B5

Cells are the building blocks of all living things.

Cells contain..

- DNA

- Organelles

DNA molecules are in the form of a double helix and contain the genetic code

Organelles = different parts of the cell's structure

Animal and plant cells = have cytoplasm, a nucleus, ribosomes and a cell membrane

Plant cells = have a cell wall, permanent vacuole, chloroplasts

Mitosis = division of body cells to produce new cells, each new cells contains the same genetic information as the parent cell.

Meiosis = division of cells in the testes and ovaries to produce gametes for sexual reproduction

Gametes = contain half the number of chromosomes as the parent cell.

During fertilisation a male gamete and a female gamete fuse together to produce a zygote.

Zygote = has one whole set of chromosomes

After fertilisation the zygote divides by mitosis to produce an embryo.

Genes = present on the chromosomes in each cell nucleus

Genes control....

- growth and development in organisms

- development of characteristics

Meiosis and sexual reproduction produce variation between offspring and parents as genetic information from two individuals is combined.

Genetic code = made up of 4 bases which hold the two strands of the DNA molecule together

Bases are always paired up in the same way...

- Adenine with Thymine

- Cytosine with Guanine

mRNA = small molecules which leave the nucleus and carry genetic information into the cytoplasm

Ribosomes = follow instructions from the mRNA to make proteins

A group of 3 base pairs codes for one amino acid in a protein chain, called a triplet code. There are 20 different amino acids that can be made.

Embryonic Stem Cells = unspecialised cells that can turn into any kind of cell

After the eight cell stage, the cells in an embryo become specialised and can form different types of tissue.

Stem cells can be used to treat diseases and disorders and repair damage to various tissues.

The three sources of stem cells are embryos, blood from the umbilical cord and adult stem cell.

New cells in plants specialise into the cells of roots, leaves and flowers.

Meristem = area of plant growth, where specialised cells divide repeatedly.

Lateral meristem = leads to increased growth

Apical meristem = leads to increase height and longer roots

Xylem tubes = transports water from roots to stem and leaves and replaces lost water.

Phloem tubes = transports dissolved food to the whole plant for respiration or storage.

New plants develop when cuttings are taken from a plant and put in a rooting hormone.

The new plants are clones of the parent plants.

Auxins = main plant hormones used in horticulture which cause cell growth and division.

Phototropism = growth of plants towards a light source.

Light source overhead - auxin spread evenly - shoot grows straight up

Light source at an angle - ausin on side furthest from light - shoot bends towards light