B5 - Growth and development

B5 - Growth and development

1. DNA is a double helix of paired bases (a DNA molecule has two strands coiled together in the shape of a double helix (two spirals).

   • 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 bases - adenine (A), thymine (T), Guanine (G) and Cytosine (C)
         • The bases hold the two stands together, they always pair up in the same way - A+T and G+C
   • STRAND -> UNITS -> MOLECULE

2. DNA controls the production of proteins in a cell.

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

3. Proteins are made by ribosomes

   • Proteins are made in the cell cytoplasm by organelles called ribosomes.
     • DNA is found in the cell nucleus and can't get out of it because it's really big.. To get the information from the DNA to the ribosomes, a copy of the DNA is made using messenger RNA. Messenger RNA is very similar to DNA but it's much shorter and only a single strand.
       • This is how it happens:
         • The DNA strands UNZIP. A molecule of messenger RNA is made by using one strand of the DNA as a TEMPLATE. Base pairing ensures that it's an exact match. The messenger RNA molecule moves out of the nucleus and JOINS with a ribosome in the cytoplasm.
           • The job of the ribosome is to stick amino acids (3 bases of DNA) together in a chain to make a protein, following the order of bases in the messenger RNA.

4. New cells are needed for growth and repair. The cells of your body divide to produce more cells. 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 cycle. Cell division doesn't just happen in humans, animals and plants do it too.

   • There are two stages of cell division -
     • 1) First the cell physically grows and duplicates
       • The cell has has to copy everything it contains so that when it splits in half, the new cell contains the right amount of material. DADDY (DNA is copied) BUYS (cell gets physically bigger - it grows) ORANGES (number of organelles increase).
     • 2) And then it splits in to two by Mitosis.
       • Mitosis
         • The cell has copies of 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 of each chromosome go to opposite ends of the cell. Membranes then form around each of the sets of chromosomes. The two ends of the chromosomes go to opposite ends of the cell. Lastly the cytoplasm divides. You now have two genetically identical cells and to the parent cell.

5. 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 body cells have two copies of the 23 chromosomes (46 in total). But gametes only have one copy of each chromosome, so when the egg and the sperm combine to form a zygote, it will contain 23 pairs of chromosomes (46 chromosomes).

6. Meiosis

   • Meiosis involves two divisions- it produces new cells that have half the original number of chromosomes. In humans Meiosis only happens in the ovaries and testes.
     • At first with meiosis the cell gets bigger, the DNA duplicates and the number of organelles increase.
       • First division = The chromosome pairs line up in the centre of the cell. The pairs are then pulled apart, so the two new cells only have one copy of each chromosome like a normal cell.
         • Second division = The chromosomes line up again in the centre of the cell, just like mitosis the cell fibres pull the arms of the chromosome apart.
           • 1 ->2 ->4
             • After two gametes join at fertilisation, the zygote grows by repeatedly dividing by mitosis.