DNA, RNA & ProteinsTranscription Translation

Chapter 3, 15 & 16

Learning Outcomes

• Explain how DNA carries the instructions for the characteristics of individuals.

• Given a sequence of DNA, write the sequence of mRNA and the resulting amino acids.

• Explain how defects in a sequence of DNA causes a defect in proteins and causes disease.

Sequences of DNA code for proteins indirectly

• Sequence of bases in DNA is used to produced mRNA

• The order of the bases in mRNA determines the order of the amino acids (mRNA codes for specific amino acids)

• Amino acids combine to make proteins

Understanding the Code

• George Gamow predicted that each word (codon) in the genetic code —the rules that specify the relationship between a sequence of nucleotides in DNA or RNA and the sequence of amino acids in a protein—would be three nucleotides long based = minimum code length that could specify the 20 different amino acids found in proteins.

How Did Researchers Crack the Code?

• The three-nucleotide sequence coding for each amino acid. Most of the 20 amino acids have multiple codons that designate their addition to a protein.

Genetic Code

• Redundant: – More than 1 triplet can code for the same amino

acid

• Not Ambiguous– No triplet codes for more than 1 amino acid

Central Dogma of Molecular Biology

• The DNA is copied or transcribed into RNA

• RNA is translated in protein

• Applies to ALL cells

Exceptions to the Central Dogma

• Many genes code for RNA molecules that do not function as mRNAs and are not translated into proteins.– These other RNAs perform important functions in the cell.

• Sometimes information flows in the opposite direction—from RNA back to DNA.– For example, some viral genes are composed of RNA and

use reverse transcriptase, a viral polymerase, to synthesize a DNA version of the virus’s RNA genes.

– HIV

Central Dogma of Molecular Biology

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The Messenger RNA Hypothesis

DNA

mRNA Do mRNAmoleculesconnect DNAto proteins?

mRNA

Ribosome Protein

DNA isfoundin thenucleus

Proteinsynthesistakes place inthe cytoplasm

The Central Dogma of Molecular Biology

• The central dogma summarizes the flow of information in cells: DNA is transcribed (copied) into RNA, and RNA is translated into protein.

DNA

• Stores information long-term• Allows for stable maintenance of information

Transcription: Making RNA from DNA

• DNA unzips• RNA polymerase synthesizes RNA by

transcribing one strand of DNA• Complementary base pairing

Initiation: How does transcription begin?

• Transcription factors-proteins• Promoters – sections of DNA where

transcription begins• RNA polymerase

Transcription Is the Synthesis of RNA from a DNA Template

InitiationPromotor- sequence of bases telling RNA Polymerase where to bind.

RNA Polymerase binds to exposed DNA template

Elongation

RNA PolymeraseMoves along DNATemplate

mRNA continues toelongate

Termination

Termination sequence is a sequence of bases telling RNA Polymerase to stop

Exons, Introns, and RNA Splicing• The transcription of eukaryotic genes by RNA polymerase generates a

primary transcript that contains exons and introns. Introns are removed by splicing .

RNA Splicing

Other Aspects of Transcript Processing: Caps and Tails

• Two additional steps of RNA processing are performed in eukaryotic cells after splicing of the primary transcript: the addition of a 5' cap and a poly (A) tail (Figure 16.7).

•The 5' cap serves as a recognition signal for the translation machinery, and the poly (A) tail extends the life of an mRNA by protecting it from degradation.

Transcription Produces

• mRNA– Messenger RNA which is code for a protein carried

from nucleus to the cytoplasm for translation

• tRNA– Transfer RNA 3-D protein transfers specific amino

acids to the ribosome

mRNA

• Shuttles information from DNA to translation machinery (ribosomes)

• Unstable• Intermediary between a sequence of DNA

(gene) and proteins

tRNA

mRNA & tRNA exit nucleus

Amino Acid

Activation

Translation – protein synthesis

• Sequence of bases in mRNA is converted to amino acid sequence

• Amino acid sequence = protein• mRNA is decoded by ribosomes

Ribosomes

• Site of protein synthesis

• Consist of large and small ribosomal subunits and rRNA

• tRNA is adapter molecule

Initiation of Translation

• Ribosome binding site• AUG start codon

Ribosome

• The A site of the ribosome is the acceptor site for an aminoacyl tRNA.

• The P site is where a peptide bond forms that adds an amino acid to the growing polypeptide chain.

• The E site is where tRNAs no longer bound to an amino acid exit the ribosome.

Initiation Phase

Termination Phase

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Post-translational modifications

• ER, Golgi • Add sugar or lipid groups• Add or remove phosphates• Insulin

– Proinsulin active hormone– C-peptide (remaining portion of proinsulin)

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