GCSE Additional Science: Growth & Development

Mitosis and Meiosis are two ways cells reproduce. By Mitosis a cell splits to create two identical copies of the original cell. In Meiosis cells split to form new cells with half the usual number of chromosomes, to produce gametes for sexual reproduction. 

DNA

Cell > Nucleus > Chromosome > DNA > Gene

DNA is a very large molecule shaped like a twisted ladder. The shape is a double helix. Long Cells of DNA make up chromosomes. These are found in the nucleus of a cell. 

Mitosis is a type of cell division, it occurs when more cells are needed. It produces two 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, it's chromosomes are copied exactly. The DNA molecule is made of two strands. As each of the two strands separate, new strands are made alongside each of them, thereby making two new copies.                                                                                                                                                                                                                                                                                                   

Meiosis is a different type 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 fertilization, 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.

The Genetic Code                                                                                                                                        Gene's are sections of DNA. Each gene is a coded instruction for making a particular protein. DNA is a chemical code or set of instructions, it controls which proteins are made (our bodies need proteins for growth and development). The code consists of four different chemicals or base always pair up in the same way.  T always pairs with A and G always pairs with C.

The order of these pair of bases along the DNA molecule codes for all the various proteins. One section of DNA which codes for one specific protein is called a Gene. Each chromosome contains thousand's of different genes. The genetic code of the DNA always stays 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.                                                                                                                                                                        

The Genetic code (continued): Each gene acts a code/ set of instructions for making a particular Protein. Some of these proteins control the cell's internal activity. They Instruct the cell what to do, give the organism it's characteristics, and determine the way it's 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 & order dictated by the number & 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.             

Cell Specialization; 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 1st division of the zygote forms 2 cells, then 4, 8 so on. Up to the 8-cell stage all of the cells are identical. It's possible for embryonic stem cells to develop into any other specialized type of cell that the growing embryo needs- for eg nerve cells, blood cells & muscle cells. But 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 type of tissue that the embryo needs.

Switching gene's off and on; Cells become specialised because the genes that are not needed are switched off. Only the gene's needed to make a specific type of cell work are switched on eg muscle cells only have the gene's needed to make muscle cell proteins switched on. All the other gene's eg those needed to make blood cell proteins & nerve cell proteins are switched off.                                                                                                                                                                                                                                                                                                                                                                                                          

Cell Specialization in Plants: Unspecialised stem cells also exist in plants. They can become specialised into the cell of roots, leaves or flowers. Unlike animal cells, some plant cells can remain unspecialised and develop into any type of plant cell.

Cloning Plants:  This is done by taking a cutting of the original plant and growing it in a special hormone called auxin. Cuttings develop much bigger root systems if they are dipped into hormone rooting powder or planted in rooting compound containing growth hormone. These hormones cause unspecialised stem cells to grow and develop. They turn into tissue's such as xylem and phloem, and organs such as roots, leaves and flowers, hence forming a new plant. This makes it possible to clone plants quickly & cheaply.

Meristems:  Plants cells differ to animal cells by another way, unspecialised stem cells in plants are grouped together in structures called meristems. Cells produced by meristems ensure that plants continue to grow in height and width throughout their life. Plant meristems divide to produce cells that increase the height of the plant, length of the roots & girth of the stem. They also produce cells that develop into leaves & flowers.

Phototropism:  Is a plant's growth response to light. When the stem grows towards the light, the plant can photosynthesis more. More food is produced so the plant can grow faster. This increases the plant's chances of survival. 

Switching genes back on : Cells can now be cloned and genes can be reactivated that have been switched off. These reactivated cells have the potential to develop into cells of all the different tissue types, such as nerve, muscle & blood. This is done by changing the genetic material in a human egg cell, but changing it's nucleus. The steps in the process are as follows:

• Nucleus taken out of a human egg cell
• Nucleus from a patients cell put into the egg cell
• Egg cell stimulated to develop into an 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: repairing damaged immune systems, making replacement heart valves, rebuilding bones & cartilage.)

Phototropism & Auxin: Auxin's and 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. 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.   

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.