B5 Science

Module B5 (Biology) OCR 21st century 

Growth and Development

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DNA - Making Proteins (1)

Is a Double Helix  of two strands, Each strand is made up on nucleotides, and each one contains a 'base'

DNA has four different bases -

A- adenine, T- thymine, C- cytosine , G- guanine , these bases hold together the two strands, they pair up and are always A-T and C-G. 

DNA controls production of proteins: A gene (a section of DNA) contains instructions for one particular protein. Cells make them by joining amino acids together in a specific order.  The order of bases in the gene tells the cell the order to put the amino acids in. 

Every set of three bases (triplet) is code for ONE amino acid.

DNA determines which genes are on/off and so which proteins the cell produces.

This can also determine what type of cell it is. 

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DNA - Making Proteins (2)

Proteins are made in the cell cytoplasm by organelles called ribosomes.

DNA is found in the nucleus and cannot ,move out of it. BUT the cell needs to get the information from the nucleus to the ribosome. A copy of DNA is made using RNA, which is basically the same as DNA but much shorter and only a single strand. RNA is like a messenger between the nucleus and cytoplasm.

The two DNA strands pull apart and the RNA molecule is made using one as a template and the bases make sure there is a exact match. Now its made the copy, it moves out of the nucleus back to the cytoplasm and joins with the ribosome. 

The ribosome sticks amino acids together in a chain to make a protein in the order of the bases in the RNA. 

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Cells divide so your body can grow, and new cells are used for growth and repair. The division to produce more cells it to replace damaged or faulty ones.

When it splits it has to copy everything. The chromosomes are copied so that they have two copies of its DNA.

The two strands of the DNA molecule split apart, and free floating nucleotides pair up with matching bases on each strand. forming two exact copies of the DNA. 

Now the cell splits into two, so the chromosomes line up at the centre of the cell and the cell fibres pull them apart and each arm goes to opposite ends of cell. Then membranes form around each other the sets, these become the nuclei of the two new cells, then finally the cytoplasm divides.  Now you have two new identical cells, containing the same DNA as each other and the parent cell. 

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All cells divide by Mitosis except in your reproductive organs and they divide to form sperm/egg cells. (gametes)

They have half the usual amount of chromosomes they only have one copy of each chromosome, (23) whereas usually  cells have two copies, meaning (46). But in the reproductive organs the egg and sperm combine the zygote and then this will contain 46 chromosomes, a set of 23 from each parent. 

As in Mitosis it duplicates its DNA, so firstly its divided into two and the pairs of chromosomes are pulled apart so each new cell has a copy and some fathers/mothers chromosomes. Then like in mitosis, the chromosomes in both new cells line up in the centre and by cell fibres pull the arms of the chromosomes apart. Then you are left with FOUR gametes each with only a single set of chromosomes in it. 

After the two gametes (sperm and egg) meet in fertalisation the zygote continually divedes but by MITOSIS. 

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Development from Single Cell/ Stem Cells

Fertalised egg, or otherwise known as a zygote, divides by mitosis and produces lots of cells.

Cells in the embryo are all the same and called embryonic stem cells.

They are undifferentianted, which means they are not specialsed to divide into any specific type of cell. In humans all cells in teh embryo are undifferentiated up to EIGHT cell stage . 

The process when they finally become specialled is called differentiation. As it starts to differentiate it develops tissues and organs. 

All cells contain the same genes, except some are not turned on (active) in each cell type. What the cell differentiates into depends on what genes are on/off and what  proteinss that the cell produces. 

In adults, stem calls can be found in bone marrow, but they are not AS versitile as the ones in the embryo since they can only differentiate into certain types. 

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Stem Cells - curing diseases

Can help to cure diseases because they can be transplanted in and can, for example, turn into new blood cells and replace faulty old ones. This is how sickle-cell anaemia is treated by- bone marrow transplant. 

Embryonic stem cells can be extracted from very early human embryos, and these again can be used to differentiate into specific cells and replace faulty cells in sick people.

It is harder than that though- since to control the differentiation of the stem cells you need to alter the conditions very precisely. 

Cloning can be used to make stem cells, you take the nucleus from an adult cell and remove a nucleus from a egg cell and put them together.


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Growth in Plants (1)

Humans/Animals STOP growing whereas plants can grow continuously throughout there life.

Instead of cell division in plants growth in height is mainly due to cell enlargement/elongation and cell division only happens in the tops of the roots/shoots.

Meristems contain plant stem cells, if the conditions are correct the cells can differentiate into plant tissues and organs.

And similarly to humans the cells contain exactly the same DNA and differentiation is a matter of switching certain genes on or off.

Meristem tissue is found in growing parts of the plant like the roots and shoots.

You can clone a plant by planting a cutting.

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Growth in Plants (2)

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Becky Edwards


this was ok but a little confusing! but keep up the good work

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