nucleic acids and protein synthesis. when? ◦ 1928 what? ◦ worked with bacteria which cause...
TRANSCRIPT
Chapter 10Nucleic Acids
and Protein Synthesis
I. Discovery of DNA (p. 193)
When?◦1928
What?◦ Worked with bacteria which cause
pneumonia (Streptococcus) Why?
◦ To figure out what substance carries genetic info from parents to offspring
How?◦ Used R-strain (non-virulent) and S-strain
bacteria (virulent)in series of 4 experiments
Griffith’s Experiments
Results?◦ Experiments 1 & 2:
S strain kills mice/ R strain does not◦ Experiment 3:
Dead S strain bacteria do not kill mice◦ Experiment 4:
Dead S strain bacteria and live R DO kill mice Conclusions?
◦ 1. heat killed bacteria release a hereditary factor which live, bacteria absorb, and use
◦ 2. termed this process “transformation” which is defined as the external transfer of genetic material from one cell or organism to another
Griffith’s Experiments
When?◦ 1940s
What?◦ Worked with same types of bacteria as Griffith
Why?◦To determine if the transformation
substance in Griffith’s experiments was DNA, RNA, or protein
How?◦ Conducted 3 experiments using different
enzymes which destroy DNA, RNA, or proteins in S cells that were heat killed and then mixed with live R cells to be injected into mice
Avery’s Experiments
Results?1. Experiment 1: protease enzyme was used to
destroy protein; mice died2. Experiment 2: RNase used to destroy RNA; mice
died3. Experiment 3: DNase used to destroy DNA; mice
lived Conclusions?
◦ 1. In experiment 1, R cells transformed into S cells and killed the mice: protein not needed
◦ 2. In experiment 2, R cells transformed into S cells and killed the mice: RNA not needed
◦ 3. In experiment 3, DNase used to destroy the DNA and mice lived; DNA needed for transformation
Avery’s Experiments
Avery’s Experiment
When?◦ 1952
What?◦ Worked with bacteriophages (viruses that infect
bacteria) *FYI – viruses are non-living*
Why?◦ Test whether DNA or protein was the hereditary
material used to infect living cells How?
◦ 1. used radioactive isotopes of phosphorus (DNA) and sulfur (protein)
◦ 2. allowed bacteriophages to infect bacteria (E. coli)
Hershey and Chase Experiment
Results?◦ 1. When bacteriophages with the phosphorus
isotopes were used, the radioactive phosphorus was found in the bacteria
◦ 2. When bacteriophages with radioactive sulfur were used, the radioactive sulfur was not found in the bacteria
Conclusions?◦ 1. little protein entered the bacterial cells when
infected with the viruses◦ 2. DNA entered the bacterial cells when they were
infected with viruses◦ 3. DNA is therefore the hereditary material
Hershey and Chase Experiment
Hershey and Chase Experiment
DNA Double Helix◦ DNA generally accepted as hereditary
carrier by the 1950s, but structure not known
◦ Franklin and Wilkins took x-rays of DNA crystals
◦ Watson and Crick deduced structure of DNA based on the diffraction images taken by Franklin DNA was double helix (twisted ladder) DNA’s width did not vary Made of 2 sides/chains Each full twist/turn has 10 nucleotide pairs
◦ Watson & Crick published article in Nature magazine and of course the rest is history!
◦ Watson, Crick and Wilkins won a Nobel Prize in Medicine in 1962 (Franklin died in 1958)
II. DNA Structure (p. 196)
Structure of DNA
Classified as nucleic acid Made of 2 chains held together by hydrogen bonds
down the center Chains are made of alternating sugar and
phosphate units Nitrogen bases line the center of DNA molecule on
each side Purines = adenine & guanine; contain 2 rings Pyrimidines = thymine & cytosine; contain 1
ring Purine always paired with a pyrimidine to
maintain DNA’s width ( A = T 2-H bonds/ C = G 3-H bonds)
Chains are made of repeating units called nucleotides which are held together by covalent bonds
◦ One nucleotide = sugar, phosphate & nitrogen base◦ Sugar is between phosphate and nitrogen base
DNA Nucleotides
DNA Nitrogen Bases
Chargaff’s rule◦ When? 1949◦ What? Analyzed DNA content of variety of
organisms◦ Why? To prove that DNA from different organisms
should reflect differences in their biological activities/ was impressed and influenced by Avery’s work
◦ How? Developed procedure which could analyze DNA using extremely small amounts since DNA was hard to come by
◦ Results? Observed that amount of guanine = amt of cytosine & amt of adenine = amt of thymine
◦ Conclusions? Base pairing rule: A +T & C +G
Complementary Bases
Defined as order of nitrogen bases in DNA sequence
Gives the organism its traits Held together by covalent bonds to ensure
continuation of genetic code
Base Sequence
Give the complementary strand to this half of DNA’s base sequence?
A -T-T-G-G-C-C-T-A-G-C-G-T-C
T-A-A-C-C-G-G-A-T-C-G-C-A-G
Can you…..
How DNA copies itself
III. DNA REPLICATION (p.200)
Replication is defined as the copying of DNA’s code in the nucleus of a cell in preparation for cell division
It occurs during S phase of the cell cycle
How DNA Replication Occurs
Precedes mitosis, meiosis, + binary fission
Results in an exact copy of the DNA material
How DNA Replication Occurs
Occurs in a series of steps◦ Special enzymes called helicases break the
hydrogen bonds between the nitrogenous bases at the center of the DNA strand
◦ Special enzymes called DNA polymerases join new nucleotides to create a complementary strand for each parent strand
◦ Result is 2 new DNA molecules each consisting of 1 parent strand and 1 new strand: semi-conservative replication
How DNA Replication Occurs
Action at the replication fork◦ A replication fork is formed when the DNA strands
separate and create a Y shaped region◦ Several replication forks form in eukaryotic
replication◦ One strand is complementary base paired in a
continuous fashion toward the replication fork◦ The second strand is made through segment
formation and requires an additional enzyme: DNA ligase
How DNA Replication Occurs
Prokaryotic cells◦ 1 circular DNA chromosome◦ Has 2 replication forks which work in opposite
directions until the entire chromosome is replicated
Eukaryotic cells◦ Have multiple linear chromosomes◦ Have several points of origins for replication to
speed up the process
Prokaryotic & Eukaryotic Replication Differs
1 per billion occurs in eukaryotic cells Errors are corrected by DNA polymerase and by
nucleases If the errors are excessive, cell division can be stopped
(remember the checkpoint before mitosis occurs in the cell cycle)
Changes that are not corrected & result in a mismatch or missing complementary base are called mutations◦ Can occur through chemicals, x-ray exposure, UV
light, etc….◦ Can lead to cancer
DNA Errors in Replication
Flow of genetic information◦ You are you (your phenotype) because of the
proteins your body makes◦ The source of your genetic information is contained
in your DNA located in the nucleus of your cells◦ RNA is responsible for copying DNA’s code in the
nucleus (transcription) and for assembling the correct amino acids in the correct order to make the proper protein (translation)
◦ Together transcription & translation are called protein synthesis & can be summarized as follows: DNA RNA Protein
IV. Protein Synthesis (p.204)
Is classified as a nucleic acid & therefore is made of repeating units called nucleotides
Contains the sugar ribose instead of deoxyribose
Uses the same bases as DNA except for thymine which is replaced by the pyrimidine uracil
Is usually single stranded rather than double stranded and not as long as DNA
RNA Structure & Function
RNA Structure & Function
Exists in 3 types:◦Messenger RNA Single straight strand Made when RNA nucleotides join through
complementary base pairing when copying DNA’s code in the nucleus
Copies only the section of DNA needed for a particular protein
Used in transcription and translation Leaves the nucleus following transcription
through the nuclear pores
RNA Structure & Function
Transfer RNA◦Hairpin/Cross/clover shaped◦Single stranded◦Reads mRNA at ribosome and then retrieves
correct amino acid for placement in protein◦Used in translation
Ribosomal RNA◦Globular form◦Makes up ribosomes (site of protein synthesis) and the nucleolus
RNA Structure & Function
RNA Structure and Function
Defined as the process in which mRNA is created when mRNA copies a specific segment of DNA’s code (rRNA & tRNA can also be made in this fashion)
Occurs in a series of steps◦ Promoter (sequence of nucleotides on DNA) initiates
the formation of RNA (denotes specific section of DNA to be copied)
◦ RNA polymerase binds to the promoter to untwist DNA and prepare the section of DNA for copying
◦ RNA polymerase is used to bind complementary RNA nucleotides together until a particular sequence of nucleotides on DNA (termination signal) is reached and the process is stopped
Transcription
Defined as the sequence of nucleotides which identify each amino acid in a protein
Based on 3 nucleotides on mRNA called a codon
Codons identify specific amino acids, starting point for a protein and the stop signal for a protein
Multiple codons can be used for a single amino acid
No codon codes for multiple amino acids
The Genetic Code
Chapter 3 Review Vocabulary
The Genetic Code
Involves all 3 types of RNA◦ mRNA = contains DNA’s code◦ tRNA = retrieves the correct amino acid◦ rRNA = composes the ribosomes
Protein structure◦ Made of individual units called amino acids
(20 different ones)◦ Sequence of nucleotides varies for each
protein◦ Each protein has a specific 3D shape which
correlates to its function
Translation
Occurs in a series of steps◦ mRNA arrives at the ribosomes◦ tRNA reads mRNA’s code & complementary base
pairs to the codon with an anticodon found at one end of the tRNA molecule
◦ tRNA retrieves the correct amino acid from the cytoplasm of the cell & takes it to the ribosome where it forms a peptide bond with the next amino acid for the protein
◦ tRNA molecules can be used over & over again◦ Process occurs from the initial start codon (often
methionine) & continues until a stop codon is reached
◦ To summarize the basic steps are: initiation, elongation, termination
Translation
Translation
Prokaryotic cells can transcribe and translate at the same time
Eukaryotic cells do not translate until transcription is complete
Translating Many Ribosomes At Once
Defined as the entire gene sequence of DNA
Contains about 30,000 genes in our 46 chromosomes
The Human Genome
