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 http://www.ceptualinstitute.com/genre/falk/protein2.gif
The Messenger RNA Hypothesis DNA mRNA Do mRNA molecules connect DNA to proteins? mRNA Ribosome Protein DNA is found in the nucleus Protein synthesis takes place in the 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
Initiation Promotor- sequence of bases telling RNA Polymerase where to bind. RNA Polymerase binds to exposed DNA template
Elongation RNA Polymerase Moves along DNA Template mRNA continues to elongate
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.
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
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.