text notes to transcription and replication

text notes to transcription and replication - Supplementary...

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Unformatted text preview: Supplementary Notes to class 5.5 Nucleoside: nitrogenous base and pentose sugar Phosphodiester linkage: between hydroxyl group on the 3' carbon of 1 nucleotide and the phosphate of the 5' carbon of the next nucleotide. It is a covalent bond. 16.2 origins of replication DNA replication proceeds in both directions from each origin Replication fork: at the end of each replication bubble, y-shaped region where the new strands of DNA are elongating Elongation at the replication fork is catalyzed by enzymes called DNA polymerases DNA polymerase adds individual nucleotides to complementary nucleotides on the template strand Each new nucleotide added actually is a nucleoside triphosphate (sugar, base, 3 phosphates) As each new nucleotide joins the growing of the DNA strand, it loses 2 phosphates. The loss of 2 phosphates is the exergonic reaction that fuels the polymerization reaction DNA polymerases add nucleotides only to the free 3' end of the growing DNA New DNA strand can elongate only in the 5'3' direction Leading strand is elongated only in the 5'3' direction Lagging strand: DNA polymerase must go in the direction away from the replication fork. It is synthesized in a series of segments called Okazaki fragments. DNA ligase joins the sugar-phosphate backbones of the Okazaki fragments DNA polymerase can't initiate synthesis, they can only add nucleotides to the 3' end of the existing strand Primer: initial nucleotide sequence Primase: starts an RNA chain from scratch. It joins RNA nucleotides together one at a time, making a primer complementary to the template strand at the location where initiation of the new DNA strand will occur. DNA polymerase adds DNA nucleotides to the 3' end of the RNA primer and continues adding DNA nucleotides to the growing DNA strand Only 1 primer needed to start synthesizing leader strand, lagging strand: each Okazaki fragment must be primed separately Another polymerase replaces RNA nucleotides of the primer w/DNA nucleotides Ligase joins the sugar-phosphate backbone of all the Okazaki fragments Helicase-untwists double helix at replication fork and topoisomerase relieves the strain After helicase separates 2 parental strands, single strand binding proteins bind to unpaired DNA strands, which stabilizes them During replication, DNA polymerases proofread Chapter 17 p. 312 codons 17.2 During transcription, the gene determines the sequence of bases along the length of an mRNA molecule mRNA base triplets are called codons are written in 5'3' direction mRNA- carries info from DNA to cell's protein synthesizing machinery. mRNA is transcribed from the template strand of a gene RNA polymerase pries the 2 strands of DNA apart and hooks together the RNA nucleotides as they base pair along the DNA template in the 5'3' direction. RNA polymerase doesn't need a primer Promoter- DNA sequence where RNA polymerase attaches and initiates transcription Terminator: sequence signaling the end of transcription Initiation: Promoter has transcription starting point and serves as binding site for RNA polymerase and determines where transcription starts, also determines which of the 2 DNA strands serves as the template In eukaryotes transcription factors (proteins) mediate the binding of RNA polymerase Elongation As RNA polymerase moves along the DNA, it continues to untwist the double helix RNA polymerase adds nucleotides to the 3' end of the growing RNA molecule. The new RNA molecule peels away from the its DNA template and the DNA double helix reforms Termination Prokaryotes: transcription goes through a termination sequence in the DNA. The terminator sequence causes the polymerase to detach from the DNA and release the transcript Eukaryotes: pre-mRNA is cleaved form the growing RNA strand while RNA polymerase continues to transcribe DNA. The RNA polymerase transcribes the polyadenylation signal (AAUAAA) in the pre-mRNA. Proteins associated with growing RNA transcript cut it from the polymerase. 17.3 p. 317 Each end of the pre-mRNA molecule is modified The 5' end (the end that was transcribed first) is capped with a modified guanine called a 5' cap The 3' end has several adenines added called a poly-A tail Why? o 1. facilitate export of mature mRNA from the nucleus o help protect the mRNA from hydrolytic enzymes o help ribosomes attach to 5' end of mRNA RNA splicing- cut & paste introns: noncoding segments of nucleic acid that lie between coding regions exons are eventually expressed RNA splicing: introns cut out and exons joined snRNPs & spliceosomes: release introns ribozymes: RNA molecules that function as enzymes ...
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