Chapter 2 An Introduction to Genes and Genomes
Prokaryotic Cell Structure Prokaryotic Cell Eukaryotic Cell
Eukaryotic Cell Structure Animal Cell Plant Cell
Let’s in on a cell! Zoom DNA Zoom Interactive
DNA Discovery (visit DNAi.org) • Miescher – identified a nuclear substance he called nuclein • Griffith – performed the first transformation • Avery, McCarty, and Macleod – identified Griffith’s transforming factor as DNA • Chargaff – proved that the percentage of the DNA bases adenine always equaled thymine and guanine always equaled cytosine • Wilkins, Franklin, Watson & Crick – demonstrated the structure of DNA
Structure of DNA • Deoxyribose Sugar • Phosphate • Nitrogen Base
Structure of DNA • Purines – double ring • Pyrimidines – single ring
Structure of DNA Nucleic Acid Overview
DNA Replication • When DNA makes an exact copy of itself
DNA Replication • The first step in DNA replication is for the enzyme, helicase, to unzip the double stranded DNA molucule.
DNA Replication • Proteins hold the two strands apart. • An RNA primer lays down on each strand of DNA.
DNA Replication • DNA polymerase extends the primer by adding complementary nucleotides. • DNA polymerase can only extend in the 5’ → 3’ direction
DNA Replication • Leading strand follows helicase. • Lagging strand must wait for replication fork to open and therefore forms discontinous Okazaki fragments. • Ligase seals the nicks in the DNA backbone between the Okazaki fragments. helicase
Let’s put it all together • Click on the animation below. • Select the button for the “whole picture”. DNA Replication Animation
Transcription • Making an RNA copy from a DNA template RNA polymerase
RNA Structure • Uracil instead of thymine • Ribose sugar instead of deoxyribose sugar • Single stranded • Can leave the nucleus
RNA Structure • mRNA – RNA copy of DNA that carries genetic information from the nucleus to the ribosomes • rRNA – makes up the ribosomes • tRNA – carries amino acids to ribosomes for protein synthesis
Transcription • RNA polymerase binds to a promoter region on double stranded DNA and unzips the double helix.
Transcription • Free RNA nucleotides pair with the complementary DNA of the template strand
Transcription • RNA is processed • Introns are spliced out • 7 methyl guanosine cap • Poly-A tail
Transcription • mRNA leaves the nucleus and travels to the ribosomes in the cytoplasm ribosome nucleus
Let’s put it all together • Transcription Animation
Central Dogma of Molecular Biology Click to see Video Animation
Translation • Making protein from mRNA
Translation • Important Definitions • A codon is composed of 3 RNA nucleotides • Each codon codes for one amino acid • Protein does the work in a cell
Translation • Asparagine, Serine, Methionine • Tryptophan, Glycine, Lysine • Proline, Leucine, Serine • Aspartic acid, Histidine, Threonine
Translation • Always begins at a start codon and ends at a stop codon. • The region between the start and stop codons is called the open reading frame (ORF)
Practice • Click on the animation to transcribe and translate a gene. Click to see animation
Translation Initiation • mRNA attaches to the small subunit of a ribosome • tRNA anticodon pairs with mRNA start codon • Large ribosomal subunit binds and translation is initiated amino acid tRNA anticodon
Translation Elongation • Anticodon of tRNA carrying next amino acid binds to codon on mRNA • A peptide bond joins the amino acids and the first tRNA is released.
Translation Termination • Amino 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 • Click on the animation below Translation Animation Translation Video
Let’s put it all together Coding: Template: mRNA: tRNA: amino acid: 5’-GATCTGAATCGCTATGGC-3’ 3’-CTAGACTTAGCGATACCG-5’ mRNA 5’-GAUCUGAAUCGCUAUGGC-3’ CUAGACUUAGCGAUACCG Asp, Leu, Asn, Arg, Tyr,Gly
Control of Gene Expression • Prokaryotes cluster genes into operons that are transcribed together to give a single mRNA molecule.
Control of Gene Expression • Lac Operon • Promoter region allows RNA polymerase to attach and begin transcription. • Operator region is in the middle of the promoter.
Control of Gene Expression • If a repressor protein is bound to the operator, RNA polymerase cannot pass to transcribe the genes.
Control of Gene Expression • When the inducer (lactose) binds to the repressor protein, it changes shape and falls off of the operator region. • Now RNA polymerase can pass and transcribe the genes into mRNA.