Chapter 12 and 13

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

PowerPoint Lectures forBiology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon

Lectures by Chris Romero

Chapter 12 and 13Chapter 12 and 13

DNA, RNA and Protein Synthesis


A. 1928 - Frederick Griffith discovers transformation in bacteria : * discovered that “something” was able to transform harmless (non – virulent) bacteria into harmful (virulent)

Discovery of the Role of DNA


B. 1944 -Oswald Avery and colleagues show that DNA can transform bacteria

C. 1952 - Alfred Hershey and Martha Chase use bacteriophage to confirm that DNA is the genetic material

Discovery of the Role of DNA (cont’d)


1

Hershey-Chase Experiment: Infected cells make more virus by injecting their DNAanimation


D. 1953 - James Watson and Francis Crick propose a structural model for the DNA molecule

Discovery of the Role of DNA (cont’d)

1. X-Ray crystallography images prepared by Maurice Wilkins and Rosalind Franklin

2. Chargraff’s Rule: # of Adenines = # of Thymines # Guanines = # of Cytosines

Based On:


DNA and RNA are Polymers of Nucleotides• Both are nucleic acids made of long chains of

nucleotide monomers

• A nucleotide (building block of a nucleic acid) has 3 parts:

1. A phosphate (PO4-)

group that is negatively charged

2. A 5-Carbon sugar (deoxyribose in DNA or ribose in RNA)

3. A nitrogen-containing base


DNA (deoxyribonucleic acid) bases:

Pyrimidines: single ring basesPurines: double ring basesComplimentary binding pattern:

• Adenine + Thymine (share 2 hydrogen bonds)• Cytosine + Guanine (share 3 hydrogen bonds)

Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

pyrimidines purines


Similar to DNA except:

• Sugar in RNA = ribose

• Base “uracil” instead of thymine

• Single stranded

Figure 10.2C, D

RNA: ribonucleic acid


The Structure of DNA• Two polynucleotide strands wrapped around each other

in a double helix

• A sugar-phosphate backbone

• Steps made of hydrogen-bound bases (A=T, C = G)

Twist


DNA REPLICATION:Starts with the separation

of

DNA strands

• Enzymes use each strand as a template to assemble new nucleotides into complementary strands…“semi-conservative” (Meselson & Stahl 1958)

• Portions to be replicated must untwist first


DNA replication begins at specific sites on double helix

1. DNA segments unwind

2. Helicase splits H bonds between bases, unzip DNA

3. Binding proteins keep unzipped DNA apart (Single Stranded Binding Proteins)

4. Primase makes a short RNA primer because DNA polymerase can only extend a nucleotide chain, not start one.

5. DNA polymerase adds new nucleotides to the 3’ end of daughter strand that are complimentary to the parent strand

6. RNase H cuts out original primers

7. DNA polymerase fills in gap of removed primers

8. DNA ligase glues S/P backbone where needed •Topoisomerase: prevents further coiling at replication fork

replication forks

Animation/tutorial

9. Two identical double helices


• Each strand of the double helix is oriented in the opposite direction (“anti-parallel”)

• “prime” #’s refer to carbons in the sugar

• At one end, the 3’ carbon has an (OH) and at the opposite, a 5’ carbon has the PO4

-

• Why does this matter? DNA polymerase can only add nucleotides to the 3’ end. A daughter strand can only grow from 5’ 3’

• Therefore, only one daughter strand is made continuously (leading strand)

• The other strand (lagging strand) is made in a series of short pieces (Okazaki fragments), later connected by DNA ligase

A Structural Problem with DNA Replication

animation

Animation/tutorial


When DNA can repair mistakes and when it can’t

DNA Repair enzymes work like a spell checker

• Cut out wrong sequences

• Undamaged strand is template

• Only 2 or 3 stable changes per year

: some severe, others are not

• Mutations Inheritable changes occur in gametogenesis

• Now the “wrong” sequences are copied

– Ex: cystic fibrosis (CF): a deletion of 3 nucleotides in a certain gene

– Ex: sickle cell anemia: one nucleotide substitution in the hemoglobin gene

• Mutagen: a mutation causing substance (can break DNA)

– Ex: X-Rays, radioactivity, nicotine


Protein Synthesis: the transfer of information from: DNA RNA Proteins “gene expression”:A gene is a linear sequence of many nucleotides. 3 Types:

1. Structural genes: have info to make proteins

2. Regulatory genes: are on/off switches for genes

3. Genes that code for tRNA, rRNA, histones

• double stranded• A T C G• deoxyribose sugar

• single stranded• A U C G• ribose sugar• 3 types of RNA:

•messenger, transfer, ribosomal

DNA vs. RNA

mRNA (messenger): copies DNA’s message in nucleus brings it to cytoplasm

tRNA (transfer): carries amino acids to mRNA so protein can be made

rRNA (ribosomal): major part of the ribosome. Helps link amino acids from tRNA’s together assemble protein


1. Transcription: The DNA of the gene is transcribed into mRNA

2. Translation: decoding the mRNA and assembling the protein

Protein Synthesis is Two Steps:


Transcription: Eukaryote• DNA sequence (message for

protein) is transcribed by mRNA

• Only one strand (non-coding strand) is needed as a template

• Steps:1. RNA polymerase splits H bonds in DNA

section2. RNA polymerase travels along non-coding

strand of DNA. RNA nucleotides join in a complimentary pattern (A=U, C=G)

3. A termination signal is reached, transcription is over

4. mRNA strip detaches from DNA, DNA helix closes up

5. mRNA is processed: Introns are cut out, Exons are glued together, cap and tail are added.

6. Mature mRNA leaves nucleus through pores cytoplasm for next step


Translation: the synthesis of proteins using mRNA, tRNA and ribosomes

• The Genetic Code: the language in which instructions for proteins are written in the base sequences

• Each triplet of mRNA bases is a “codon” because it will “code” for 1 amino acid

– Ex: AUG GUC CCU AAU CCU

Met – Val – Pro – Asn – Pro

– Original coding strand of DNA (the actual gene): ATG GTC CCT AAT CCT

• Only difference: U is substituted for T

– Use the Genetic Code chart to “decode” mRNA message


–

–Nearly all organisms use exactly the same genetic code

– More than one codon for most amino acids = degenerate nature…a change (mutation) in gene does not always mean a different amino acid.

– what does CAU code for? ACU? UAU? GCC?

– how many codons for Leu?

– what is special about AUG and it’s amino acid, Methionine?

– what is special about UAA, UAG, and UGA?

The Genetic Code is the Rosetta Stone of Life


An exercise in translating the genetic code:

A AGT A G T T T A GT

Step 1: fill in corresponding DNA bases to dark blue strand (non-coding)

Step 2: Transcribe the dark blue strand into mRNA (pink)

Step 3: Translate the codons into correct amino acids (use chart)

Coding strand (gene) transcription

translation


An exercise in translating the genetic code: answers

Step 1: fill in corresponding DNA bases to dark blue strand (non-coding)

Step 2: Transcribe the dark blue strand into mRNA (pink)

Step 3: Translate the codons into correct amino acids (use chart)


Need: tRNAs and ribosomes (rRNA)

tRNA: single stranded RNA, folded up

– 2 parts: anticodon and aa attachment site

How Does Translation Happen?

Ribosome: 2 protein subunits and ribosomal RNA

• allows aa’s to attach by making peptide bonds

• travels along mRNA strip, tRNA’s join and bring correct amino acids

• 3 sites on ribosome: • A site – where new tRNA’s and amino acids join• P site – where protein is growing• E site – where empty tRNA’s exit ribosome

Translocation: as ribosome moves, tRNA’s move from A site to P site. “A” site is now open for new tRNA with attached amino acid to join

animation


Put It All Together:


Mutations can change the message of genes

Mutations: • changes in DNA base sequence

• caused by errors in DNA replication, recombination, or by mutagens• substituting, inserting, or deleting nucleotides also alters a gene

“frame-shift mutation”…most devastating to protein structure

“point mutation”…may or may not alter amino acid sequence


MUTANTS –

• Mutant Animals!

Chapter 12 and 13

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