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Chapter 12 DNA and RNA What is DNA again? Deoxyribonucleic acid Long double-stranded molecule of nucleotides Stores genetic code that is transferred to future generations and cells Codes for synthesis of proteins What is a nucleotide A nucleotide has 3 parts: ◦ A sugar (deoxyribose for DNA) ◦ A phosphate group ◦ A nitrogen base Nitrogen base pairing The order of nitrogen bases are what determines your genetic code The order of the bases determines the proteins that are made 4 different bases exist: ◦ Purines: Guanine, Adenine Grade A Pure ◦ Pyrimidines: Cytosine, Thymine Pairing is also called Chargaff’s Rule Chargaff’s Rule Result: A=T, G=C But what is the structure? In the early 1950s Rosalind Franklin used X-ray diffraction took a “picture” of DNA ◦ Difficult to tell what the structure is from the image ◦ Strands twisted around one another But what is the structure? James Watson and Francis Crick used Franklin’s images and their models to determine the correct structure: double helix ◦ Two strands wound around one another ◦ “Twisted ladder” ◦ Later discovered that hydrogen bonds hold the two sides of the ladder together ◦ Can only from between adenine and thymine, or guanine and cytosine (A=T, G=C) ◦ The base pairing explained Chargaff’s Rule DNA Structure DNA Structure DNA is very long; E. coli’s contains 4,639,221 base pairs (length =1.6 mm) ◦ Must fit in an organism 1/1000 its length ◦ Must be packed very tightly to fit Human cell contains almost 100 times the base pairs ◦ Chromatin consists of DNA packed around proteins called histones ◦ These compact together during mitosis to create our visible chromosomes DNA Structure DNA replication Remember that before a cell divides it has to copy its DNA ◦ If we separate each strand, we can use it to make a copy of the other ◦ If we have an A, we match it with a T; if a G, we match it with a C (called complementary) In prokaryotes, replication begins at one point and proceeds typically in both directions In contrast, eukaryotes begin replication in hundreds of locations (due to its longer length) DNA Replication DNA replication Two replication forks formed, each strand has a new complementary strand added ◦ TACGTT ATGCAA ◦ Results in two identical DNA molecules ◦ One strand is new, the other old DNA was unzipped (breaking of hydrogen bonds) ◦ DNA polymerase (enzyme) joins individual nucleotides to produce a DNA molecule What is RNA? Ribonucleic acid Intermediate molecule between DNA and proteins Differs from DNA in 3 ways: ◦ Single stranded (vs. double) ◦ Ribose instead of deoxyribose ◦ Utilizes uracil instead of thymine (A-U, G-C) Type of RNA There are 3 types of RNA: ◦ messenger RNA (mRNA)- carries copies of the protein instructions ◦ ribosomal RNA (rRNA)- makes up the ribosome, along with several proteins ◦ transfer RNA (tRNA)- take the appropriate (base pairing anti-codon to codon) amino acid to the ribosome Types of RNA Protein synthesis: how we make proteins? Two processes or steps: ◦ Transcription- mRNA is made from the DNA and travels to the cytoplasm to find a ribosome ◦ Translation- tRNA brings to the ribosome the amino acids to build the primary structure of a protein and the result is a free polypeptide that will then fold up into the shape of the protein Remember: amino acids are held together by peptide bonds Transcription Transcription RNA polymerase binds to DNA and separates the strands RNA polymerase uses one strand to serve as a template to create a strand of RNA RNA polymerase only binds to regions of DNA called promoters (landing pad) The RNA is next edited, removing the introns, while leaving the exons (exons=expressed) RNA Splicing Translation Proteins are made by joining amino acids (1 of 20) into long chains (polypeptides) ◦ How do 4 letters lead to so many different amino acids? ◦ Read 3 letters at a time, called a codon ◦ UCGCACGGU ◦ UCG-CAC-GCU ◦ Serine-Histidine-Glycine ◦ Some codons code for the same amino acid ◦ AUG is the start codon Translation Codon – 3 nucleotides that code for an amino acid. Found on the mRNA. Anticodon - Fit the codon. Found on the tRNA DNA: master plan; never brought to work site mRNA: blue prints made from master plan Ribosomes: building site tRNA: trucks unloading materials Codons Codons Translation Transcription and Translation Mutations Mutations – changes made in the genetic material; can occur during copying Point mutation – effect only one nucleotide. Frameshift mutation – an insertion or deletion that effects the remaining string of nucleotides. Chromosomal mutations – changes in the number or structure of chromosomes. Polyploidy – when an organism has extra sets of chromosomes. Mutations THE FAT CAT ATE THE RAT ◦ Delete H (Frameshift / deletion) TEF ATC ATA TET HER AT ◦ Change H to L (Point mutation) TLE FAT CAT ATE THE RAT Mutations DNA: TAC GCA TGG AAT RNA: AUG CGU ACC UUA AA: Met Arg Thr Leu ◦ Substitution DNA: TAC GTA TGG AAT RNA: AUG CAU ACC UUA AA: Met His Thr Leu ◦ Insertion DNA: TAT CGC ATG GAA T RNA: AUA GCG UAC CUU A AA: Ile Ala Tyr Leu