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Summary Life Sciences: Grade 12 notes
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These notes cover the Grade 12 Syllabus for Life Sciences
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Topic 1: DNA- The Code of LifeUNIT 1: DEOXYRIBONUCLEIC ACID
Location of DNA in the cell, chromosomes, genes and
extranuclear DNA
DNA- Location in the cell
- Located in the nucleus of a cell and forms part of the chromosomes
structure
- Chromosomes:
● Long strands, made of proteins (histones) and DNA in the
nucleus of a cell, which contains genetic information
DNA- Chromosomes and genes
- When a cell is not dividing = chromosomes become long and thin
- Along each chromosome are sections of DNA called genes
- Genes provide the ‘instructions’ that determine the structure and
functions of cells
Extranuclear DNA
- DNA is also found in the mitochondria of animal and plant cells as
well as chloroplasts in plants
- DNA found outside of the nucleus → Extranuclear DNA
- DNA found in the mitochondria → DNA or mtDNA
- mtDNA:
● Passed on only by the female to her offspring
● Useful in tracing genetic relationships
, Structure of DNA
DNA
- DNA is a long, twisted molecule
- Made up of two strands of material that are twisted into a shape
called a double helix
Nucleotides
- Double helix DNA → Made up of nucleotides
- Nucleotides are the building blocks of DNA
- Consist of a nitrogen-containing purine or pyrimidine base linked to a
sugar and phosphate group
- About 3 billion nucleotides in the human cell
- Made up of:
1. A sugar called deoxyribose
2. A phosphate molecule
3. A nitrogenous base
, Nitrogenous Base
- Consists of adenine, thymine, guanine and cytosine
- Divided into two groups → purines and pyrimidines
- A purine base always pairs with a pyrimidine base
- The amount of each nitrogenous base is different in different
organisms
- There are equal number of adenine and thymine bases and
equal numbers of guanine and cytosine in a DNA molecule
- The order of the bases determine the hereditary characteristics
- Genes → Made up of groups of nucleotides
- The DNA molecule is made up of two parallel strands of nucleotides
- These strands are joined together to form the ladder-like structure
, Discovery of the structure of DNA by Watson, Crick, Franklin
and Wilkins
- In the mid-19th century, scientists knew that genetic material found in
cells was responsible for inherited characteristics
- Middle of the 20th century they discovered it was DNA
- By the early 1950s, scientists discovered the following about DNA:
● DNA consists of molecules of deoxyribose sugar, phosphate
molecules and four nitrogen bases: Adenin, thymine, cytosine
and guanine
● DNA has equal numbers of adenine and thymine bases and
equal number of guanine and cytosine bases
- By 1953 scientists (Maurice Wilkins and Rosalind Franklin) found a
method using X-rays to work out the shape and structure of large
molecules such as DNA
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