12.2 Replication of DNA DNA replication is the process of copying a DNA molecule. Semiconservative replication - each strand of the original double helix (parental molecule) serves as a template (mold or model) for a new strand in a daughter molecule.

Semiconservative Replication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5′ 3′ G C G C G A T A region of parental DNA double helix T A C G DNA polymerase enzyme A T A T G G T C G C G C G C A A C A T C G C region of replication: new nucleotides are pairing with those of parental strands T G A A T T A T T A G C T C A T A G A T A T A A G A G T G T region of completed replication A C C C A G C 3′ G n e w o l d A s t r a n d s t r a n d 5′ daughter DNA double helix old strand new strand daughter DNA double helix

Replication of DNA Replication requires the following steps: Unwinding, or separation of the two strands of the parental DNA molecule Complementary base pairing between a new nucleotide and a nucleotide on the template strand Joining of nucleotides to form the new strand Each daughter DNA molecule contains one old strand and one new strand 3

Replication of DNA Prokaryotic Replication Bacteria have a single circular loop of DNA Replication moves around the circular DNA molecule in both directions Produces two identical circles The process begins at the origin of replication Replication takes about 40 minutes, but the cell divides every 20 minutes A new round of replication can begin before the previous round is completed

Replication of DNA Eukaryotic Replication DNA replication begins at numerous points along each linear chromosome DNA unwinds and unzips into two strands Each old strand of DNA serves as a template for a new strand Complementary base-pairing forms a new strand paired with each old strand Requires enzyme DNA polymerase

Replication of DNA Eukaryotic Replication Replication bubbles spread bidirectionally until they meet The complementary nucleotides are joined to form new strands. Each daughter DNA molecule contains an old strand and a new strand. Replication is semiconservative: One original strand is conserved in each daughter molecule, i.e., each daughter double helix has one parental strand and one new strand.

Prokaryotic versus Eukaryotic Replication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. origin replication is complete replication is occurring in two directions a. Replication in prokaryotes replication fork replication bubble parental strand new DNA duplexes daughter strand b. Replication in eukaryotes

Aspects of DNA Replication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. OH P Is attached here base is attached here 5′ CH2 O OH 4′ C C H H 1′ 1 H C H 3′ C 2′ OH H D e o x y r i b o s e m o l e c u l e 2 DNA polymerase attaches a new nucleotide to the 3 ′ carbon of the previous nucleotide. 5′ e n d P T P A P P G C 3′ e n d P P C G 5′ P P T A 3′ P C G P template strand P 3 ′end DNA polymerase 5′ end 4 leading new strand template strand new strand Direction of replication 3′ 3 helicase at replication fork RNA primer template strand 6 Okazaki fragment 5 lagging strand 3′ 5′ 5′ parental DNA helix 7 DNA ligase DNA polymerase 3′ Replication fork introduces complications

Replication of DNA Accuracy of Replication DNA polymerase is very accurate, yet makes a mistake about once per 100,000 base pairs. Capable of identifying and correcting errors

12.3 The Genetic Code of Life Genes Specify Enzymes Beadle and Tatum: Experiments on the fungus Neurospora crassa Proposed that each gene specifies the synthesis of one enzyme One-gene-one-enzyme hypothesis

The Genetic Code of Life The mechanism of gene expression DNA in genes specify information, but information is not structure and function Genetic information is expressed into structure and function through protein synthesis The expression of genetic information into structure and function: DNA in a gene determines the sequence of nucleotides in an RNA molecule RNA controls the primary structure of a protein

The Central Dogma of Molecular Biology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3' 5' A G C G A C C C C DNA T C G C T G G G G 3' 5' 12

The Central Dogma of Molecular Biology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. nontemplate strand 5 3 A G C G A C C C C DNA T C G C T G G G G 3 5 template strand transcription in nucleus 5 3 A G C G A C C C C mRNA 13

The Central Dogma of Molecular Biology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. nontemplate strand 5 3 A G C G A C C C C DNA T C G C T G G G G 3 5 template strand transcription at ribosome 5 3 A G C G A C C C C mRNA codon 1 codon 2 codon 3 14

The Central Dogma of Molecular Biology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. nontemplate strand 5 3 A G C G A C C C C DNA T C G C T G G G G 3 5 template strand transcription in nucleus 5 3 A G C G A C C C C mRN A translation at ribosome codon 1 codon 2 codon 3 O O O polypeptide N C C N C C N C C R1 R2 R3 15 Serine Aspartate Proline

The Genetic Code of Life RNA is a polymer of RNA nucleotides RNA nucleotides contain the sugar ribose instead of deoxyribose RNA nucleotides are of four types: uracil (U), adenine (A), cytosine (C), and guanine(G) Uracil (U) replaces thymine (T) of DNA Types of RNA Messenger (mRNA) - Takes a message from DNA in the nucleus to ribosomes in the cytoplasm Ribosomal (rRNA) - Makes up ribosomes, which read the message in mRNA Transfer (tRNA) - Transfers the appropriate amino acid to the ribosomes for protein synthesis

RNA polymerases 3 RNA polymerase enzymes RNA polymerase 1 only transcribes rRNA genes makes ribosomes RNA polymerase 2 transcribes genes into mRNA RNA polymerase 3 only transcribes tRNA genes each has a specific promoter sequence it recognizes

Structure of RNA 5′ end G U S A base is uracil instead of thymine C S Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5′ end P G U S A base is uracil instead of thymine P C S P S P S ribose 3′ end one nucleotide

RNA Structure Compared to DNA structure

The Genetic Code of Life The unit of the genetic code consists of codons, each of which is a unique arrangement of symbols Each of the 20 amino acids found in proteins is uniquely specified by one or more codons The symbols used by the genetic code are the mRNA bases Function as “letters” of the genetic alphabet Genetic alphabet has only four “letters” (U, A, C, G) Codons in the genetic code are all three bases (symbols) long Function as “words” of genetic information Permutations: There are 64 possible arrangements of four symbols taken three at a time Often referred to as triplets Genetic language only has 64 “words”

Messenger RNA Codons First Base Second Base Third Base U C A G UUU Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First Base Second Base Third Base U C A G UUU phenylalanine UCU serine UAU tyrosine UGU cysteine U UUC phenylalanine UCC serine UAC tyrosine UGU cysteine C U UUA leucine UCA serine UAA stop UGA stop A UUG leucine UCG serine UAG stop UGG tryptophan G CUU leucine CCU proline CAU histidine CGU arginine U CUC leucine CCC proline CAC histidine CGC arginine C C CUA leucine CCA proline CAA glutamine CGA arginine A CUG leucine CCG proline CAG glutamine CGG arginine G AUU isoleucine ACU threonine AAU asparagine AGU serine U AUC isoleucine ACC threonine AAC asparagine AGC serine C A AUA isoleucine ACA threonine AAA lysine AGA arginine A AUG (start) methionine ACG threonine AAG lysine AGG arginine G GUU valine GCU alanine GAU aspartate GGU glycine U GUC valine GCC alanine GAC aspartate GGC glycine C G GUA valine GCA alanine GAA glutamate GGA glycine A GUG valine GCG alanine GAG glutamate GGG glycine G

The Genetic Code of Life Properties of the genetic code: Universal With few exceptions, all organisms use the code the same way Encode the same 20 amino acids with the same 64 triplets Degenerate (redundant) There are 64 codons available for 20 amino acids Most amino acids encoded by two or more codons Unambiguous (codons are exclusive) None of the codons code for two or more amino acids Each codon specifies only one of the 20 amino acids Contains start and stop signals Punctuation codons Like the capital letter we use to signify the beginning of a sentence, and the period to signify the end