CP BioChapter 12-13 DNA Function

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

CP Bio Chapter 12-13 DNA Function


History: Learning about DNA

Frederick Griffith 1928

a. Tried to make a vaccine for pneumoniab. Used mice and two strains of bacteria

- one harmless (“R type”)- one caused pneumonia (“S type”)

c. Live R alone and dead S alone did not cause immune response

d. Mixed live R with dead S mice got sick and DIED


How did harmless bacteria turn into deadly?

Live R (‘rough’) – no diseaseLive S (‘smooth’) – pneumoniaMix Live R and dead S - pneumonia


Griffith’s conclusionSomething from dead S cells transformed

living R cells into living S cells


Avery, McLeod, and McCartey 1944What cellular substance changed Griffith’s bacterial cells from harmless to deadly?

Used a series of enzymes on bacterial cultures

- destroyed specific molecules in the cells

- carbs, proteins, lipids, DNA

Found that when DNA was destroyed, bacterial cells did NOT change

CONCLUSION: DNA is the substance that can change bacterial cells


The Hershey-Chase experiment 1952Background: Viruses enter cells and change them - make cells produce copies of virus.

Problem: Which part of virus enters cells? Is it the protein coat? Or the DNA?

Experiment: Grow bacteria, add phage virus tagged with radioactive isotope

- use sulfur radioactivity in proteins (capsid)

- use phosphorus radioactivity in DNA

Which enters bacterial cells?


Hershey-Chase Experiment

Grow bacteria in culture with tagged

phage. Virus infects bacteria

Is radioactivity in the liquid (virus), or

in the cells (bacteria)?

Centrifuge separates cells

from culture liquid

Blender shakes phage loose from

bacterial cells.


Which part enters cells?

When phage was tagged with phosphorus bacterial cells became radioactive

Conclusion: DNA entered cells, but protein did not


Finding the structure of DNA

Franklin & Wilkins 1952- X-ray pictures of DNA

crystals- showed double helix shape

Watson & Crick 1953 discovered structure of DNA

Erwin Chargaff 1950 - base-pairing rules


• Two strands, held together by hydrogen bonds – Between complementary bases

• four nitrogen bases: adenine, thymine, cytosine, guanine• Deoxyribose sugar

DNA is a double helix


DNA replication:

• For cell division• Starts in several places

at once• Each original strand is

template for a new strand• Proceeds until entire

strands are duplicated• Copies stay together at

centromere


Ch. 13 DNA Function – “Gene Expression”

Flow of genetic information: DNA to RNA to protein

• DNA (genotype) codes for proteins

– Proteins make the phenotype

• A gene is one section on a DNA molecule• Has instructions to make one polypeptide

• CODE is the sequence of DNA bases


RNA

• ONE strand• Ribose sugar• Uracil base (no thymine)


Three kinds of RNAMessenger RNA (mRNA)– carries code from DNA in nucleus to ribosome

Ribosomal RNA (rRNA) – makes up ribosome, along with proteins

Transfer RNA (tRNA) – carries one amino acid to ribosome and matches to mRNA code


1. The DNA of the gene is transcribed into RNA

2. The gene is translated into a polypeptide

Figure 10.6A

DNA

Transcription

RNA

Protein

Translation

Two stages in protein synthesis


mRNA synthesis - writes the gene onto a messenger molecule

RNApolymerase

RNA nucleotides

Direction of transcription Template

Strand of DNA

Newly made RNA

TC

AT C C A A T

TG

G

CC

AATTGGAT

G

U

C A U C C AA

U

Stage 1: Transcription – in nucleus

DNA unzips

mRNA leaves nucleus

Copy ONE side of DNA


Transcription – writes DNA code onto mRNA


1. In the nucleus, the DNA unzips

2. RNA nucleotides line up along one strand of the DNA, follow base pairing rules

3. Messenger RNA (mRNA) is single strand, detaches from DNA

4. leaves the nucleus


RNA is processed before leaving the nucleus• Noncoding segments called introns are edited out

• Coding segments called exons are spliced together

• A cap and tail are added to the ends


13.2 How is the genetic code read?• The “words” of the DNA “language” are

written in sets of three bases – codons

• Codons spell the amino acid sequence - primary structure of a protein

Stage 2: Translation- in ribosome


Translating the genetic code

T A C T T C A A A A T C

A T G A A G T T T T A G

A U G A A G U U U U A G

Transcription

Translation

RNA

DNA

Met Lys PhePolypeptide

Startcondon

Stopcondon

Strand to be transcribed


UUG

All organisms use the same code


1. Ribosome attaches to mRNA

2. Transfer RNA (tRNA) brings amino acids to ribosome

How RNA helps


3. Begins at “start” codon

4. Amino acids are set in sequence, according to the code on mRNA

5. Ends at “stop” codon


Genetic information: DNA to RNA to protein

Sequence of codons primary structure of protein


change in the DNA base sequence• Errors in replication or by mutagens

Mutations can change the meaning of genes


Ch. 13- What turns genes ON and OFF?In bacteria, control genes are next to code genes

Lac operon – gene is OFF when lactose absent

- ON when lactose presentRepressor protein on DNA blocks RNA polymerase - Lactose removes repressor transcription


Gene Control in EukaryotesControl genes are NOT near code genes

Many proteins interact to help mRNA form

• Transcription Factors

• Control genes may be on different chromosomes from coding genes

CP BioChapter 12-13 DNA Function

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