in size and duplicates its contents.
2. Number of organelles increase.
3. Chromosomes are copied so there are two sets of DNA.
4. The chromosomes line up (along the imaginary metaphase plate) and fibres pull them apart; cells now divide for the only time in the mitosis sequence. (Metaphase)
5. Membranes form around each of the sets of chromosomes. (Anaphase)
6. Cytoplasm divides and nuclear membrane re-forms; chromosomes de-condense (telophase).
6. Two daughter cells are formed.

The cycle repeats itself until the cells can no longer divide.

Gametes have half the usual number of chromosomes (23).
Meiosis produces cells which have half the normal number of chromosomes.
Meiosis is cell division only for sexual reproduction.
Only takes place in testes and ovaries.

During fertilisation, a male gamete (sperm cell) and female gamete (egg cell) fuse to produce a single body cell called a zygote. One chromosome come from male, one from female.

How it works:
1. Cell grows physically in size and duplicates its contents.
2. Number of organelles increase.
3. Chromosomes are copied so there are two copies of DNA.
4. Chromosomes line up (along imaginary metaphase plate) and fibres get pulled apart; this is the first cell division of the sequence.
5. Chromosome pairs line up again and are pulled apart. Membranes re-form around chromosomes. 4 gametes with the a single set of genetically varied chromosomes are produced; four daughter cells are formed.

Embryonic stem cells:

Embryo's at the 8 cell stage have cells that can differentiate into any type of cell; embryonic stem cells are unspecialised.

Adult stem cells:

Can still be used for growth and repair but only for certain tissues. Used to cure diseases e.g. Sickle cell anaemia which can be treated by bone marrow transplants (bone marrow contains stem cells that can turn into blood cells and replace faulty ones).

Plant stem cells:
Meristems contain last stem cells; they are mitocially active (used for growth and repair). Found in roots and shoots and are similar to embryonic stem cells but are active throughout the plants lifetime. Plant stems cells form tissues like xylem and phloem. These can group together to form organs like leaves.

A clone is a genetically identical organism.

Cloning can be used to make stem cells.

Clones of plants can be produced from cuttings.

Cuttings are taken from parent plants with desirable characteristics. Cutting reproduces by asexual reproduction; growth and new cells are produced; a genetically identical organism is produced.

Rooting powder helps cutting to grow into complete plants as they contain auxins that produce roots. This produces clones more quickly.

Phototropism is the growth towards or away from light.

Positive- needs sunlight for photosynthesis so grows towards the light.

Negative- needs soil for nutrients so roots grow away from light down into the soil so they can absorb more water.

Auxins are plant growth hormones produced in the tips of plants and diffuse backwards to stimulate the cells elongation behind the tips. If the tip of the pant is removed, the plant cannot grow.

Auxin hormones are not produced in exposure to light, therefore auxins are only released on the shaded side of the plant, meaning only the dark side of the plant grows, hence why plants bend toward the light.