Cell specialisation in animals
In organisms that are multicellular, cells are specialised to do different jobs. Cells of the same type are grouped into tissues eg. muscle cells --> muscular tissue. Different tissues are grouped together and work together in organs. For example, the heart has muscular tissue, epithelial (lining and covering) tissue, blood and nervous tissue. Organs work together as body systems eg. the circularatory system.
Organisms begin life as a zygote (a fertilised egg). The zygote divides by mitosis in 2, 4, 8 etc. to form an embryo. In humans up to and including the 8 cell stage, the cells are identical. These cells are embryonic stem cells which means they have the ability to become any type of cell in the body. After the 8 cell stage, cells become specialised (differentiation), and different tissues form. In specialised cells, only the genes are needed to enable the cell to function, as that type of cell is switched on. In embryonic stem cells, any gene can be switched on.
Cell specialisation in plants
Specialised plant cells form tissues such as the xylem, which transports water and mineral salts, and phloem, which transports the products of photosynthesis. Tissues are organised into organs eg. stems, leaves, roots and flowers.
Cells in regions called meristems are unspecialised. When meristem cells divide into two, the new cell produced can differentiate into different cell types (the other stays as a mersitematic cell). In plants, the only cells that divide are in meristems. Meristems produce growth in height and width (by division of meristem cells followed by enlargement of one of the daughter cells).
At a certain point, animals stop growing and only certain cells are able to produce new cells. Unlike animals, plants keep growing for their whole lifetime.
New plants can be grown by placing the cut end of a shoot in water or soil. Roots grow at the base of the stem, while the shoot continues to grow. Root growth in cuttings is promoted by plant hormones (using hormone rooting powder). Plants grown in this way include garden plants and houseplants. The presence of a meristem (as sources of unspecialised cells) is necessary to clone a plant from cuttings. Pieces of plants eg. plant stems, that have meristems and are used to produce clones are called cuttings. Cuttings can be used to produce new plants with the same desirable features as the parent that are genetically identical to them.
Another method of cloning is called tissue culture. This is when a small piece of tissue, or a few cells taken from a plant root or stem are placed on agar jelly containing nutrients and plant hormones. Enzymes are used to separate the cells before they are placed on the nutrient jelly. Each will grow into a small plant or plantlet and can then be transferred to compost to grow normally..
Plant hormones called auxins are included in the agar for tissue culture and in hormone rooting powder. Auxins increase cell division and cell enlargement, promoting the growth of the plant tissue.
Plant growth and development
Plant growth and development is affected by the environment. Plants' response to the direction of light is called phototropism. Plants grow towards light so they are positively phototrophic.
The plant hormone auxin is produced in the growing tip of plant shoots. It moves down the shoot and produces growth below the tip.
If a plant is illuminated from one side:
- the auxin produced in the tip is distributed towards the shaded side
- the auxin produces growth on the shaded side
- the shoot grows towards the light
Mitosis is a type of cell division that takes placve when an organisms grows, and cells divide to repair tissues. Mitosis results in the production of two daughter cells that are genetically identical. Before mitosis, the DNA in each chromosome is copied. Each chromosome is now a double chromosome with two DNA molecules. During mitosis, each double chromosome separates, so that two nuclei and two cells are produced.
The events between and leading up to the cell division, and cell divison itself, are called the cell cycle. The main processes of the cell cycle are:
- Cell growth: the cell increases in size; numbers of organelles increase; the DNA in each chromosome is copied.
- Mitosis: two daughter cells, each identical to the parent cell and containing an identical set of chromosomes are produced as the strands of each double chromosome separate and two nuclei are formed.
1. Parent cell
2. Chromosomes make identical copies of themselves
3. They line up along the centre
4. They move apart
5. Two daughter cells form with identical chromosomes to the parent cell
Meiosis is the type of cell division used to produce gametes (sex cells-eggs and sperm in animals; eggs and pollen grains in flowering plants). In humans, gametes contain half the number of chromosomes (23) as body cells which contain 46 or 23 pairs.
At fertilisation, gametes (sperm and eggs) join to form a zygote with 46 chromosomes. It's important that the gametes each contain 23 chromosomes or the zygote would end up with 92 chromosomes.
Meiosis produces four daughter cells with half the number of chromosomes.
(see next card for diagram)
Chromosomes, genes & DNA
Chromosomes are thread like structures found in the nucleus. They are made from a DNA molecule and can be grouped into pairs (humans have 23 pairs).
The DNA molecule is a double helix and is made up of two strands facing each other. The strands of DNA are made up of units linked by chemicals called bases. There are four bases: A, T, G & C. A only pairs with A and G only pairs with C. The order of bases in a gene makes up the genetic code. This is the code that gives instructions for the assembly of a protein (the amino acids that are in the protein and the order they are arranged in).
In plant and animal cells, the genetic code that carries the instructions for protein synthesis is on the DNA in the nucleus. Protein synthesis occurs in the cytoplasm. Genes do not leave the nucleus, so in order to carry the genetic code to the cytoplasm, messengar RNA (mRNA) is produced in the nucleus, using the DNA as the template (during which time the two strands of the DNA separate). mRNA carries the instructions for the assembly of proteins (protein synthesis) into the cytoplasm. Proteins are assembled on organelles in the cytoplasm called ribosomes.
The number and sequence of amino acids determines the type of protein and its properties. The sequence of amino acids in the protein is determined by the genetic code. The bases work in threes (base triplets) to code for an amino acid. mRNA is a copy of the base sequence of the DNA that makes up a gene. The mRNA leaves the nucleus and attaches to a ribosome. Transfer RNAs (tRNA) ferry amino acids to the ribosome, where they are bonded together.
KEY STEPS TO PROTEIN SYNTHESIS:
- DNA 'unzips'
- mRNA copies DNA
- mRNA moves to ribosome where proteins are made
- Bases code for amino acids which form proteins
The cell only produces the proteins it needs to carry out its function. The genes to make these proteins are switched on; the others are switched off. These specialised cells begin to make specific proteins. They usually change shape and structure. In embryonic stem cells, meanwhile, any gene can be switched on, so they can produce any type of cell. Stem cells therefore have the potential to replace damaged or diseased cells.
Adult stem cells are found at various locations in the body such as the bone marrow. These cells can be used to produce a limited number of cell types. For examples, bone marrow cells will differentiate to produce types of blood cells.
Using embryonic stem cells raises ethical issues since, in removing cells, the embryo is destroyed. Embryonic stem cells are usually removed from surplus embryos from IVF. The creation of embryos produced with the intention of destroying them would be even more contraversial. Work with stem cells is therefore subject to governemnt regulations.
Using chemical treatment, scientists have managed to transform mammalian body cells into stem cells. Using this technique, inactive genes in the nuclei of body cells have been reactivated. The hope is that the transformed cells will be able to form cells of all cell types.
Therapeutic cloning overcomes some ethcial issues of using embryonic stem cells. It involves replacing the nucleus of an egg with the nucleus of a body cell, then stimulating the egg cell to divide to produce an 'embyro'. The technique does not require fertilisation, and the cells will be genetically identical to the patient's (so will not be rejected by the immune system). The 'embryo' produced is still destroyed after the stem cells are extracted.