Biology B3

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B3 Living and Growng notes

Cell Structure


Ribosomes- where proteins are made. 

Nucleus- contains DNA. It controls the activities of the cell. 


Cell Membrane- controls what enters and leaves the cell. 

Cytoplasm- chemical reactions take place in this. 

Mitochondria- where respiration takes place. 

A very active cell that needs lots of energy (eg muscle cell) will have lots of mitochondria.

A cell that makes lots of protein (eg pancreas cell making insulin) will have lots of ribosomes.

Ribosomes are tiny (about 20nm) and cannot be seen with a light microscope.


nucleus ---> chromosome ---> DNA (deoxyribonucleic acid) 

A gene is a section of DNA that codes for a particular protein. 

Erwin Chargoff- analysed DNA samles using chromotography. He found that the amount of thymine (T) and the amount of cytosine (C) equalled the amount of guanine (G). -1951. 

In 1953, Rosalind Franklin and Maurice Wilkins worked on the X-ray crystallography of DNA. They used a technique called X-ray diffraction to work out how the atoms inside DNA were arranged. The technique involved firing a beam of electrons into crystals of DNA. The X-rays hit the atoms and are deflected making a pattern on a photograpic plate. 

In 1953, James Watson and Francis Crick finally worked out the three-dimensional structure of DNA. They used X-ray diffraction photographs of Franklin and Wilkins, and Chargaff's result to create their 3D model. 

DNA Structure

DNA is a double helix. 


A= adenine

T= thymine

C= cytoseine

G= guanine

A always pairs with T

C always pairs with G. 

This is known as complementary base pairings. 


DNA controls the synthesis of protein in the form of the genetic code.

This code is the particular sequence of bases and it is read in three's= triplet code

The code is copied on to mRNA (messenger RNA) and the proteins are assembled at the ribosomes. 

1. DNA double helix is broken. 

2. mRNA is made using strand = Thymine. 

3. mRNA leaves nucleus and enters cytoplasm. 

4. mRNA joins onto a ribosome. 

5. The mRNA acts as a code for tRNA = Thymine. 

6. Amino acids on neighbouring tRNA join together. 

7. A chain of amino acids is a poly-peptide. 

8. The poly-peptide chain folds to form a protein which is used in the cell or            exported to the body. 


Examples of proteins: 

  • Structual proteins eg keratin (hair), collagen. 
  • Hormones eg insulin.
  • Carrier proteins eg haemoglobin- carries oxygen around the body in red blood cells. 
  • Enzymes- biological catalysts. 

'Breaker' enzyme reaction:

enzyme and substrate (separate) --> substrate in active site --> enzyme (on own) --> products released

'Builder' enzyme reaction:

enzyme and broken substrate (separate) --> broken substrate in active site --> enzyme and substrate (separate)

Five properties of enzymes: 

1. They are all


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