Class notes Life science

GRADE 12 LIFE SCIENCES: DNA THE CODE OF LIFE SUMMARY-PART 1

-All living organisms have DNA and RNA
1.DNA

● DNA - Deoxyribonucleic acid -Is mainly found on chromosomes in the nucleus.
● Gene is a segment of DNA that carries genetic information of a particular
characteristics.(Also carries code for proteins).
● Three places in which DNA can be found:
- Nucleus: Known as nuclear DNA ;makes up genes on chromosomes.
- Mitochondria: Mitochondrial DNA ; used for tracing ancestry
- Chloroplasts: Chloroplast DNA ;only found in plants(humans cannot photosynthesise
therefore do not have chloroplasts.)
● Discovery of DNA
-In 1952 ,Rosalind Franklin and Maurice Wilkins researched the structure of DNA using X-ray
diffraction images.
-The diffraction images were used by Watson and Crick to create a 3-D double helix model for
DNA in 1953.
-Watson and Crick received the Nobel Prize for the discovery of the structure of DNA in 1962

The structure of DNA-Natural shape is a double helix
Each strand of the double helix is made up of a sequence of DNA nucleotides.The nucleotides
of the DNA molecules consist of:
1. Phosphate
2. Deoxyribose sugar(THE PENTOSE SUGAR FOR DNA)
3. Nitrogenous base:
a.Adenine(A)-Purine
b. Thymine(T)Pyrimidine
c.Cytosine(C)-Pyridine
d.Guanine(G)-Purine
4. Weak hydrogen bonds between the nitrogenous bases




● PURINES-DOUBLE RING STRUCTURE
● PYRIMIDINES-SINGLE RING STRUCTURE

● The nitrogenous bases are complementary to each other:

Adenine(A) is complementary to Thymine(T)
(Apple Tree)
Cytosine(C) is complementary to Guanine(G)