DNA, RNA, and Protein Synthesis - PowerPoint Presentation

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DNA, RNA, and Protein Synthesis

Chapters 16 and 17

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Before the end of the semester we will be covering…

Historical DNA experiments

Structure of DNA/RNA

DNA Replication

Protein Synthesis (Transcription and Translation)

Mutations

Gene Expression (if time)

No more labs this semester!

Your final will be comprehensive

 Both multiple choice and short answer…more info to come!

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Important Historical Experiments

In

the 1940s

little was understood about inheritance and how it worked.

It

was believed the genetic material was either DNA or protein

.

It was understood that chromosomes are made of both DNA and protein

Initial experiments suggested it was

protein…little was understood about DNA’s structure or function (proteins were identified as being more complex)

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Avery, McLeod, McCarty1944

goal was to identify if inherited substance was either DNA, RNA, or protein

used chemicals and bacteria that only allowed one of the above to be active at a time

Results determined that the transforming agent was DNA

Scientific community still skeptical

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Hershey and Chase 1952

used bacteriophages to confirm DNA is the genetic materialbacteriophages were tagged with radioactive isotopes (DNA--P; protein--S)it was shown that the DNA was able to “infect” the bacteriophages, not the protein by tracking the radioactivity

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Watson and Crick 1953

discovered the shape of DNA molecule was a double helixusing pictures of the molecule, they built a model sugar/phosphate backbonenitrogen bases in the interiorstrands are antiparallelhelix uniform in diameter (base-pairing rules)

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Wilkins and Franklin 1953

Franklin used x-ray diffraction to photograph DNA (her pictures were used by Watson and Crick)Wilkins was working closely with Franklin in her lab and allegedly showed Watson and Crick the photograph that helped them build their modelWatson, Wilkins, and Crick were awarded the Nobel Prize for science in 1962. Rosalind Franklin died in 1958 of cancer and was never given a Nobel Prize.

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Using the original papers published in

Nature

in 1953, identify the characteristics of a DNA molecule.

What important characteristics did they (Watson, Wilkins, Crick, and Franklin) discover? List/highlight as many as you can.

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Deoxyribonucleic Acid (DNA)

Classified as a nucleic acid (biological molecule)Genetic material organisms inherit from their parentsCopied prior to cell division (mitosis/meiosis)Shape of a double helixMade up of nucleotides (building blocks)5-carbon sugar (deoxyribose), phosphate, nitrogen base

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Base-Pairing Rules: ATCGDirectionalityComplementary strandsAntiparallel: each strand runs in an opposite directionDesignated 5’ and 3’ ends (carbon on sugar)Two types of basesPurines: two carbon ringsGuanine, AdeninePyrimidines: one carbon ringCytosine, Thymine, Uracil

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Ribonucleic Acid (RNA)

mRNA (messenger RNA)—instructions (from DNA) for making proteintRNA (transfer RNA)– carries amino acidsrRNA(ribosomal RNA)—makes up ribosomes5-carbon sugar (ribose)Single stranded (one gene)Uracil instead of thymineAU

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Both RNA and DNA…

Have adenine, cytosine, and guanineMade of nucleotidesSugar and phosphate backboneNitrogen bases perpendicular to backboneHeld together by hydrogen bonds

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Biochemical Gymnastics: DNA Replication

Occurs during S-phase of Interphase in the cell cycleSemi-conservative process. Each new strand of DNA produced is made of one parental and one new strand (described by Watson and Crick)Each strand serves as a template for the new strandIn prokaryotes DNA is circularIn eukaryotes DNA is linear

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DNA Replication(Overview)

Begins at the origin of replication (specific sequences of DNA nucleotides)Proteins recognize this sequence and attach to the DNA and separate the two strands creating a “bubble”At either end of this “bubble” is the replication fork Replication then proceeds in both directions from the origin until both strands are copied.In prokaryotes replication starts in one spot, in eukaryotes multiple spots

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The Players.

Helicase

: enzyme that unwinds and unzips the helix at the replication forks.

Single-strand Binding Protein (SSBP’s)

: binds to unpaired DNA strand to keep them from re-pairing

Topoisomerase

: enzyme that relieves tension ahead of the replication fork (from untwisting of strand)

Primase

: enzyme that synthesizes the RNA primer for replication

DNA Polymerase

: several enzymes that catalyze the synthesis of new DNA (in eukaryotes there are 11 total); also checks for errors

Ligase

:

links new fragmented DNA segments together

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Replication only occurs in the 5’ to 3’ directionNucleotides added only to 3’ end of molecule…This is problematic for one side of the DNA moleculeLeading strand: continuous strandSingle RNA primerLagging strand: discontinuous in fragments (Okazaki fragments) multiple RNA primers

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The steps…

1. Origin of replication is located

2. Bubble forms in DNA helix by

Helicase

3.

Primase

synthesizes primer to begin replication

Need RNA primer to have something to add nucleotides to.

4. DNA Nucleotides are added by DNA polymerase to primer to begin new strand

5. Replication proceeds in the 5’ to 3’ direction on both sides of the molecule

Leading and lagging strands

6. Replication continues until the entire molecule is copied

http://highered.mheducation.com/sites/0035456775/student_view0/chapter12/dna_replication.html

http://www.dnalc.org/resources/3d/04-mechanism-of-replication-advanced.html

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Ending Replication…

After every round of replication some of the DNA molecule is lost due to polymerase not being able to replicate it.To avoid excess loss of DNA, the ends of eukaryotic chromosomes have telomeres (long repeating sequences)Excess DNA nucleotides (no genetic info)Acts as a buffer to actual genes (does shorten over time—thought to be evidence of aging)

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Protein Synthesis (gene expression)

Chapter 18

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Gene Expression: process by which DNA directs the synthesis of proteinsoccurs in two parts: Transcription and Translationthis process dictates the presence of specific traits (genotype/phenotype)occurs in all organismsWatson and Crick describes this as the “central dogma” (DNARNAProtein)

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Transcription

synthesis of RNA (mRNA) using DNA as a template (protein instructions)occurs in nucleus (eukaryotes) or cytoplasm (prokaryotes)Prokaryotes can begin translation before transcription is finishedEukaryotes have an extra step during transcription before translation can begin

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DNA is a template strand, which is used to produce mRNA instructions (for protein)mRNA is complementary to DNAuses different nucleotides (uracil)RNA polymerase unzips DNA and joins complementary RNA nucleotides to copy instructionsreads 5’ to 3’ , no primer neededpromoter: DNA sequence where RNA polymerase attaches and initiates transcription

http://www.dnalc.org/resources/3d/13- transcription-advanced.html

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3 Stages

Initiation

RNA polymerase joins to the promoter and begins to unwind helix

helped by transcription factors (proteins), creates a “transcription-initiation complex”

Elongation

RNA polymerase unwinds/untwist 10-20 nucleotides at a time

nucleotides added to 3’ end

as mRNA is built, the molecule peels away from DNA and the double helix reforms

Termination

in prokaryotes there is a terminator sequence (stop signal)

eukaryotes transcribe a specific sequence to stop transcription (creates pre-mRNA)

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RNA Modifications (eukaryotes only)

RNA processing

: enzymes in the nucleus modify pre-mRNA

To help protect from

degradation

5’ cap (modified G sequence)

3’ (poly-A tail)

RNA splicing

: removal of large portions of the RNA molecule (cut and paste)

eukaryotes have long stretches of non-coding DNA interspersed with coding segments

introns

: non-coding segments

exons

: coding segments eventually expressed

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RNA Splicing…introns are removed and exons are joined togethersnRNPs: join together to form a spliceosomeOnce finished, completed mRNA leaves nucleus to begin translation

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Translation

synthesis of a polypeptide using mRNA as instructionsoccurs on ribosomes (rRNA) in cytoplasmtRNA: transfers amino acids to growing proteineach associated with a particular amino acidanticodon: complementary RNA sequence to mRNA

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instructions for producing a protein uses three letters on mRNA (triplet code)codon: mRNA triplet (3 nucleotides)methionine is “start”“stop” codons UAA, UAG, UGA

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3 stages

InitiationmRNA, tRNA, and ribosome start with amino acid methionine (initiator tRNA)“translation initiation complex”Elongation amino acid added to chain via sites on ribosome (A-P-E)“elongation factors” help process; reads 5-3’requires energyTerminationstop codons end synthesis, codes for “release factor”release factors bind and protein is released via hydrolysisProtein is then folded into appropriate shape with help of chaperonin proteins

http://www.dnalc.org/resources/3d/16-translation-advanced.html

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Oops!

Mutation: change to genetic informationUltimate source of new genesMay be spontaneous or result from mutagensPoint mutations: change in single nucleotide (substitution)silent: change doesn’t alter amino acid sequencemissense: changes one amino acid into another (minor changes)nonsense: change codon for amino acid into a stop codon (premature end to translation)Frameshift mutation: add/lose nucleotides resulting in change to reading frame of codons (not multiples of 3)Insertion or Deletion

http://www.bozemanscience.com/mutations

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