- Slides: 28
DNA & GENES
DNA: the molecule of heredity • DNA ultimately determines an organism’s traits. • Within the structure of DNA is the complete instructions for manufacturing all the proteins for an organism.
• DNA is a very long molecule. • It can hold a lot of information. • It is made out of repeating units called nucleotides.
Nucleotide • Composed of 3 parts. • Simple sugar • phosphate group • nitrogen base
Simple Sugar • In DNA the simple sugar is ‘DEOXYRIBOSE’
Phosphate group • Composed of one atom of phosphorus surrounded by 4 atoms of oxygen.
Nitrogen Base • A carbon ring structure, containing some nitrogen. • Four different nitrogen bases:
Adenine • Symbol = A • Classed as a Purine.
Guanine • Symbol = G • Classed as a Purine
Thymine • Symbol = T • Classed as a Pyrimidine
Cytosine • Symbol = C • Classed as a Pyrimidine
• Nucleotides join together to form long chains, with the phosphate group of one nucleotide bonding to the deoxyribose sugar of an adjacent nucleotide.
• In DNA the amount of Adenine is always equal to the amount of Thymine and the amount of Guanine is always equal to the amount of cytosine.
1953 • James Watson and Francis Crick discovered what DNA looked like thus enabling scientists to understand how DNA stored information and worked.
Double Helix • Watson and Crick discovered that: • Adenine always bonded with Thymine. • Guanine always bonded with Cytosine.
• The two bonded bases are called complementary base pairs. • This pairing of the bases causes the DNA to twist like a coiled spring (Helix).
• Since DNA is composed of two strands and it has a helix shape it is referred to as a Double Helix.
• All living things have DNA. • The DNA is the same in every living thing. • What makes living things different is the sequence of the DNA
• The sequence of nucleotides forms the unique genetic information of an organism. • A-T-T-G-G-C-T-A-C carries different information than: • T-C-G-G-C-A-T-T-G
• The more similar the DNA sequence between organisms the closer their evolutionary relationships.
DNA Replication • DNA copies itself during the S phase of Interphase. • Watson and Crick discovered that Adenine always pairs with Thymine and Guanine always pairs with Cytosine.
• Step 1 • DNA replication begins as an enzyme breaks the hydrogen bonds between nitrogen bases that hold the two strands together.
• Step 2 • Nucleotides that are floating free in the surrounding medium (inside the nucleus) bond to the single strands by base pairing.
• Step 3 • This process continues until the entire molecule has been replicated. • The result are two identical strands of DNA.
ASSIGNMENT SA 11. 1 problems 1 -6