Growing and Changing
- In cell is an organism is specialized to do a particular job.
- Tissues are groups of specialized cells of the same type working together to do a job.
- As an embryo or plant grow, the tissues arrange themselves into organs.
- A zygote is a fertilized egg cell and contains instructions for making all cells in the right place and at the right time- DNA.
- A fetus is 2 months old- has main organs.
- There are cells in the early embryo that can develop into complete individuals- embryonic stem cells.
- For living things to grow, some cells must divide to make new cells.
- Plants grow taller by making new cells and they have rings of dividing cells in stems and roots to grow wider- meristem cells.
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- Plant meristem cells are unspecialized and plants keep them throughout their lives.
- Stem cells are also back up cells; they divide and grow, developing into any cell.
- Meristems are used to grow new plants by taking cuttings.
- Rooting powder contains plant hormones called auxins and are used to make cuttings grow better.
- All cuttings taken from one plant have identical DNA- they are clones.
- Taking cuttings is a good way of reproducing a plant with the features you want.
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A Look Inside the Nucleus
- Red blood cells develop from stem cells in bone marrow. As they grow, they make more haemoglobin and by the time they leave the bone marrow, they are full and their nuclei have broken down.
- Different species have different numbers of chromosomes and genes.
- The molecule of DNA has a particular structure that allows it too-
- 1. Make exact copies of itself.
- 2. Provide instructions so that the cell can make the right proteins at the right time.
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Making New Cells
- When new body cells are made, they contain the same number of chromosomes as each other. They also contain the same cell parts- organelles.
- Before a cell divides, it must grow and make copies of-
- 1. Other organelles- such as ribosomes and mitochondria.
- 2. It nucleus- chromosomes.
- Mitosis is cell division. Copies of chromosomes separate and the whole cell divides.
- In mitosis, a complete set of chromosomes goes to each end of cell and forms 2 nuclei. Organelles and cytoplasm go and divide to form 2 new cells.
- Mitosis is used in asexual reproduction to produce cells for a new individual.
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- Males and females produce sex cells or gametes which join at fertilization.
- Gametes are made by a special kind of division called meiosis.
- In humans, meiosis makes gametes that-
- 1. Have 23 single chromosomes (one from each pair).
- 2. Are all different- no 2 gametes have the same genetic information.
- Offspring from sexual reproduction are different from each other and from their parents- they show genetic variation.
- Meiosis starts with normal body cells, it only happens in sex organs.
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The Mystery of Inheritance
- Crick and Watson worked out the double helix structure of DNA.
- There are 4 bases- adenine (A), thymine (T), guanine (G) and cytosine (C).
- A always pairs with T, C always pairs with G- Base pairing.
- All of the 4 bases are equal in amounts- this is true no matter where the DNA is found.
- Base pairing means that it is possible to make exact copies of DNA:
- 1. Weak bonds between the bases split, opening up the DNA from one end to form 2 strands.
- 2. Immediately, new strands start to form from free bases in the cell.
- 3. As A always pairs with T, and G always pairs with C, the 2 new chains are identical to the original.
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Specialized Cells- Special Proteins
- Structural proteins make up the frameworks of cells and tissues.
- Other proteins are essential for the chemical reactions that keep our bodies working (eg. enzymes speed up chemical reactions).
- Each of your cells has a copy of your genes. Something must make the stem cells specialize into different cells- gene switches.
- The one-gene-one-protein theory states that each gene controls the making of 1 protein.
- Not all of these genes are active in every cell- as cells grow and specialize, some genes switch off.
- An early embryo is made of stem cells, all genes are switched on. As it specializes, some switch off.
- The differences in the amounts of proteins in different parts of the egg cell will affect which genes are active.
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- Stem cells could be used for:
- 1. The treatment of some diseases.
- 2. The replacement of damaged tissues.
- Embryonic stem cells are the most useful because the cells are not specialized and the genes are turned on.
- Tissues from embryonic stem cells do not have the same genes as the person getting the transplant. The transplanted tissue is rejected if your body recognises that the cells are not from your body.
- Therapeutic cloning-The nucleus of a zygote is replaced with the patients nucleus from a body cell. The new embryo and stem cells would have the same genes as the patient.
- If adult genes can be switched back on, the person's own stem cells could be used to produce any cells or tissues.
- It is not easy to reactivate genes and the bone marrow (where stem cells come from) is full of other cells; hard to separate.
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- The shape of a protein effects how it works. Cells make proteins from about 20 different amino acids.
- Different combinations of the 4 bases could produce 64 triplet codes- more than one code for each amino acid.
- There are also codes for start and stop, they mark the beginning and end of a gene.
- DNA contains the genetic code for making proteins, but ribosomes make them.
- A small molecules passes through the pores of the nuclear membrane and transfers the genetic code to the ribosomes. The smaller molecule is called messenger RNA (mRNA).
- Differences between DNA and mRNA:
- 1. mRNA has only one strand.
- 2. mRNA has the base U is place of DNA's base T.
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- The direction of the light effects the direction of plant growth- phototropism.
- Darwin found that young shoots of grasses-
- 1. Normally grew towards the light.
- 2. Remained straight when he covered the tips.
- When the light come directly downwards, the auxins spread out evenly; the plant grows straight up.
- The auxins move to the shaded side. The shoot grows faster on the shaded side, bends towards the light.
- Phototropism increases a plants chances of survival as they are able to access more light, increasing the rate of photosynthesis and growth.
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