Chapter 12 DNA and RNA Class Lecture Notes

12-1 First Discoveries 1950’s- British scientist, Rosalind Franklin used x-ray diffraction to study DNA Aimed x-rays at DNA samples – recorded scattering pattern of x-rays on film Showed the shape being twisted like a coil spring or a helix – angle of “x” suggested 2 strands 1953 – American scientist, Watson and British scientist, Crick (both biochemists) proposed a model for structure of DNA Model was double helix – 2 strands wound around each other

12-1 Structure of DNA Nucleic acid is one of the four major organic molecules that make up all living things. DNA (deoxyribonucleic acid) consists of 2 long nucleotide chains that twist around each other to form a double helix (spiral staircase shape) Weak bonds (hydrogen bonds) between chains hold nucleotides to each other Nucleotide = 3 parts (see fig.12-5 pg. 291 & fig.12-7 pg. 294) 1.) nitrogen base, 2.) phospate group, 3.) molecule of deoxyribose (a 5-carbon sugar) Nitrogen bases are of 2 types: 1 = purines: adenine (A), guanine (G) 2 = pyrimidines: cytosine (C), thymine (T)

12-2 DNA Replication Before a cell divides it replicates its DNA When does this occur in cell cycle? Why? DNA separates into 2 strands (zipper unzips) (fig 12-9 p.296) Bonds between nitrogen bases are broken Each strand serves as a template to create the new strand from Each replicated DNA strand will have one original strand and one new Enzyme called DNA polymerase assists in creating the new nucleotides, also “proof reads” each new strand

In the double helix, the nitrogen bases in the first strand pair up and form bonds with the bases in the second strand holding the two strands together The pairing of bases is not arbitrary. The base adenine(A) always pairs up with thymine (T) and cytosine (C) always pairs up with guanine (G). Test yourself, let’s see how well you remember: Type answers and save file in student drive under “DNA Unit” folder. What 3 parts make up a DNA nucleotide? What type of bonds hold each strand of DNA together?

12-2 DNA and Chromosomes Nucleus of human cell contains more than 1 meter of DNA (long chains of nucleotides make up DNA) Smallest human chromosome has 30 million base pairs of DNA How is so much DNA folded into tiny chromosomes? Chromosomes contain both DNA and protein tightly packed together to dorm chromatin Chromatin is DNA tightly coiled around proteins called histones. (fig 12-12 p.299) They form a system of loops and coils with a beadlike structures = nucleosomes Nucleosomes pack with each other to form a thick fiber or chromosome

12-3 RNA and Protein Synthesis To transcribe something means to write a copy of it DNA is transcribed to produce a molecule of RNA How RNA is different from DNA: Sugar in RNA is ribose not deoxyribose RNA is single-stranded RNA uses the base uracil (U) instead of thymine (T) RNA helps create amino acids which are then formed into proteins Proteins are structural components that make up your body and form enzymes that direct every chemical reaction in your body

RNA = a blueprint of DNA in a 4-letter language There are 3 types of RNA: Messenger RNA, Ribosomal RNA, Transfer RNA mRNA (messenger) is found in the nucleus and cytoplasm and carries the instructions (messages) for creating amino acids into proteins rRNA (ribosomal) is found in the nucleolus and the ribosomes, and is involved in construction of ribosomes tRNA (transfer) helps to transfer amino acids, produced by mRNA instructions, to the ribosome

Transcription is the process in which mRNA is manufactured, it occurs in the nucleus Enzyme, RNA polymerase binds to a strand of DNA, causing bonds between nitrogen bases to break and separates strands Individual mRNA nucleotides floating around in the nucleus bind with one unraveled DNA strand Uses DNA strand as a template to create the RNA strand Starts transcription at a spot on DNA called the promoter, which is a signal for where the RNA should start making its strand After RNA is created, it is proof read and edited (like the rough draft of a paper). Pieces called introns are cut out and remaining portions called exons are spliced back together.(fig 12-15 p. 302)

mRNA genetic code is read 3 letters at a time (like a word) mRNA genetic code is read 3 letters at a time (like a word). This is called a codon. The four letters in the RNA alphabet, A, U, C, and G are combined in various combinations, each unique codon representing a specific amino acid. (64 possible combinations which represent 20 amino acids in total) Some amino acids are specified by more than one codon A string of several amino acids creates a protein (aka: polypeptide) See fig 12-17 p. 303 Special codons: start = AUG, stop = UGA, UAA, UAG Translation – to express something in another language Genetic code language is translated into amino acid/protein sequence

A ribosome, out in the cytoplasm, reads the instructions and assembles the proteins mRNA,leaves the nucleus through a pore in the nuclear membrane and finds its way to a ribosome The ribosome binds to the start codon of the mRNA tRNA then binds to the mRNA with complementary base pairs called anticodons. These carry the correct amino acids to make the protein strand.

Practice Questions: Type answers and save file in student drive under “DNA Unit” folder. The nucleotide sequence of an unraveled section of DNA is ATGCCCTAAGTG. What is the corresponding mRNA sequence? The nucleotide sequence of a strand of mRNA is AUGUGCAAUUUUUAA. What is the amino acid sequence of the protein that this mRNA codes for? Use wheel chart on p. 303. Note that amino acid names are abbreviated using the first three letters of the amino acid. The nucleotide sequence of an unwound strand of DNA is TACATGCTAGCCGCTATC. What is the amino acid sequence that the DNA codes for?

12-3 Mutations Sometimes, even with the proof reading, mistakes are made by inserting the wrong base or skipping a base These changes in DNA or RNA sequence affect genetic information and are called mutations . Mutations that affect a single gene = gene mutations Mutations that affect a single base and therefore a single amino acid = point mutations If a base is deleted or an extra is inserted it can throw off the whole chain since they are read in groups of three. Frame shift mutation is when a bas is deleted or extra is inserted so that the reading frame is shifted, therefore affecting every amino acid following the mutation. Mutations that affect an entire chromosome = chromosomal mutations

4 types of chromosomal mutations: 1.) deletion – loss of all or part of a chromosome 2.) duplication – segment is repeated 3.) inversion – part of a chromosome gets oriented in the reverse of its normal direction 4.) translocation – part of a chromosome breaks off and attaches to another non-homologous chromosome Chromosomal mutations occur by failure to separate properly during meiosis. Some may even have an extra chromosome. This is called polyploidy. Down Syndrome – when person has 3 copies of chromosome # 21. Cystic Fibrosis – caused by a deletion of 3 bases in the middle of a sequence for a protein on chromosome #7. (Causes digestive problems, breathing problems, mucus clogs lungs)