Copyright © 2009 Pearson Education, Inc. Head Tail fiber DNA Tail

THE STRUCTURE OF GENETIC MATERIAL

Sugar-phosphate backbone DNA nucleotide Phosphate group Nitrogenous base Sugar DNA polynucleotide DNA nucleotide Sugar (deoxyribose) Thymine (T) Nitrogenous base (A, G, C, or T) Phosphate group

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DNA REPLICATION

DNA replication depends on specific base pairing ◦ DNA replication is semiconservative –The two DNA strands separate –Each strand is used as a pattern to produce a complementary strand, using specific base pairing Parental molecule of DNA Nucleotides Both parental strands serve as templates Two identical daughter molecules of DNA

DNA replication proceeds in two directions simultaneously ◦ Continuous strand replication: leading strand ◦ Discontinuous strand replication: lagging strand ◦ Proteins involved in DNA replication –DNA polymerase adds nucleotides to a growing chain –DNA ligase joins small fragments into a continuous chain

Parental DNA 3 5 DNA polymerase molecule DNA ligase 3 5 Overall direction of replication Daughter strand synthesized continuously Daughter strand synthesized in pieces

Copyright © 2009 Pearson Education, Inc. DNA  RNA  PROTEIN Transcription and Translation

DNA genotype is expressed as a protein which determines phenotype ◦ A gene is a sequence of DNA that directs the synthesis of a specific protein –DNA is transcribed into RNA –RNA is translated into protein Copyright © 2009 Pearson Education, Inc. Cytoplasm Nucleus DNA Transcription RNA Translation Protein

Transcription produces genetic messages in the form of RNA ◦ Transcription: –The two DNA strands separate –One strand is used as a pattern to produce an RNA chain, using specific base pairing –What special nucleotide is found in RNA?? –RNA polymerase catalyzes the reaction –Result is mRNA (protein message) or tRNA or rRNA Copyright © 2009 Pearson Education, Inc.

Polypeptide Translation Transcription DNA strand Codon Amino acid RNA

Genetic information written in codons is translated into amino acid sequences –Translation involves switching from the nucleotide “language” to amino acid “language” –Each “word” is a codon, consisting of three nucleotides –Each amino acid is specified by a codon –64 codons are possible –Some amino acids have more than one possible codon Copyright © 2009 Pearson Education, Inc.

First base Third base Second base

Polypeptide A site 1 Codon recognition Codons Amino acid Anticodon P site mRNA 2 Peptide bond formation 3 Translocation New peptide bond Stop codon mRNA movement Ribosomes perform translation to make proteins ◦ Translation occurs on the surface of the ribosome –Ribosomes have two subunits: small and large –Each subunit is composed of ribosomal RNAs and proteins Copyright © 2009 Pearson Education, Inc.

Polypeptide A site 1 Codon recognition Codons Amino acid Anticodon P site mRNA 2 Peptide bond formation 3 Translocation New peptide bond Stop codon mRNA movement

Strand to be transcribed DNA Start codon RNA Transcription Stop codon Polypeptide Translation Met Lys Phe

Genetic Engineering: GENE CLONING Copyright © 2009 Pearson Education, Inc.

–Gene cloning: production of multiple identical copies of a gene-carrying piece of DNA –Recombinant DNA: joining DNA sequences from two different sources –The gene of interest can be combined with a gene carrier or vector –Plasmids (small, circular DNA molecules independent of the bacterial chromosome) are often used as vectors Copyright © 2009 Pearson Education, Inc.

◦ Restriction enzymes cut DNA at specific sequences called restriction sites –Many cuts leave staggered ends that produce restriction fragments with “sticky ends” –Fragments with complementary sticky ends can associate with each other, forming recombinant DNA ◦ DNA ligase joins DNA fragments together Copyright © 2009 Pearson Education, Inc.

Restriction enzyme recognition sequence 1 2 DNA Restriction enzyme cuts the DNA into fragments Sticky end 3 Addition of a DNA fragment from another source 4 Two (or more) fragments stick together by base-pairing DNA ligase pastes the strands Recombinant DNA molecule 5

How do we use gene cloning? GENETICALLY MODIFIED ORGANISMS Copyright © 2009 Pearson Education, Inc.

Examples of gene use Recombinant DNA plasmid E. coli bacterium Plasmid Bacterial chromosome Gene of interest DNA Gene of interest Cell with DNA containing gene of interest Recombinant bacterium Clone of cells Genes may be inserted into other organisms Genes or proteins are isolated from the cloned bacterium Harvested proteins may be used directly Examples of protein use Gene of interest Isolate plasmid 1 Isolate DNA 2 Cut plasmid with enzyme 3 Cut cell’s DNA with same enzyme 4 Combine targeted fragment and plasmid DNA 5 Add DNA ligase, which closes the circle with covalent bonds 6 Put plasmid into bacterium by transformation 7 Allow bacterium to reproduce 8 9