DNA & RNA. Chapter Twelve and Thirteen Biology One

Transcription

1 DNA & RNA Chapter Twelve and Thirteen Biology One I. DNA Structure A. DNA monomers = nucleotides *1. sugar bonded to PO4 & one of four possible nitrogen bases 2. bases = Adenine, Guanine, Cytosine, Thymine a. A & G = purines; 2 rings in structure b. C & T = pyrimidines; 1 ring 3. backbone = sugar & PO4 group a. DNA s sugar = deoxyribose B. DNA Researchers 1. Rosalind Franklin (early 1950s) a. used x-rays to show shape of molecules b. major ideas of her work 1) helix shape 2) double strands 3) bases are in middle

2 2. Francis Crick & James Watson a. 3 main contributions 1) built 3-D model in April 1953 a) used clues from Franklin s X-rays 2) discovered that hydrogen bonds hold the 2 strands together 3) explained pairing of nitrogen bases d. Base pairing rules 1) a nitrogen base on 1 strand can only pair w/ one specific base on the corresponding strand 2) one purine & one pyrimidine in each pair a) Adenine pairs to Thymine b) Cytosine pairs to Guanine II. Chromosomes & DNA replication A. DNA & Chromosomes 1. Prokaryotic DNA a. found in 1 circular chromosome in the cytoplasm 1) some DNA is in separate plasmids 2) lacks histone proteins 3) only have 1 copy of each gene

3 2. Eukaryotic DNA a. found in multiple chromosomes in cell nucleus 1) some DNA is also found in mitochondria & chloroplasts 2) made of DNA & histone proteins 3) only visible as chromosomes during cell division b. somatic cells have 2 copies of each gene c. up to 1000x more DNA than in prokaryotes d. Found as chromatin during interphase a. coils into chromosomes in Prophase

4 B. DNA Replication 1. copying of DNA prior to cell division a. ensures daughter cells get same genetic info 2. double helix shape eases replication a. strands are complementary 1) have opposite bases 2) ex: ATCGGCTA TAGCCGAT b. enzymes control replication 3. To copy, strands separate into replication forks a. the enzyme helicase breaks hydrogen bonds, unwinds & unzips helix b. each strand is a template to create new DNA 1) leading strand copies continuously toward the opening (3 to 5 ) 2) lagging strand copies away from opening (5 to 3 ) in small pieces (Okazaki fragments) that are later joined 4. Strands are proofread to ensure right bases have paired 5. Enzyme DNA ligase binds DNA into double strands a. will coil into helix automatically 6. Replication is semi-conservative a. each strand is ½ old and ½ new 1) old = ACGTTGA new = TGCAACT

5 C. Telomeres 1. ends of eukaryotic chromosomes 2. these are difficult to replicate regularly a. the enzyme telomerase copies these 1) adds short repeated DNA sequences b. prevents damage to or loss of genes in quickly dividing cells 3. usually not functioning in adults, but can contribute to cancer growth III. RNA & Protein Synthesis A. RNA structure 1. monomers = nucleotides a. contain ribose (sugar) & the base uracil in place of thymine 2. single stranded helix 3. copies gene info from DNA *a. gene = coded DNA instructions for producing proteins

6 B. RNA forms 1. messenger RNA (mrna) a. made in nucleus based on DNA b. carries instructions (codons) for making proteins to ribosomes in cytoplasm 2. ribosomal RNA (rrna) a. part of ribosome structure b. site of protein synthesis 1) puts amino acids together to create proteins 3. transfer RNA (trna) a. found in cytoplasm b. carries in amino acids to ribosomes 1) has an anticodon that pairs with codon sequence on mrna 2) assures correct amino acid is added to protein being made C. Transcription *1. process that forms mrna from a complementary sequence of DNA 2. transcription factors bind to DNA at promoter sequences to begin process 3. DNA strands open & one strand is copied into mrna by RNA polymerase a. complementary bases pair up 1) adenine on DNA pairs w/ uracil on RNA 2) T, C & G on DNA match as before 4. when terminator sequence is reached, mrna is complete & ready to edit

7 D. RNA Editing 1. RNA copied both types of gene sequences found on DNA a. exons code for proteins 1) expressed (coding) sequences b. introns don t code for proteins 1) intervening & noncoding 2. Introns must be removed before mrna can leave nucleus a. remaining exons join in final RNA E. The Genetic Code 1. genes are codes to make proteins a. located on chromosomes 1) proteins are basis of cells 2. mrna copies the genetic code a. written as (triplet) codons *1) 3 consecutive nucleotides that specify one amino acid 2) ex: UGG = tryptophan b. amino acids join to make proteins 1) 20 different amino acids exist

8 3. Genetic code = 64 codons a. some amino acids are made by more than 1 codon 1) ex: 6 codons make leucine b. AUG = initiation codon 1) signals starting point for protein synthesis 2) if found later in gene, codes for methionine c. UGA, UAA & UAG = stop codons 1) don t code for amino acids F. Translation 1. decoding mrna and assembling proteins on ribosomes a. order of codons determines order amino acids are put together to build proteins 1) wrong order = mutations occur 2. Process a. edited mrna leaves nucleus 1) attaches to a ribosome (rrna) b. ribosome reads code on mrna c. trna carries correct amino acid to ribosome 1) trna s anticodon pairs w/ mrna s codon d. ribosome forms peptide bonds to bind amino acids into proteins e. at stop codon, protein is released 1) mrna is broken down in cytoplasm 2) rrna & trna are reused

9 IV. Mutations & their effects mutare to A. Mutations Latin: move or change 1. heritable changes in genetic material a. wrong bases may be inserted b. bases may be skipped during copying c. may be caused by physical or chemical mutagens 2. two main forms a. gene or chromosomal B. Gene mutations (a.k.a. point mutations) 1. changes to one or a few nucleotides 2. occur at one point in DNA sequence 3. 3 main types a. substitution changing 1 base is to another 1) affects just 1 amino acid b. insertion adding a base c. deletion removing a base

10 4. Insertions & deletions cause frameshift mutations a. every amino acid after the mutated point is changed 1) shifts where codons are read 2) may make unusable proteins or prevent translation C. Chromosomal mutations 1. change to number or structure of chromosomes a. may change # of copies of a gene b. may change where genes are located 2. Five main types a. Deletion loss of all or part of a chromosome b. Duplication extra copies of genes on a chromosome c. Inversion reversing the direction of genes on part of a chromosome d. Translocation part of one chromosome breaks off & attaches toa non-homologous chromosome

11 e. Nondisjunction failure of homologous chromosomes to properly separate during cell division 1) causes aneuploidy an abnormal number of chromosomes D. Significance of Mutations 1. many are neutral no visible effects 2. some are harmful a. change protein structure or gene activity in bad ways 1) ex: Sickle Cell disease mutation causes misshaped red blood cells b. some are lethal (cause death) 3. some are helpful a. changes aid survival 1) ex: long necks in giraffe ancestors 4. Mutations are only passed to offspring if carried in the gametes a. mutations in body cells only affect that individual 5. Polyploidy can benefit plant breeders a. having multiple sets of chromosomes b. results in larger &/or strong crops