L85. In eukaryotes, chromosomes are linear and as such, pose a problem for DNA replication. As replication nears the end of the chromosome, there is no place for the primase to land as
such, DNA polymerase would be unable to synthesize a DNA fragment for this end. Without a mechanism to fill this gap, the chromosomal end will remain unpaired and will be lost and over several rounds of replication, the ends would get progressively shorter ultimately compromising the organism's ability to survive. Telomerase solves this problem by attaching to the end of the chromosome by complementary base pairing between the RNA of telomerase and the DNA template. The RNA is used to elongate the DNA and once it is sufficiently elongated, primase will create a primer followed by DNA polymerase which can then add nucleotides complementary to the ends of chromosomes thus replicating their ends. L86. The source of genetic variation is mutation because changing the DNA sequence can produce different genes. DNA polymerase is able to proofread DNA for any errors and when an error is detected, they can remove and replace bases. The tradeoff for fixing error is that you can have a potentially positive mutation that can help in evolution however if you fix it, you are essentially holding yourself back. L87. The central dogma states how DNA becomes a protein. When DNA goes to DNA, this is known as replication. When DNA goes to RNA this is known as transcription, and when RNA goes to protein this is known as translation. The key components of replication include, DNA polymerase I/III, primase, ligase, helicase, topoisomerase. Key components of transcription include RNA polymerase, a promoter, the initiation and termination sites. Key components of translation include, mRNA, charged tRNA, ribosome binding site (Only for prokaryotes), start codon (AUG), and stop codons (UAA, UAG, UGA). L88. The structure of RNA differs from DNA primarily in the presence of hydroxyl groups in the carbon molecule. RNA has hydroxyl groups on the 2’ and 3’ end of the carbon molecule whereas DNA lacks a hydroxyl group on the 2’ end. L89. RNA synthesis occurs during transcription and made possible by RNA polymerase. In transcription, a promoter has a DNA sequence onto which transcription machinery bind and initiate transcription. The promoter in Eukaryotic cells is typically called a TATA box. Transcription factors recruit the RNA polymerase to the promoter. Initiation of transcription begins when the RNA polymerase binds to the promoter. In elongation, the RNA polymerase moves along the DNA template adding nucleotides by base pairing with the DNA template. The product of this is almost identical to the DNA non-template strand which is called the coding strand (just replace the U’s with T’s). RNA polymerase needs a termination signal to end transcription.
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