A zygote is a structure that forms when a sperm fertilises an egg.
The zygote then divides many times by mitosis to form an embryo. The first division of the zygote forms two cells, the next four, the next eight, and so on.
Up to the eight-cell stage, all of the cells are identical. They are called embryonic stem cells. It is possible for embryonic stem cells to develop into any other specialised type of cell that the growing embryo needs - for example, nerve cells, blood cells and muscle cells. However, once the embryonic stem cells become specialised, they can't change into any other type of cell.
The specialised cells can form all the different types of tissue that the embryo needs. Groups of different types of tissues are arranged together to form organs.
Switching Genes On and Off
Cells become specialised because the genes that are not required are switched off. Only the genes needed to make a particular type of cell work are switched on. So muscle cells only have the genes needed to make muscle cell proteins switched on. All the other genes, such as those needed to make blood cell proteins and nerve cell proteins, are switched off.
Cell Specialisation in Plants
Unspecialised stem cells also exist in plants. They can become specialised into the cells of roots, leaves or flowers.
Unlike animal cells, some plant cells can remain unspecialised and develop into any type of plant cell
Whenever you eat a Golden Delicious apple, you are eating a clone. In fact, most of the fruits that we buy in shops and supermarkets are clones. This is because the plants they come from have been grown from cuttings.
Cuttings develop much bigger root systems if they are dipped in hormone rooting powder or planted in rooting compound containing growth hormone. These hormones cause unspecialised stem cells to grow and develop. They turn into tissues such as xylem and phloem, and organs such as roots, leaves, stems and flowers, thereby forming a complete new plant.
This makes it possible to clone plants quickly and cheaply.
Plants cells are different to animal cells in another way. Unspecialised stem cells in plants are grouped together in structures called meristems. Cells produced by meristems ensure plants continue to grow in height and width throughout their life. Animals stop growing in size once they become adults.
Plant meristems divide to produce cells that increase the height of the plant, length of the roots and girth of the stem. They also produce cells that develop into leaves and flowers.
Phototropism is a plant's growth response to light. When the stem grows towards the light, the plant can photosynthesise more. More food is produced, so the plant can grow faster. This increases the plant's chances of survival.
Auxins are plant hormones that make some parts of a plant stem grow faster than others. The result is that the plant stem bends towards the light.
You may have noticed that a houseplant grows towards the window and turns its leaves towards the light. It does this because light coming from the window side of the plant destroys the auxin in that side of the stem. So growth on that side slows down.
On the shaded side of the plant there is more auxin. So growth on this side speeds up. The result is that the shoots and leaves are turned towards the light for photosynthesis.
Auxin is produced in the tip of growing shoots.
If the tips are removed, they cannot produce auxin, so phototropism cannot occur. If the tips are covered, light cannot break down the auxin, so phototropism cannot then occur either.
Phototropism increases the plant's chances of survival
Mitosis is a type of cell division. Mitosis occurs wherever more cells are needed. It produces two new cells that are identical to each other, and to the parent cell. The process of growth and division is called the cell cycle.
The cycle starts as the number of organelles - the different parts of the cell - increases. This is to ensure that each of the two new cells receives copies of all the organelles.
Before a cell divides, its chromosomes are copied exactly. The DNA molecule is made of two strands. As each of the two strands separates, new strands are made alongside each of them, thereby making two new copies.
Meiosis is a different kind of cell division. It is used to produce male and female gametes. A human body cell contains 46 chromosomes arranged in 23 pairs. The gametes are sperm or eggs, and only contain half as many chromosomes (23). This is why meiosis is sometimes called reduction division.
At fertilisation, the nuclei of the sperm and an egg join to form the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg, thereby creating the correct number of 46 chromosomes for all body cells. It also means the zygote contains a complete set of chromosomes from each parent.
DNA is a very large molecule shaped like a twisted ladder. The shape is a double helix.
Long strands of DNA make up chromosomes. These are found in the nucleus of a cell.
DNA is a chemical code, or set of instructions. Our bodies need proteins for growth and development, and the DNA controls which proteins are made. The code consists of four different chemicals, or bases, that always pair up in the same way.
T always pairs with A
G always pairs with C
The order of these pairs of bases along the DNA molecule codes for all the different proteins. A section of DNA that codes for one particular protein is called a gene. Each chromosome contains thousands of different genes.
The genetic code of the DNA always remains safe inside the nucleus. But the proteins are made outside the nucleus in the cytoplasm of the cell. For this to happen, a copy of the genetic code of a gene is made. This copy then passes out of the nucleus into the cytoplasm where the protein is made.
Each gene acts as a code, or set of instructions, for making a particular protein. Some of these proteins control the cell's internal chemistry. They tell the cell what to do, give the organism its characteristics, and determine the way its body works.
To enable genes to code for proteins, the bases A, T, G and C get together not in pairs but in triplets. This is how it works:
Each protein is made up of large numbers of amino acid molecules
Each triplet of bases codes for one particular amino acid
So amino acids are made in the number and order dictated by the number and order of base triplets
Finally, the amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein results.
Proteins are made on small organelles, called ribosomes, in the cytoplasm. The genetic code is transferred to the ribosomes by a small molecule called messenger RNA (mRNA). A protein is made in the following way:
- The gene unzips, mRNA bases pair with DNA bases forming a strand of mRNA
- RNA moves out of the nucleus to one of many available ribosomes within the cytoplasm
- The ribosome becomes attached to one end of the mRNA, gradually moving along the strand reading the genetic code allowing the amino acids to be joined in the correct order
- The protein is released by the ribosome into the cytoplasm
Switching Genes Back On
Scientists can now clone cells and reactivate genes that have been switched off. These reactivated cells have the potential to develop into cells of all the different tissue types, such as muscle, nerve and blood. This is done by changing the genetic material in a human egg cell, by changing its nucleus. The steps in the process are as follows:
- Nucleus taken out of a human egg cell
- Nucleus from a patient's cell put into the egg cell
- Egg cell stimulated to develop into an embryo
- Stem cells taken from the embryo
- Stem cells grown in a container of warm nutrients
- Stem cells treated to develop into required cell types
Stem cells have the potential to produce cells that could be used to replace damaged tissue, such as:
- Making new brain cells to treat people with Parkinson's disease
- Rebuilding bones and cartilage
- Repairing damaged immune systems
- Making replacement heart valves