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Chapter 12

Chapter 12. DNA and RNA. Discovery of DNA. How do genes work? Several scientists from 1928-1954 began investigating the chemical nature of genes. DNA was determined to be the molecule responsible for coding all genes. What is DNA?.

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Chapter 12

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  1. Chapter 12 DNA and RNA

  2. Discovery of DNA • How do genes work? • Several scientists from 1928-1954 began investigating the chemical nature of genes. • DNA was determined to be the molecule responsible for coding all genes.

  3. What is DNA? • Deoxyribonucleic acid (DNA) is a long molecule made up of units called nucleotides. • Nucleotides are made up of: • 5-carbon sugar (deoxyribose) • Phosphate group • Nitrogenous base (adenine, guanine, cytosine, or thymine) • All the genetic information for an organism is carried on its DNA.

  4. Nucleotides Form DNA Nucleotide Structure

  5. Structure of DNA • DNA wounds around itself, forming a twisted ladder (double helix) • Discovered in 1950s by Rosalind Franklin using X-ray diffraction. • “Double Helix” term coined by Watson and Crick in the 1950s as well.

  6. Groups of Bases • Purines (larger) • Adenine • Guanine • Pyrimidines (smaller) • Cytosine • Thymine

  7. Structure of DNA Cont’d • Chargaff’s Rules: • The four bases (adenine, guanine, thymine, cytosine) pair up in a certain way • A – T • C – G • The bases are held together by hydrogen bonds (hydrogen bonds are weak bonds)

  8. Storage of DNA • Prokaryotes – DNA is found in the cytoplasm • Most have single circular DNA molecule • Eukaryotes – Much more DNA than in prokaryotes (up to 1000x as much!) • Condensed into chromosomes • Chromosome number varies wildly from one species to the next. (humans have 46)

  9. DNA Replication • Before a cell divides, it must copy its DNA in a process called replication. • During DNA replication, DNA separates into two strands. DNA polymerase makes complimentary strands using base pairing. • Each strand of the original DNA molecule is used as a template for the new DNA.

  10. The area where DNA unwinds is called a replication fork.

  11. RNA • RNA is ribonucleic acid. • Like DNA: • Made up of nucleotides • Codes for proteins • Unlike DNA: • Nucleotides are made of ribose (instead of deoxyribose) • RNA is single-stranded (DNA is double) • The base Thymine is replaced with Uracil in RNA

  12. 3 Types of RNA • Messenger RNA (mRNA) • RNA that carries copies of DNA instructions • Ribosomal RNA (rRNA) • Make up ribosomes (along with proteins) • Transfer RNA (tRNA) • Transfers or carries each amino acid to the ribosomes as coded for by the mRNA

  13. Transcription • RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence of RNA • DNA strands are separated, RNA polymerase uses one strand of DNA as a template and assembles RNA • Uses base pairing to assemble RNA • A – U (instead of T) • C - G

  14. Genetic Code • The “language” of mRNA is sometimes called the genetic code. • The genetic code is read 3 letters (or bases) at a time, called codons. • A codon is made up of 3 nucleotides that specify for a single amino acid • Amino acids are strung together to form proteins (polypeptides)

  15. Genetic Code, cont. • Ex: • UCGCACGGU Would be read 3 bases at a time… • UCG-CAC-GGU Using the code → Serine-Histadine-Glycine

  16. Start and Stop Codons • AUG codon codes for the assembly of amino acids to begin. It also codes for methionine • Three codons code for the assembly of amino acids to stop – UAA, UAG, and UGA

  17. Protein Synthesis (Translation) • Proteins are assembled on ribosomes using information from mRNA • Steps: • 1 – mRNA transcribed • 2 – mRNA attaches to ribosome. tRNA matches anticodons to codons and brings in amino acids. • 3 – amino acids are joined together by peptide bonds. Free tRNA molecules float away. • 4 – polypeptide chain grows until stop codon.

  18. 12-4 Mutations • Gene mutations: • Point mutations – involve changes in one or a few nucleotides • Substitutions • Insertions • Deletions • Frameshift mutations – shift the “reading frame” of the genetic message • Ex: AUG UGG CCU UAC → AUG UGG ACC UUA C

  19. DNA: TAC GCA TGG AAT mRNA: AUG CGU ACC UUA Amino Acids: Met-Arg-Thr-Leu SUBSTITUTION DNA: TAC GTA TGG AAT mRNA: AUG CAU ACC UUA Amino Acids: Met-His-Thr-Leu DNA: TAC GCA TGG AAT mRNA: AUG CGU ACC UUA Amino Acids: Met-Arg-Thr-Leu INSERTION DNA: TAT CGC ATG GAA T mRNA: AUA GCG UAC CUU A Amino Acids: Ile-Ala-Tyr-Leu Point Mutations

  20. Chromosomal Mutations • Deletions • Duplications • Inversions • Translocations

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