B2 Topic 2

Revision cards for B2 topic 2 (divide and develop) -most of my notes are from the cgp book.

HideShow resource information
  • Created by: shamB
  • Created on: 10-11-11 18:53

Measuring Growth.


you can measure the growth of an organism in these three ways:

1) size - you can measure its height, length, width or circumference

2) wet weight - the weight of the organism depends on how much water it has gained or lost. (e.g through drinking or sweating), the wet weight of the organism is its weight including all the water in its body *it can vary from one day to the next*

3) dry weight - the weight of an organism with no water in its body, this doesnt vary in the same way as wet weight, but you can only measure it once the organisms dead. the dead organism is dried out by leaving it in a hot oven overnight and whatevers left is weighed

1 of 34



  • individual organisms of the same species vary in size e.g humans arent all the same height.
  • each species has a range of size which most individuals fall within e.g most adult humans are between 4'8" and 6'8" in height.
  • when idividuals within a species have a characteristic which varies along a range like this, its called continuous variation. size (e.g height, length) is an example this.
  • individuals also have characteristics that fall into distinct categories e.g sex can either be male or female - this is called discontinous variation.
  • lots of factors affect how an individuals organisms grow and what size they become e.g how tall humans are able to grow is influenced by their genes, hormones and diet.
2 of 34



1) a few animals have the ability to regenerate (regrow) parts of their body if it is damaged e.g - if a young spider loses a leg, it can grow a new one whereas adult spiders cannot

2) the ability to regenerate is very rare and it tends to happen in fairly simple or very young animals which still contain a lot of stem cells

3 of 34

Growth Factors in Sport.


  • growth factors are chemicals which stimulate the body to grow and to make extra muscle
  • some athletes have used growth factor drugs (e.g steroids) to improve their performance at sport. this is banned because it gives them an unfair advantage over other competitors and has health risks such as: bad side effects for health (reduce fertillity, increase the risk of heart disease and can sometimes lead to mental illnesses like depression) and some cause women to develop male characteristics such as a deeper voice
  • athletes are given random tests for growth factor drugs
4 of 34

Mitosis Makes New Cells For Growth and Repair


  • human body cells are diploid. this means they have two versions of each chromosome (one from the persons mother and one from the persons father)
  • when a cell divides it makes two cells identical to the original cell - each with a nucleus containing the same number of choromosomes as the original cell
  • this type of cell is called mitosis. its used when humans and animals and plants want to grow or to replace cells that have been damaged
5 of 34


*in a cell that is not dividing, the DNA is spread out in long strings.

*if the cell gets a signal to divide, it needs to duplicate its DNA so that theres one copy for each new cell. the DNA is then copied and forms X-shaped chromosomes. each 'arm' of the 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 chromosome go to opposite ends of the cell.

*membranes form around each of the sets of chromosomes. these become the nuclei of the two cells.

*lastly the cytoplasm divides.

you now have two new cells containing exactly the same DNA - theyre identical.

6 of 34

Hayflick Limit.


  • most cells have a limit to the number of times they can divide. this is called the hayflick limit
  • after they reach this limit the cell stops dividing. the human limit is about 52 divisons
  • stem cells and cancer cells have no hayflick limit. they can divide as many times as they want to and this is what makes cancer cells dangerous - they continue growing uncontrollably and form tumours
7 of 34



  • gametes are sex cells. theyre called ova (single, ovum) in females and sperm in males. during sexual reproduction, two gametes combine to form a new cell which will grow to become a new organism
  • gametes are haploid (they only have one copy of each chromosome) this is so that when the gametes combine in sexual reproduction, the cell that is created (zygote) has the right number of chromosomes
8 of 34


* before the cell starts to divide it duplicates its DNA with one arm of each chromosome an exact copy of the other arm

*in the first division of meiosis the chromosome pairs line up in the centre of the cell

*theyre then pulled apart so each new cell has only one copy of each chromosome. some of the fathers chromosomes and some of the mothers chromosomes go into each new cell

*each new cell will have a mixture of the mothers and fathers chromosomes. mixing up the genes in this way creates variation in the offspring. this is a huge advantage of sexual reproduction compared to asexual reproduction

*in the second stage of division the chromosomes line up again in the centre of the cell. the arms of the chromosomes are then pulled apart

you get four gametes, each with only a single set of chromosomes in it

9 of 34

Embryonic Stem Cells.


1) a fertilised egg can divide by mitosis to produce a bundle of cells - the embryo of the new organism

2) to start with the cells in the embryo are undifferentiated (all the same) they are called embryonic stem cells

3) stem cells are able to divide to produce either more stem cells or different types of specialised cells e.g blood cells

4) the process of stem cells becoming specialised is called differentiation

5) adult humans only have stem cells in certain places like the bone marrow. these stem cells arent as versatile as the stem cells in embryos - they can only differentiate into certain types of cell

10 of 34

Stem Cells.


1 - doctors use adult stem cells to cure some diseases e.g sickle cell anaemia can sometimes be cured with a bone marrow transplant (containing adult stem cells which produce new blood cells)

2 - scientists have experimented by extracting stem cells from very early human embryos amd growing them. under certain conditions the stem cells will differentiate into specialised cells

3 - it might be possible to use stem cells to create specialised cells to replace the ones which have been damaged by disease or injury

11 of 34

Ethical Concerns Of Stem Cell Research.

  • some people are against embryonic stem cell research as they argue thhat human embryos shouldnt be used for experiments because each one is a potential human life. they think scientists should find other sources of stem cells
  • other people think that the aim of curing patients who are suffering should be more important than the potential life of the embryos. they also point out that the embryos that are often used are from fertility clinics so if they werent used for research they would most likely be destroyed
  • in some countries stem cell research is banned. it is allowed in the UK under strict guidelines
12 of 34



after the 8th week of pregnancy the embryo starts to look a bit more like a human and is then called a foetus. in britain a termination (abortion) is legal until a foetus is 24 weeks old and if two doctors agree that termination is necessary. a termination can be carried out at a later stage but only if the pregnancy is putting the mothers health at serious risk or if there is a major foetus abnormality. the limit is 24 weeks because this is the age that the foetus can usually survive with medical help outside the womb.

13 of 34

Ethical Concerns Of Termination.

  • some people argue that abortion at any stage of the pregnancy is unethical. they argue that human life starts at fertilisatin and ending the pregnancy is the same as killing a human being
  • other people argue that the foetus doesnt become human until it is conscious. e.g until it can feel pain. they argue that abortion should be allowed until this point.
14 of 34

Growth and Distribution of Plants.


[1] soil nutrients: the soil contains nutrients essential for plant growth

[2] light: plants need sunlight for photosynthesis

[3] temperature: plants grow best when its warm but not too hot. the reactions for photosynthesis and respiration happen quicker in warmer temperatures - so the rate of growth increases. but if it gets too hot then enzymes involved in the reactions will be destroyed and growth stops

[4] carbon dioxide: plants need CO2 for photosynthesis. if there isnt enough, plant growth slows down

[5] oxygen: plants need oxygen for respiration (which provides energy for growth) plants absorb oxygen from the atmosphere and also create it as a by-product of photosynthesis

[6] plant hormones: they can stimulate growth

15 of 34

Specially Adapted Plants.

some environments arent perfect for plant growth. plants that grow in these areas will often be specially adapted to be able to survive in poor conditions e.g

Environment: Sand Dunes.

Conditions: Poor soil containing little water.

Specially Adapted Plant: Maram grass has curled up leaves to reduce water loss

Environment: Woodland.

Conditions: low light intensity - trees overshadow smaller plants

Specially Adapted Plant: Bluebells store food over winter so that it can grow quickly in spring before the trees are in leaf

16 of 34



1 - auxins are plant hormones that control growth near the tips of shoots and roots

2 - auxin is produced in the tips and diffuses backwards to stimulate the cell enlargement process which occurs in the cells just behind the tips

3 - if the tip of the shoot is removed then no auxin is available and the shoot may stop growing

4 - auxins are involved in the responses of plants to light, gravity and water

17 of 34

Auxins Change Direction of Root and Shoot Growth.

extra auxin promotes growth in the shoot but actually inhibits growth in the root - but this produce the desired result in both cases



1 - when a shoot tip is exposed to light, more auxin accumulates on the side thats in the shade than the side thats in the light

2 - this makes the cells grow (elongate) faste on the shaded side, so the shoot bends towards the light

3 - the growth of a plant in response to light is called phototropism

18 of 34

Auxins Change Direction of Root and Shoot Growth.


1 - when a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip with more auxin on the lower side

2 - this causes the lower side to grow faster, bending the shoot upwards

19 of 34

Auxins Change the Direction of Root and Shoot Grow


1 - a root growing sideways also has more auxin on its lower side

2 - but in a root the extra auxin inhibits growth. this means the cells on top elongate faster and the root bends downwards

20 of 34

Auxins Change The Direction of Root and Shoot Grow


1 - an uneven amount of moisture either side of a root produces more auxin on the side with more moisture

2 - this inhibits growth on that side causing the root to grow in that direction (towards the moisture)

21 of 34

Seedless Fruits.


  • fruit (with seeds in the middle) normally only grow on plants which have been pollinated by insects. if the plant doesnt get pollinated the fruit and seeds dont grow
  • if growth hormones are applied to the unpollinated flowers of some types of plant, the fruit will grow but the seeds wont. some seedless citrus fruit can be grown this way
  • hormones are also used in the first production of seedless grapes (although these are usually fertilised first)
22 of 34

Selective Breeding.

selective breeding is when humans select the plants or animals that are going to breed and flourish according to what we want from them. its also called artificial selection

this is the process:

  • from existing stock, the organisms which have the best characteristics are selected
  • theyre bred with each other
  • the best of the offspring are selected and bred
  • this process is repeated over several generations to develop desired traits
23 of 34

Advantages of Selective Breeding (For Farmers)

  • farmers can improve the quality of milk from cattle
  • farmers can increase the number of offspring in sheep
  • farmers can increase the yield from dward wheat
24 of 34

Disadvantages of Selective Breeding.

  • only some of the original population is bred from so theres less variety in the gene pool of the organisms
  • all the organisms in the crop/herd will be closely related and have similar characteristics, this includes their level of disease-resistance. some diseases might be able to wipe out the whole lot
  • some of the characteristics encouraged by selective breeding are beneficial for humans but not for the organisms themselves
25 of 34

Cloning/Genetic Modification.

cloning is using an organisms DNA to create a genetically identical new organism

genetic modification is changing the DNA of an organism

26 of 34

Dolly the Sheep.

  • dolly was made by taking a sheeps egg cell and removing its nucleus (leaving the egg without any genetic material) the nucleus removed from the egg cell was haploid (containing half the normal number of chromosomes) because the egg cell was a gamete
  • another nucleus was inserted into the egg cell in its place. this was a diploid nucleus from a cell of the parent sheep and it contained the full number of chromosomes. this process is called nucleur transfer
  • the cell was stimulated so it started dividing by mitosis, as if it was a normal fertilised egg cell. it formed an embryo
  • the embryo was implanted into the uterus of a female sheep which carried and gave birth to it. the new sheep was genetically identical to the parent sheep which the nucleus was taken from
27 of 34

Cloned Mammals Dont Live As Long.

1 - dolly the sheep only lived for 6yrs (half as long as many sheep) she was put down because she had lung disease and arthritis.

2 - the risks and problems associated with cloning:

  • the cloning process often fails, it took hundreds of attempts to clone dolly
  • clones are often born with genetic defects
  • cloned animals' immune systems are sometimes unhealthy so they suffer from more diseases
28 of 34

Genes Can Be Transferred.

useful genes can be transfered into the DNA of animals and plants at a very early stage of their development-shortly after fertilisation. this is called genetic modification.(GM)

potential uses:

  • GM plants have been developed that are resistant to viruses and herbicides (chemicals used to kill weeds) and long life tomatoes can be made by changing the gene that causes the fruit to ripen
  • genes can also be inserted into animal embryos so that the animal grows up to have more useful characteristics
  • genetic disorders like cystic fibrosis are caused by faulty genes. scientists are trying to cure these diseases by inserting working genes into sufferers. this is called GENE THERAPY
29 of 34

Concerns of Geneic Modification.

  • some people strongly believe that we shouldnt mess with genes because its not atural
  • there are also worries that changing an organisms genes ight accidently create unplanned problems which could then get passed onto future generations
30 of 34

Inherited Disorders are Caused By Faulty Genes.

  • each gene in your DNA codes for the production of a specific protein
  • if a gene is faulty it might produce the wrong protein or not produce a protein at all
  • this can cause genetic disorders e.g cystic fibrosis. these disorders are hereditary - parents can pass the gene on to their children
  • sometimes a single faulty gene causes a disorder
  • other diseases are thought to be caused by a combination of inherited genes and the environmental factors e.g types of cancer
31 of 34

Gene Therapy.

  • gene therapy is a new experimental treatment for genetic disorders
  • it can involve inserting a new, functional version of a faulty gene into a patients cells. these cells would then be able to make the correct protein and the symptoms would dissapear
  • the gene therapy would be targeted at areas that were badly affected by the disease
  • in the long term scientists hope to make these changes to the patients DNA permanent but so far the trials testing gene therapy have only shown very temporary improvements
  • gene therapy still needs a lot of research and testing. potential dangers of gene therapy have already been found e.g 'suicide genes' could be taken up by healthy genes
32 of 34

Treating Cancer With Gene Therapy.

  • genetic material could be introduced that improves the bodys natural immune system e.g it could make the cancer cells more obvious to the cells of the immune system
  • the gene therapy could be targeted at the cancer cells to cause their death or stop them from growing e.g a gene for a drug could be introduced into the cancer cells which would produce the drug and kill the cell. (suicide genes)
  • the cancer cells could be targeted with genes that will make them more sensitive to treatments like chemotherapy and radiotherapy
  • if a person carries a version of a gene that makes them more likely to get cancer, a normal copy of the gene could be introduced. this may prevent the cancer from developing in the first place but it couldnt be used to treat an existing disease
33 of 34

Faulty Genes Will Still Be Passed On.

  • even if someone is treated with gene therapy their children will still inherit the faulty gene
  • this is because gene therapy is targeted to an are of the body the disease affects. the working healthy gene in incorporated into those specific cells and not into the reproductive cell that makes eggs and sperm. it would be very difficult to target the reproductive cells using gene therapy
  • to remove or destroy the disease from a family you would need to select a healthy embryo during IVF or genetically engineer the embryo to replace the faulty gene (which is illegal in the UK)
34 of 34


No comments have yet been made

Similar Biology resources:

See all Biology resources »