Structure & Function of DNA

1. By C. Kohn, Waterford WI Structure & Function of DNA

2. What are genes? • You now know that genes encode for specific traits like eye color, ear lobes, and milk production. • A gene is simply a section of DNA that creates the proteins responsible for a specific trait. • Genes are found in DNA; chromosomes are made of DNA

4. Structure of DNA • DNA has several key components • A Phosphate Molecule • A Sugar Molecule • A Nitrogenous Base (A,T,G,C)

5. Classification • The sugar and phosphate molecules comprise the ‘skeleton’ or ‘backbone’ of DNA • The nitrogenous base is used to encode the actual information on the gene needed to create the protein (a base is the C,G, T, or A)

6. Nucleotides • A nucleotide is a subunit (or building block) of DNA consisting of a base, a phosphate, and a ribose sugar. Nucleotide

7. Bases • DNA has four nitrogenous bases • Adenine (A) • Thymine (T) • Guanine (G) • Cytosine (C) • All information encoded in DNA exists through different combinationsof these four letters.

8. Bases (cont) • The DNA bases always exist in the same kinds of combinations • A always pairs with T • G always pairs with C • “Great Combinations, Always Together”

9. Bases (cont) • A-T and G-C combos must occur for two main reasons • 1. This is the only way they will fit inside the framework of the DNA molecule • 2. This is the only way that their binding sites will match up

10. Pyrimidines vs. Purines • The bases are grouped into two categories • Pyrimidines • Purines

11. Pyrimidines vs. Purines • Two pryrimidines would be too small to fit inside the structure of DNA • Two purines would be too big to fit inside the structure of DNA Too small Too big

12. Base Bonding • C-G and T-A are also necessary because of binding sites • T and A have 2 binding sites • C and G have 3 binding sites • They wouldn’t match up any other way

14. Specific Combos • Because of size, G and A would be too big together, and C and T would be too small together • Because of binding sites, G only matches up with C and T only matches with A

15. Why does this matter? • Knowing these facts are HUGE! • This feature enables the structure of DNA to enable its function • In other words, because of the G-C, T-A combination, DNA can be read and replicated.

16. Review • DNA has 3 main components • A phosphate molecule • A ribose sugar • A Base (C,T,G,or A) • A phosphate, sugar, and base together is called a nucleotide, the building block of DNA • C-G and A-T are only possible because… • This is the only way they fit inside DNA • This is the only way their bonding sites match up

17. To make a protein… • To make a protein, we have to make a copy strand of DNA and send it to a ribosome • The copy strand is called mRNA

18. RNA vs. DNA • While our genetic information is encoded in double-stranded DNA, copies of this information are encoded in single-stranded RNA. • RNA is a primitive version of DNA. • DNA and RNA are very similar; the key differences are that… • 1. RNA can be single stranded • 2. RNA replaces a T with a U (uracil) • 3. Also, the sugar is slightly different (extra -OH molecule)

19. Transcription • The process of creating an mRNA copy of DNA is called Transcription. • Think of “transcript” of a TV program – it’s just a copy • Transcription has three stages: • 1. Initiation – DNA is unwound by helicaseenzyme and a polymerase enzyme binds to the DNA strand • 2. Elongation – nucleotides are added by polymeraseto the developing mRNA strand • 3. Termination – polymerase and mRNA are released from the DNA strand; the strand is re-closed

21. Transcription Animation C G A T C G A T C G A T G C T A G C T A G C T A C G A U C G A U C G A U Step 1: Helicase opens and unwinds the DNA strand Step 2: Polymerase adds a complementary base for each nucleotide Step 3: The newly created mRNA strand goes to a ribosome to be read Step 4: The DNA strand is closed and re-wound

22. Key Note: 5  3 • Transcription always occurs in a 5 > 3 direction. • The sugar molecule has 5 carbon atoms • The 5th carbon atom is ‘inside’ the nucleotide, while the 3rd carbon atom is at the ‘lower’ edge • NOTE – there is no top or down in DNA, so use these terms carefully! • Just remember: 5 > 3 5 3

23. Translation • Once an mRNA copy has been made, the next step is Translation. • Translation is when the information in the mRNA is ‘translated’ into the creation of a protein by a ribosome, or rRNA.

24. How Translation Works • The mRNA copy strand’s base letters are read in groups of three • E.g. if our mRNA strand was AUGGCAAAGGACCAUit would be read as AUG GCA AAG GAC CAU • Each group of three is called a codon. • i.e. AUG is a codon; GCA is a codon; etc.

25. 1 Codon = 1 Amino Acid • Each codon codes for a specific amino acid. • An amino acid is the building block of a protein • For example, GGG codes for Glycine • AUA codes for Serine • CUA codes for Leucine • Each codon will specific which amino acid is added next in order to create a protein

27. Translation Animation Arg Arginine C G A U C G A U C G A U Ser Protein Serine Iso Isoleucine Asp Asparagine

28. tRNA • Amino acids are determined by the strand of mRNA and brought to the ribosome by tRNA • tRNA will only bind to a complementary codon; e.g. ACG will bind the UGC–form of tRNA.

29. Amino Acids  Proteins • A protein is a long string of amino acids. • The type of amino acids in a protein, and their order, determine the function of the protein • For example, insulin is shown here at the right • As you can see, it is simply a long chain of amino acids

32. DNA –> RNA –> Protein -> Traits