Biology AQA Genetics


Genetics - DNA

  • Shape - Double stranded, twisted into a double helix and held together by hydrogen bonds.
  • Sugar - Deoxyribose 
  • Bases - Adenine Thymine Guanine Cystosine
  • Hydrogen bonds can be broken for replication
  • Semi conservative 
  • Double helix and phospate-sugar backbone give protection
  • Codes for proteins

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  • Shape - Single stranded
  • Sugar - Ribose
  • Sugars - Adenine, Guanine, Uracil, Cytosine.
  • DNA is copied into RNA which then joins with a ribosome in the cytoplasm for translation (protein synthesis)
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  • Shape - Single stranded clover, with hydrogen bonds
  • Sugar - Ribose 
  • Bases - Adenine, Guanine, Uracil, Cytosine.
  • Has a specific anticodon and an amino acid binding site
  • Hydrogen bonds hold it in shape
  • Found in the cytoplasm
  • Carries amino acids used in translation to make proteins
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Protein Synthesis - Transcription

1. Start with some double stranded DNA

2. RNA Polymerase attaches at the beginning of a gene

3. The DNA uncoils as the hydrogen bonds break

4. One of the two strands is then used to make an mRNA copy

5. The RNA polymerase then lines up free mRNA bases

6. A complementary strand of mRNA has been formed

7. As the RNA polymerase moves along the Hydrogen bonds re-form behind it

8. On reaching a stop signal the RNA polymerase detaches

9. Splicing occurs to remove the non-coding "introns"

10. mRNA moves out of the nucelus through a pore and attaches to a ribosome 

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Protein Synthesis - Translation

1. mRNA attaches to a ribosome

2. tRNA carries amino acids to the ribosome

3. tRNA with a complementary anticodon attaches to the 1st codon on the mRNA

4. Another tRNA molecule binds with specific base pairing to the 2nd mRNA codon

5. The amino acids carried by these two tRNA's join with a peptide bond

6. The 1st tRNA moves on, leaving it's amino acid behind

7. A 3rd tRNA binds to the mRNA and a peptide bond forms again

8. The 2nd tRNA then moves away leaving the amino acid behind

9. The process continues until a stop codon is reached.

10. The polypeptide chain now moves away

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The Genetic Code

The code is non-overlapping, each base triplet is read in sequence and is separate to the one before it.

The code is degenerate, there is more than one possible combination for most amino acids.

The code is universal, the same triplets code for the same amino acids in all living things.

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Transcription Factors

  • Transcription factors control the expression of genes. They're very specific. 
  • They bind to the DNA in the nucleus at the start of their target gene
  • They control expression by controlling the rate of transcription 
  • Some are called "activators" and speed up the rate of transcription by making it easier for RNA polymerase to bind.
  • Others are called "repressors" and slow down transcription by preventing RNA polymerase from binding
  • siRNA is Small Interfering RNA, it cuts up the target mRNA into tiny pieces that can no longer be translated. This is known as translation interference. 
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There are two types of mutation that can occur:

  • Deletion - which results in a frame shift  
  • Substitution - only one amino acid will be changed

Mutations can be caused by errors during DNA replication or by mutagenic agents such as UV radiation, chemicals and some virus'

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Tumour suppressor genes can be inactivated if a mutation occurs in the DNA sequence. 

When functioning normally these tumour suppressor genes slow down cell division by producing proteins, that stop cells dividing or make them self destruct.

If the tumour suppressor is not working properly then the protein isn't produced and the cells can divide uncontrollably. 

Proto-oncogenes can also cause cancer. If a mutation occurs they can be enhanced, which turns them into oncogenes. 

When functioning normally they stimulate cell division by producing proteins that cause the cell to divide. 

When a mutation occurs the gene can become over active. Which can stimulate an excess of protein to be produced and the cells to divide uncontrollably. 

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Cancer Continued

Cancer can be both acquired (lifestyle related) and hereditary. It can also be due to hereditary genetic mutations. 

Diagnosis of cancer usually happens during routine scans of after symptoms have appeared. 

Treatment is often different for different mutations, some can be treated with surgery, others with drugs and some with radiotherapy. 

To prevent acquired cancer (lifestyle related) you can protect yourself from UV radiation by wearing sunscreen, be vaccinated against harmful virus' and live a healthy lifestyle, 

Hereditary Mutations can be treated and prevented by removing the organ or area that the cancer is known to develop in. 

Some people are unlucky enough to inherit genetic disorders that result in cancer. These people are closely monitored with screenings and  can be treated with gene therapy.  To prevent further generations having the mutations the embryos can be screened before implanting them for IVF.

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Stem Cells

Multi-cellular organisms are made up of many highly specialised cells.

These cells all originate from stem cells which have the power to specialise in any area. 

Stem cells are found in the embryo where divide and specialise. They can also be found in some adult tissue where they specialise into cells that need replacing. 

Stem cells that can divide into any type of cell are called totipotent. 

Totipotent stem cells are only found in the embryos of humans, adult stem cells are limited as to what cells they can specialise into. 

Plants have a much better deal, all stem cells in plants are totipotent. 

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Stem Cells in Medicine

Stem cells are very useful, they can be used to repair damaged tissue and can be used to treat leukemia and sickle cell anaemia. 

They could also be used to treat nerve damage, heart disease, bladder conditions, respiratory problems and even to grow new organs. 

Despite their potential there are many ethical issues surrounding the use of stem cells.

  • it involves the destruction of an embryo which has the potential to be a foetus. 
  • Some believe that from the moment of fertilisation an individual has the right to life
  • fewer objections are raised if the embryo is unfertilised
  • Some believe only adult stem cells should be used, these however are not totipotent

The decisions made in society should take into account everyone's views

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