Chapter 2 An Introduction to Genes and Genomes

Introduction to Molecular Biology

Prokaryotic Cell Structure Prokaryotic Cell Eukaryotic Cell

Eukaryotic Cell Structure

Animal Cell Plant Cell

Let’s in on a cell! DNA Zoom Interactive

DNA Discovery (visit DNAi.org) zMiescher – identified a nuclear substance he called nuclein zGriffith – performed the first transformation zAvery, McCarty, and Macleod – identified Griffith’s transforming factor as DNA zChargaff – proved that the percentage of the DNA bases adenine always equaled thymine and guanine always equaled cytosine zWilkins, Franklin, Watson & Crick – demonstrated the structure of DNA

Structure of DNA Deoxyribose Sugar Phosphate Nitrogen Base

zPurines – double ring zPyrimidines – single ring Structure of DNA

Nucleic Acid Overview

Structure of DNA

DNA Replication zWhen DNA makes an exact copy of itself

DNA Replication

zThe first step in DNA DNA Replication replication is for the enzyme, helicase, to unzip the double stranded DNA molucule.

DNA Replication zProteins hold the two strands apart. zAn RNA primer lays down on each strand of DNA.

zDNA polymerase extends the primer by adding complementary nucleotides.  DNA polymerase can only extend in the 5’ → 3’ direction DNA Replication

zLeading strand follows helicase. zLagging strand must wait for replication fork to open and therefore forms discontinous Okazaki fragments. zLigase seals the nicks in the DNA backbone between the Okazaki fragments. helicase

Let’s put it all together zClick on the animation below. zSelect the button for the “whole picture”. DNA Replication Animation

Transcription zMaking an RNA copy from a DNA template RNA polymerase

RNA Structure zUracil instead of thymine zRibose sugar instead of deoxyribose sugar zSingle stranded zCan leave the nucleus

RNA Structure zmRNA – RNA copy of DNA that carries genetic information from the nucleus to the ribosomes zrRNA – makes up the ribosomes ztRNA – carries amino acids to ribosomes for protein synthesis

Transcription zRNA polymerase binds to a promoter region on double stranded DNA and unzips the double helix.

Transcription zFree RNA nucleotides pair with the complementary DNA of the template strand

Transcription zRNA is processed  Introns are spliced out  7 methyl guanosine cap  Poly-A tail

Transcription zmRNA leaves the nucleus and travels to the ribosomes in the cytoplasm ribosome nucleus

Let’s put it all together zTranscription Animation

Practice

Central Dogma of Molecular Biology Click to see Video Animation

Translation zMaking protein from mRNA

Translation zImportant Definitions  A codon is composed of 3 RNA nucleotides  Each codon codes for one amino acid  Protein does the work in a cell

Translation

First BaseSecond Base UCAG U UCAGUCAG phenylalanine serine tyrosine cysteine phenylalanine serine tyrosine cysteine Leucine serine (stop) Leucine serine (stop) tryptophan C UCAGUCAG leucine proline histidine arginine leucine proline histidine arginine leucine proline glutamine arginine leucine proline glutamine arginine A UCAGUCAG isoleucine threonine asparagine serine isoleucine threonine asparagine serine isoleucine threonine lysine arginine met (start) threonine lysine arginine G UCAGUCAG valine alanine apartic acid glycine valine alanine apartic acid glycine valine alanine glutamic acid glycine valine alanine glutamic acid glycine Third Base

Translation

Asparagine, Serine, Methionine Tryptophan, Glycine, Lysine Proline, Leucine, Serine Aspartic acid, Histidine, Threonine

Translation zA lways begins at a start codon and ends at a stop codon. xThe region between the start and stop codons is called the open reading frame (ORF)

Practice zClick on the animation to transcribe and translate a gene. Click to see animation

Translation Initiation zmRNA attaches to the small subunit of a ribosome ztRNA anticodon pairs with mRNA start codon zLarge ribosomal subunit binds and translation is initiated tRNA anticodon amino acid

Translation Elongation zAnticodon of tRNA carrying next amino acid binds to codon on mRNA zA peptide bond joins the amino acids and the first tRNA is released.

Translation Termination zAmino acid chain continues until a stop codon is read. The amino acid chain is released and all of the translation machinery is recycled to translate another protein.

Let’s put it all together zClick on the animation below Translation Video Translation Animation

Let’s put it all together 5 ’-GATCTGAATCGCTATGGC-3’ 3’-CTAGACTTAGCGATACCG-5’ mRNA 5’-GAUCUGAAUCGCUAUGGC-3’ CUAGACUUAGCGAUACCG Asp, Leu, Asn, Arg, Tyr, Gly Coding: Template: mRNA: tRNA: amino acid:

Control of Gene Expression

zProkaryotes cluster genes into operons that are transcribed together to give a single mRNA molecule.

zLac Operon  Promoter region allows RNA polymerase to attach and begin transcription.  Operator region is in the middle of the promoter. Control of Gene Expression

zIf a repressor protein is bound to the operator, RNA polymerase cannot pass to transcribe the genes. Control of Gene Expression

zWhen the inducer (lactose) binds to the repressor protein, it changes shape and falls off of the operator region. zNow RNA polymerase can pass and transcribe the genes into mRNA. Control of Gene Expression

Let’s put it all together zClick on the animation below. Video of lac operon Animation of lac operon

Mutations zMutations are changes in the DNA sequence. zMutations can be inherited or acquired.

Mutations zMutagens are agents that interact with DNA to cause mutations. zExamples are chemicals and radiation.

Mutations zPoint mutation changes a single base zPoint mutations can be silent, meaning they code for the same amino acid.

Mutations zPoint mutations can also code for a structurally similar amino acid.

Mutations zPoint mutations are not always harmless. zIf the mutation occurs on a critical amino acid in the active site of the protein, it can be detrimental, as in the case of sickle cell anemia.

Mutations zFrameshift mutations cause a shift in the reading frame by adding or deleting nucleotides.

Mutations zAn example of a deletion causing a premature stop codon.

Mutations