DNA: Molecule of LifeHistory

DNA Structure

Protein Synthesis

Gene Regulation

History of DNA• By the 1940’s, scientists knew that

chromosomes consisted of both DNA and protein but did not know which was the genetic material.

• Scientists wanted it to be protein since they knew more about its structure.

• The following experiments shed some light.

HISTORY

Disease-causing bacteria (smooth

colonies)

Harmless bacteria (rough colonies)

Heat-killed, disease-causing bacteria (smooth colonies)

Control(no growth)

Heat-killed, disease-causing bacteria (smooth colonies)

Harmless bacteria (rough colonies)

Dies of pneumonia Lives LivesLive, disease-causing

bacteria (smooth colonies)

Dies of pneumonia

Harmless, live R strain somehow became disease causing S strain – transformation HOW?

Animation here

Bacteriophage with phosphorus-32 in DNA

Phage infectsbacterium

Radioactivity inside bacterium

Bacteriophage with sulfur-35 in protein coat

Phage infectsbacterium No radioactivity inside

bacterium

Hershey Chase Experiment

DDNNAA

SSTTRRUUCCTTUURREE

Hydrogen bonds- very weak

Nucleotide

Sugar-phosphate backbone

Key

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

Nucleotide: the monomer (repeating unit) of DNA

Phosphate group

Nitrogen bases

Deoxyribosesugar

What limitations exist in our model of DNA?

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

•• Chromatin:Chromatin: DNA unassociated with structural proteins.

•• ChromosomeChromosome is made up of a DNADNA-HistoneHistoneproteinprotein complex.

• Compacts a very long (2m) sequence of DNA.

DDNNAA

SSTTRRUUCCTTUURREE

DNA packing (ExpLIFE 12.1)

DDNNAA

RREEPPLLIICCAATTIIOONN

DNA Replication Animation

Helicase unwind and separate the double helix, breaking weak H-bonds: replicationfork

DNA polymerasesrebuild each strand adding nucleotides to each side according to base-pairing rules.

Each new double helix consists of one old strand linked to one new DNA strand.

DNA

REPLICATION

• Errors sometimes occur (about 1 error/10,000 pairs)• If a mismatch occurs, the DNA polymerase can

backtrack, remove the incorrect nucleotide, and replace it with the correct one.

• Eukaryotic chromosomes are so long that it would take 33 days to replicate a typical human chromosome if there were only one origin of replication.

• Human chromosomes are replicated in about 100 sections that are 100,000 nucleotides long, each section with its own starting point.

• Because eukaryotic cells have multiple replication forks working at the same time, an entire human chromosome can be replicated in about 8 hours.

Prokaryotic vs. Eukaryotic

What observations can you make between the Pro/Eukaryotic DNA?

• What are the critical characteristics of DNA that allow both the lasting storage of information and the transfer of information through copying (replication)?

• What are the dis/advanages of an information transfer system that uses as physical pattern, or template?

How does the DNA store and transmit these messages?

A particular sequence (a gene) along a strand of DNA codes for the production of a specific protein. In this activity, you will explore the production of one protein produced by your stomach. This protein, pepsinogen, is an inactive form of an enzyme that aids in digestion.

Exp Life CD: 11.4

Ribonucleic Acid (RNA)

UracilThymine

RiboseDeoxyribose

Single StrandDouble Strand

Click to play the video.

RNARNADNADNA

Gene Transcription and Translation

Prokaryotes vs. Eukaryotes Gene processing animation

RNA processing

DNA

RNA

DNA unwinds

DNA template TAC AAG TTT GAC CATTAC AAG TTT GAC CATNon-template ATG TTC AAA CTG GTAATG TTC AAA CTG GTA

Click to play the video.

Transcription: DNA ���� RNA

UUC AAA CUG GUAUUC AAA CUG GUARNA strand AUGAUG

RNA polymerase copies at a rate of 60 nucleotides/sec

mRNAmRNA

RNADNA

RNApolymerase

Start codon

RibosomeRibosome

Methionine

Phenylalanine tRNAtRNALysine

Nucleus

mRNAmRNA

Translation observation (BSCS DVD 3 )

Translation details

The Polypeptide “Assembly Line”The ribosome joins the two amino acids—methionine and phenylalanine—and breaks the bond between methionine and its tRNA. The tRNA exits, allowing the ribosome to bind to another tRNA. The ribosome moves along the mRNA, binding new tRNA molecules and amino acids.

mRNARibosome

Translation direction

LysinetRNA

tRNA

Ribosome

Growing polypeptide chain

mRNA

Completing the PolypeptideThe process continues until the ribosome reaches one of the three stop stop codonscodons . The result is a growing polypeptide chain.

Start codon

Codons in mRNA

Mutation - changes in DNA

• Substitution codon 2 as GTA

• Substitution codon 2 as GCG

• Insertion codon 1 TATC

• Deletion nucleotide

DNA template TAC GCA TGG AAT ACTATG CGT ACC TTA TGA

RNA transcript AUG CGU ACC UUA UGAAmino acids Met– Arg– Thr– Leu stop

Point Mutation

Frame Shift Mutation

1 2 3 4 5

Silent Mutation

• Transcribe and translate the two DNA sequences.

• What kind of mutation does this represent?

• Explain why the correct sequence is important to protein synthesis.

• Assume the base sequence is GGG CTT CTT AAAWould this result in sickled hemoglobin? Explain.

Normal hemoglobin GGG CTT CTT TTT

Sickled hemoglobin GGG CAT CTT TTT

As a result of this mutation, the proteins are also misshapen causing sickled cells to clog capillaries and prevent normal flow of blood to body tissues, causing severe pain.

Protein structure

Deletion

Duplication

Inversion

Translocation

Chromosomal Mutations (ExpLIFE 12.2)

How can the movement of genes take place?

Which mutation variety would be most damaging?

Regulation of Protein Synthesis• Every cell in your body, with the exception of

gametes, or sex cells, contains a complete copy of your DNA. Why, then, are some cells nerve cells with dendrites and axons, while others are red blood cells that have lost their nuclei and are packed with hemoglobin? Why are cells so different in structure and function? If the characteristics of a cell depend upon the proteins that are synthesized, what does this tell you about protein synthesis?

Regulatory sites

Promoter(RNA polymerase binding site)

Start transcription

DNA strand

Stop transcription

Typical Gene Structure

• lac operon (ExamView)

• Exploring a stretch of code of DNA

•Chromosome 11 Fly-over

Complexities of Gene Expression• The relationship between genes and their effects is

complex. Despite the neatness of the genetic code, every gene cannot be simply linked to a single outcome.

• Some genes are expressed only at certain times or under specific conditions.

• Variations and mistakes can occur at each of the steps in replication and expression.

• The final outcome of gene expression is affected by the environment of the cells, the presence of other cells, and the timing of gene expression.

Section 12-5

1. Do you think that cells produce all the proteins for which the DNA (genes) code? Why or why not? How do the proteins made affect the type and function of cells?

2. Consider what you now know about genes and protein synthesis. What might be some ways that a cell has control over the proteins it produces?

3. What type(s) of organic compounds are most likely the ones that help to regulate protein synthesis? Justify your answer.