GCSE Biology Module B5

module b5

DNA is a double helix of paired bases

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1) A DNA molecule has two strands entwined together in the shape of a double helix2) Each strand is made up of lots of small units called nucleotides3) Each nucleotide contains a small molecule called a base.4) DNA has just 4 bases, adenine (A), cytosine (C), guanine (G) and thymine (T)The two strands are held together by the bases which always pair up in the same way, A-T and C-G. This is base pairing.

DNA controls the production of proteins in a cell1) a gene is a section of DNA that contains the instructions for one particular protein2) Cells make proteins by joining amino acids together in a particular order 3) Its the order of bases in a gene that tells the cell what order to put the amino acids together4) each set of three bases codes for one amino acid.

Proteins are made by ribosomesProteins are made in the cell cytoplasm by organelles called ribosomes. DNA is found in the cell nucleus and can't move out of it because it is very big. To get the information from the DNA to the ribosome, a copy of the DNA is made using a molecule called messenger RNA. Messenger RNA is very similar to DNA but its much shorter and only a single strand.1) The two DNA strands unzip. A molecule of messenger RNA is made using one strand of the DNA template. Base pairing ensures an exact match.2) The messenger RNA molecule moves out of the nucleus and joins with a ribosome in the cytoplasm.3) the job of the ribosome is to stick amino acids together in a chain to make a protein following the order of bases in the messenger RNA.

module b5

Cell Mitosis

New cells are needed for growth and repairThe 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.

First the cell physically grows and duplicates its contentsThe cell has to copy everything it contains so that when it splits in half the two new cells will contain the right amount of material.1) The number of organelles increases during cell growth2) the chromosomes are copied so that the cell has two copies of its DNAMolecule of DNA splitsBases on free floating nucleotides pair up with matching bases on the DNACross links form between the bases and the old DNA strands and the nucleotides are joined together. So two molecules are identical to the original are formed.

...and 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 replicate of each 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 two new cells.-Lastly, cytoplasm divides.-You now have two new cells, containing exactly the same DNA. They're genetically identical to each other and the parent cell.

Cell Meiosis

Gametes have half the usual number of crisis1) During sexual reproduction an egg and a sperm combine to form a new cell, called a zygote2) All human body cells have two copies of the 23 chromosomes but gametes just have 23.3) When the egg and sperm combine the zygote will contain 46 chromosomes.

Genes are produced by meiosisMeiosis involved two divisions. It produces new cells that only have half the original number of chromosomes.Meiosis produces cells which have half the normal number of chromosomesAs 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 divisionIn the first division in meiosis 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 chromosomes go into each new cell.Second divisionIn the second division the chromosomes line up again in the centre of the cell. It's a lot like mitosis. The arms of the chromosomes are pulled apart.You get four gametes with only a single set of chromosomes in it.After two gametes join at fertilization, the zygote grows by repeatedly dividing by mitosis.

module b5

Animal development

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

1) A fertilised egg divides by mitosis to produce a bundle of cells- the new embryo2) To start with all the cells are the same- embryonic stem cells3) These are undifferentiated. This means they're able to divide to produce any type of specialised cell4) In humans all the cells in the embryo are undifferentiated until the 8 cell stage.5) The process  of stem cells becoming specialised is called differentiation. After the 8 cell stage most of the stem cells in a human embryo start to differentiate. The embryo then begins to develop tissues and organs6) Adult humans only have stem cells in certain places like the bone marrow. Adult stem cells can become specialised but they aren't as versatile as embryonic stem cells- they can only differentiate into certain cell types.7) all body cells contain the same genes but in specialised cells most of the genes are not active- they only produce the specific proteins they need. Stem cells can switch on any gene during their development- the genes that are active determine the type of cell a stem cell specialises into.

Stem cells

Stem cells may be able to cure diseases

Adult stem cells- Are already used to cure disease- Blood diseases such as sickle cell anemia can be treated by bone marrow transplants.- Bone marrow contains stem cells that can turn into new blood cells to replace the faulty old ones

Embryonic stem cells- embryonic stem cells can be extracted from very early human embryos- could be made to differentiate into specific cells to replace faulty cells in sick people- to get one specific type of cell scientists try to control differentiation of the stem cells by altering the conditions to activate certain genes- Some people think it is unethical to use embryonic stem cells because the embryos used to provide the stem cells are destroyed and they could have become a person.- therefore it is controlled by the government.

Cloning can be used to make more stem cells1) basically you take an egg cell and remove its genetic material2) a nucleus from a body cell of the adult you're cloning is then inserted into the empty egg cell3) Under the right conditions, inactive genes in the nucleus of the body can be reactivated so that an embryo forms4) embryonic stem cells can then be extracted from the embryo. These stem cells could then be controlled to form any type of specialised cell.

module b5

Plant development

Meristems contain plant stem cells1) In plants the only cells that are mitotically active are found in plant tissues that are called meristems2) Meristem tissue is found in the areas of a plant that are growing, such as roots and shoots3) Meristems produce unspecialised cells that are able to divide and form any cell type in the plant- they act like embryonic stem cells. Unlike human stem ells, these cells can divide for the plants lifespan.4) The unspecialised cells can become specialised cells and form tissues like xylem and phloem.5) These tissues can group together to form organs like leaves, roots, stems and flowers.

Clones of plants can be produced from cuttings

1) A cutting is part of a plant that has been cut off it2) cuttings taken from an areas that are growing will contain unspecialised meristems which can make any cell3) this means a whole new plant can grow from the cutting which will be a clone of the parenting plant.4) 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 plants1) if you stick cuttings in the soil they won't always grow2) if you add rooting powder which contains plant hormones they'll produce roots rapidly and start growing as  new plants3) This helps growers to produce lots of clones of a really good plant quickly.

Phototropism 

Phototropism is growth towards or away from light1) some parts of a plant, e.g. roots and shoots can respond to light by growing in a certain direction- this is called phototropism2) shoots are positively phototropic, they grow towards light3) roots are negatively phototropic, they grow away from light4) phototropism helps plants to survive.

Positive phototropismPlants need sunlight for photosynthesis. Without sunlight, plants can't photosynthesise, and don't produce food they need for energy and growth. Photosynthesis occurs mainly in the leaves, so its important for plant shoots which will grow leaves, to grow towards light.

Negati

Negative phototropismPlants need nutrients and water from the soil to grow. Negative phototropism means roots grow away from light, down into the soil where they can absorb the water and nutrients the plant needs for a healthy growth

Auxins

Auxins are plant growth hormones

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1) Auxins are chemicals that control growth near the tips of shoots and roots2) Auxins are produced in the tips and diffuse backwards to stimulate the cell elongation process, which occurs in the cells just behind the tip.3) If the tip of a shoot is removed, no auxins are available and the shoots will stop growing.4) Auxons are involved in the responses of plants to light, gravity and water.

Auxins make shoots grow towards light

1) when a shoot tip is exposed to light, more auxins accumulate on the side that is in the shade that the side thats in the light.2) this makes the cells grow faster on the shaded side so the shoot grows towards the light.

DNA & MESSENGER RNA

MITOSIS AND MEIOSIS CELL DIVISION

animal development & StEM cells

plants, phototropism & clones