B5

B5

1. Development of organisms

   • Similar cells form a tissue
     • Groups of tissues form an prgan
       • Groups of organs make up systems within the whole organism
   • Mitosis
     • The division of body cells to produce new cells
     • Each new cells has...
       • Identical sets of chromosomes as the parent cell
       • The same number of chromosomes as the parent cell
       • The same genes as the parent crll
     • Mitosis occurs...
       • For growth
       • For repair
       • To replace old tissues
     • Cells go through a cycle of growth and then division
       • The cycle repeats itself until the cell can no longer divide
     • Parent cell with two pairs of chromosomes
       • Each chromosome copies itself
         • The copies are pulled apart and the cell divides for the only time
           • Two "daughter" cells are formed
   • Meiosis
     • Only takes place in the testes and ovaries
       • Special type of cell division that produces gametes
     • Gametes contain half the number of chromosomes as the parent cell
     • Cell with two pairs of chromosomes
       • Each chromosome replicates itself
         • Chromosomes part company and move to opposite sides with their "copies"
           • Cell divides for the first time
             • Copies separate and the second cell division takes placd
               • Four gametes, each with half the number of chromosomes as the parent cell

2. Fertilisation

   • A male gamete (sperm) and a female gamete (egg) fuse together to produce a single body cell, a ZYGOTE
   • Gametes only have half the number of chromosomes as the parent cell so the zygote produced has a whole set
   • In each new pair of chromosomes, ones from the father and ones from the mother
   • The zygote divides by MITOSIS to produce a cluster of cells called an embryo
     • The embryo continues to divide by mitosis after which the cells become specialised

3. Genes

   • Variation
     • Meiosis and sexual reproduction produce variation between offspring and parents
     • When gametes fuse, genetic information from two individuals is combined
     • For each gene, just each of the parents alleles is passed on
     • Each offspring can have a different combination of alleles from either parent
     • The offspring have different characteristics from each other
   • Genes are present in the chromosomes in each cell nucleas
   • They control...
     • Growth and development in organisms
     • The development of characteristics e.g. Eye colour
   • Genetic code
     • Genes control characteristics by providing instructions for the production of proteisn
     • The instructions are in the form of a code, made up of four bases that hold yeh two strands of the double helix of the DNA molecule together
     • Adenine (A) always pairs with Thymine (T)
     • Cytosine (C) always pairs with guanine (G)
     • A gene is a small section of DNA within the chromosome
       • The order of the bases in the DNA section is the genetic code for the production of a particular protein
         • This is how the code determines the end characteristics of any organism
   • Controlling growth and repair
     • DNA is too large to leave the nucleus
       • The genes therefore stay inside the nucleus but the production of proteins takes place outside the nucleus, in the cytoplasm
         • Information stored in the genes has to be transferred into the cytoplasm
     • The relevant section of DNA is unzipped
       • Instructions are copied onto smaller molecules
         • The smaller molecules are called messenger RNA (mRNA)
           • These molecules leave the nucleus and carry the instructions to the ribosomes
             • The ribosomes follow the instructions to make the relevant protein
     • The sequence of bases in a gene determines the order in which amino acids are joined together to make a particular prouein
     • A group of three base pairs codes for one amino acid in a protein chain is called a triplet code
       • There are 20 different amino acids that can be mafe
     • DNA unravels at the correct gene
       • A copy of the coding strand is made to produce mRNA
         • The mRNA copy moves from the nucleus into the cytoplasm
           • The triplet code is decoded by the ribosomes
             • Amino acids are joined together to form a protein

4. Development of new organisms

   • Up until the 8 cells stage, all cells in a human embryo are unspecialised
     • They can have any gene switched on to form any kind of specialised cell
     • These cells are known as embryonic stem cells
   • After the 8 cell stage cells become specialised and form different types of tissues
     • The cells contain the same genes but many genes are not active (switched off)
       • The cell only produces the proteins in needs to carry out its role
     • In specialised cells, only the genes needed for the cell to function are active as it only requires specific proteins/enzymes
   • Stem cells
     • Potentially used to...
       • Help treat diseases and disorders
       • Repair damage to various tissues
     • Three sources
       • Embtyos
       • Blood from the umbilical cord
       • Adult stem cells from bone marrow
     • Only embryonic stem cells are completely unspecialised and can be used to form any cells type
       • Ethical decisions need to be taken when using embryonic stem cells and this work is monitored by the government
     • In therapeutic cloning...
       • The nucleus is removed from an egg cell and replaced with a nucleus from one of the patients cells
       • The egg cell is then stimulated so that it starts to divide as if it were a xygote
       • At the 8 cell stage, cells can be removed as they're still unspecialised
   • Adult stem cells will only produce cells of a certain type
     • Cells creating blood cells in bien narrow have been encouraged to grow back more of the type of cells by reactivating inactive genes in the nuclei
     • Advantages for using adult cells for growing replacement tissues is that they can be taken from the patient so yeh immune system won't reject it
   • Replacement tissue can be grown in a laboratory
     • Sometimes a "host animal" like a mouse is used to maintain a blood supply during growth

5. Differentiation in plants

   • Plant cells divide by mitosis
   • New cells in plants specialised into the cells of...
     • Roots
     • Leaves
     • Flowers
   • Unlike animals, plants continue to grow in height and width throughout their lives
   • Meristems
     • Plant growth only occurs in areas called meristems
     • Sites where unspecialised cells are dividing by mitosos
       • The cells then differentiate and become specialesed
     • Two types of meristem
       • Lateral - leads to increased girth
         • Located in stem
       • Apical - leads to increased height and longer roots
         • Located in shopt
     • Some plant cells remain unspecialised and can develop into any type of plant cell
       • These cells allow clones of plants with desirable features to be produced from cuttings
       • If the hormonal conditions in their environment are changed, the unspecialised cells can develop into...
         • Tissues like xylem and phloem
         • Organs likes leaves, roots and flowers
   • Xylem and phloem
     • Xylem tubes are used by the plant to...
       • Transport water and soluble mineral salts form the roots to the stem and leaves
       • Replace water lost during transpiration and photosynthesis
     • Phloem tubes are used by the plant to transport dissolved food to the whole plant for respiration or storage

6. Cuttings

   • Plants can be reproduced in the following way...
     • Cuttings are taken from a plant
       • The cuttings are put in a rooting hormone
         • Roots start to form and the new plants develop
   • The new plants are genetically identical to the parent plant (they are clones)
   • Auxins are the main plant hormones used in horticulture
     • They affect cell division at the tip of the shoot
     • They cause cells to grow in size just under the tip so that the stem or roots grow longer

7. Phototropism

   • Where plants respond to light by changing the direction in which they grow
     • They grow towards a light source as they need light to survive
   • Auxin is produced at the shoot tip and moves down the shoot, making cells further down grow
   • When light shines on a shoot, auxin near the light source is slowly destroyed
     • This means there's more auxin on the far side away from the light
       • This causes the cells in the far side to lengthen faster than those near the light, causing the shoot to bedn
   • When a light source is DIRECTLY overhead..
     • The auxin is evenly spread through the agoor
     • The shoot grows straight ip
   • When a light source is at AN ANGLE...
     • Auxin nearest the light sources destroyed
     • The auxin is concentrated on the side furthest away from the light
     • The shoot bends towards the light
   • If the tip of the shoot is removed or covered in opaque material, the plant will continues to grow upwards
   • If the tip is covered with transparent material, then it still grows towards the light psutce
     • The same thing will happen if an opaque cylinder is wrapped around the stem, leaving the tip exposed