Lecture 22Outline of Last Lecture I. The Central DogmaII. Semi-Conservative ReplicationIII. DNA Replication ProcessIV. Eukaryotic DNA ReplicationV. Prokaryotic DNA ReplicationOutline of Current Lecture I. RNA and DNAII. RNA TranscriptionIII. Three Classes of RNAA. Messenger RNAB. Transfer RNAC. Ribosomal RNAIV. Transfer RNA (tRNA) StructureV. Ribosomal RNA SubunitsCurrent LectureThis lecture will focus on RNA and how it relates with DNA. Various organisms have various genomes that can be broken down into nuclear DNA and organellar DNA. Tiny viruses have genomes that can be as smallas 5,000 base pairs. Humans have a genome 5,000,000,000 base pairs long. The total length of human DNA in the body is a total of 2.0 x 1010 km long. The DNA template is transcribed into RNA through the enzyme RNA polymerase. RNA polymerase will polymerized Uracil and not Tyrosine. Only certain DNA stretches are actually transcribed. Scientists do not understand yet what purpose the DNA that doesn’t encode information has. The focus for this biochemistry class is on the portions of DNA that will be used to code for protein synthesis. When the DNA template is transcribed, RNA polymerase needs to know where to start the transcription process. A small section called the promoter lies upstream at the 5’ end and shows RNA polymerase where to start. After the DNA strand is transcribed into RNA, some touch-ups need to be done. Sections called introns don’t encode for proteins and are spiced out of the RNA. Sections called exons do encode for proteins and are stitched together to form the final RNA strand. Exons can be mixed and matched to form protein variants. How often a strand is transcribed leads to weak or strong expression. This is based up a weak or strong promoter. A weak promoter will not allow the strand to be transcribed often. This causes that gene to have weak expression. This class will focus on three classes of RNA. Messenger RNA (mRNA) makes up only 5-10 percent of cellular RNA. Messenger RNA is what holds the genetic information needed to synthesize proteins. Transfer RNA (tRNA) makes up around 10-15 percent of cellular RNA. This type of RNA is used during the synthesis of proteins to decode the mRNA sequence. Ribosomal RNA (rRNA) makes up 75-80 percent of cellular RNA and is BCHM 307 1st Editionthe last type this class will look at. Ribosomal RNA forms a complex together with proteins. It is a structural component of ribosomes. Ribosomes are what synthesize the mRNA into proteins.Transfer RNA plays an important part in transcription. There is at least one tRNA for each amino acid. tRNAs are small and only 80-100 nucleotides long. tRNA folds itself up through hydrogen bonding in base-pair interactions. tRNA looks like a “clover leaf” when drawn 2-D. The bottom loop formed is called the anticodon loop. This loop is what decodes the mRNA strand. tRNA is used to transfer amino acids to the ribosome, so protein synthesis can take place. Amino acids are “charged”, or linked, to the 3’ end of the tRNA. The 3’ end of tRNA always contains the sequence CCA. tRNA complexes are also special, because they contain bases beyond what we have previously learned. Two examples of such bases are inosine and methylguanosine. This other bases can pair with and recognize more than one codon. Ribosomal RNA can form hydrogen bonds with itself to fold up. Ribosomal RNAs are around 120-2500 nucleotides long. Ribosomes usually have many different proteins and rRNAs within them. Prokaryotic and Eukaryotic rRNAs are each composed of two subunits. The units are named based upon their size in terms of Svedberg units (S). Prokaryotic rRNA have a 50S and 30S subunits. Eukaryotic rRNA have 60S and 40S subunits. These are more complex in
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