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 helicase enzyme and a polymerase enzyme binds to the DNA strand • 2. Elongation – nucleotides are added by polymerase to the developing mRNA strand • 3. Termination – polymerase and mRNA are released from the DNA strand; the strand is re-closed • Transcription involves two key enzymes: • Helicase: the enzyme the opens the DNA strand • Polymerase: the enzyme that creates the mRNA copy

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

30. The final functional protein is the quarternary structure. The order and type of amino acids is the primary structure. The arrangement of amino acids will create either a helix spring or a pleated sheet. The combination of springs and sheets is the tertiary structure of a protein.

31. Transcription involves making the mRNA copy of DNA. Transcription occurs in the nucleus. Translation occurs in the ribosomes. DNA and mRNA are a part of transcription. mRNA, rRNA, and tRNA are a part of translation. Translation involves using the mRNA copy to make a functional protein out of amino acids in the ribosome.

32. DNA –> RNA –> Protein -> Traits