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One Gene-one polypeptide: - Each gene codes for the production of a specific polypeptide -Beadle and Tatum first showed a direct relationship between genes and enzymes, which they put forward as the one gene-one enzyme hypothesis -Since a different gene encodes each distinct polypeptide, their hypothesis was restated as the one gene-one polypeptide hypothesis Template strand: is one of the two DNA strands that are used in transcription to copy the DNA into an RNA strand, and only part of the DNA sequence of the genome is copied in any cell at any given time Messenger RNA: the RNA strand that is transcribed from a protein-coding gene Genetic code: written in 3 letter words using a four letter alphabet. The nucleotide bases for DNA are Adenine, Thymine, Guanine, and Cytosine (A, T, G,C). The RNA nucleotide bases are Adenine, Uracil, Guanine, and Cytosine (A, U, G, C). There are four RNA bases, but there are 20 amino acids. The genetic code is the nucleotide information that specifies the amino acid sequence of a polypeptide. Codon: the three-letter word (triplet) of the code. The three-letter codons in DNA are first transcribed into complementary three-letter RNA codons. Understanding codons is more important for understanding translation. Degeneracy: when an amino acid is represented by more than one codon, some by as many as 6. Comma less: the code can be read correctly only by starting at the right place—at the first base of the first three-letter codon at the beginning of a coded message—and reading three nucleotides at a time from this beginning codon. Start codon: start codon is AUG, which is the first codon read in an mRNA in translation in both prokaryotes and eukaryotes Stop codon: acts as periods, indicating the end of a polypeptide-encoding sentence. When a ribosome reaches one of the stop codons, the polypeptide synthesis stops and the new polypeptide chain is released from the ribosome Introns: non-protein coding sequences that interrupt the protein-coding sequence. Exons: the amino acid—coding sequences that are retained in finished mRNAs Promoter: a control sequence, that initiates transcription, RNA polymerase binds to the promoter, unwinds the DNA in that region, and starts synthesizing a new RNA molecule at the transcription start point. The part of the gene that is copied into RNA is called the transcription unit. Initiation: RNA polymerase binds to the promoter, unwinds the DNA, and initiates the transcription at the start point Elongation: RNA polymerase moves along the DNA, unwinding it and adding new RNA nucleotides to the transcript in the 5’—3’ direction. Behind the enzyme, the DNA strands reform into a double helix Termination: the complete RNA molecule is released from the template DNA, RNA polymerase leaves the DNA, and the double helix reforms Alternative splicing: when pre-mRNAs are processed by reactions that join exons in different combinations to produce different mRNAs from a single gene. This greatly increases the number and variety of proteins encoded in the cell nucleus without increasing the size of the genome. As a result of the alternative splicing process, the number of proteins produced far exceeds the number of genes, and it is proteins that direct an organisms functions. This increases the coding capacity of existing genes. Exon shuffling: a process by which existing protein regions or domains, already selected for their functions by the evolutionary process, are mixed into novel combinations to create new proteins. This generates new proteins. Eukaryotic and prokaryotic transcription is very similar. Major difference is the RNA processing and that transcription occurs in the nucleus of a eukaryote and in the cytosol of a prokaryote. Similarities between DNA replication and RNA transcription 1. Only one of the 2 DNA strands acts as a template to make a complementary RNA copy, instead of both as in replication 2. Only a small part of a DNA molecule—the sequence encoding a single gene—serves as a template, rather than all of both strands as in replication 3. RNA polymerases catalyze the assembly of nucleotides rather than DNA polymerases in replication 4. RNA molecules resulting from replication a single-stranded rather than double as in replication 5. Where adenine appears in the DNA template chain, a uracil is matched to it in the RNA transcription instead of thymine as in DNA replication Transcription 1. An RNA polymerase molecule binds to the DNA at the beginning of the gene to be transcribed…and the DNA begins to unwind at the front of the RNA polymerase, which begins moving along the DNA 2. During transcription, RNA nucleotides are base paired one after another with the template DNA bases 3. The RNA copy is released when the entire gene has benn transcribed. The unwound region of the DNA rewinds into a double helix mRNA splicing—occurs in the nucleus and removes introns from pre-mRNAs and joins the exons together. This is part of pre-mRNA processing which takes place in the nucleus of eukaryotes. Takes place in a spliceosome. The snRNPs are what actually removes the introns. The snRNPs that have small nuclear RNA or snRNA, recognize the pair with sequences at the junctions of the intron with the adjacent exons. The complex then recruits the other snRNPs, producing a larger complex that loops out the intron and brings the two exon ends close together. At this point the active spliceosome has been formed. The spliceosome cleaves the pre-mRNA at the junction between the 3’ end of the exon and the 5’ end of the intron, and the intron loops back to bond itself near its 3’ end. The spliceosome then cleaves the pre-mRNA at the junction between the 3’ end of the first intron and the exon, releasing the intron and joining together the two exons. Enzymes then degrade the intron and the snRNPs are released. RNA molecule elongates as new nucleotides are added one by one. Elongation continues until the RNA synthesis terminates and the completed RNA transcript and RNA polymerase are released from DNA. In prokaryotes, the promoters are immediately upstream of the site where transcription initiates. RNA polymerase itself recognizes the key DNA sequences in the promoter, binds to the promoter and begins transcription. In eukaryotes, RNA polymerase II transcribes protein-coding genes. RNA polymerases I and III transcribe genes for non-protein coding RNAs, such as rRNAs in the ribosomes, and tRNAs.