BLG 143 Study Guide - Final Guide: Dna Replication, Okazaki Fragments, Deoxyribonucleotide
27 Apr 2018
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14.4 Replicating the Ends of Linear Chromosomes
Telomere region at the end of a linear chromosome
Problem of replication of telomeres:
• When the replication fork reaches the end of a linear chromosome, DNA polymerase
can synthesize the leading strand to the end of the parent DNA template
• The leading strand synthesis results in a normal copy of the DNA molecule
• On the lagging strand, primase adds a RNA primer close to the tip of the
chromosome
• A DNA polymerase synthesizes the final Okazaki fragment on the lagging strand
• Another DNA polymerase removes the primer
• DNA polymerase is unable to add DNA near the tip of the chromosome because
there’s not enough room for primase to add a new RNA primer
• The single stranded DNA that is left must stay single stranded
The single stranded DNA that remains at the end is eventually degraded, which results in
the shortening of the chromosome. If this process were to continue unabated, every
chromosome would shorten by 50-100 deoxyribonucleotides on average each time DNA
replication occurred.
Over time, linear chromosomes would be expected disappear completely.
How do eukaryotes maintain the integrity of linear chromosomes?
1. Telomeres don’t contain genes that code for products needed in the cell
2. Enzyme telomerase is involved in replication telomeres
Telomeres consist of short stretches of bases that’re repeated over and over.
Telomerase catalyzes the synthesis of DNA from an RNA template. The enzyme carries an
RNA molecule with it that acts as a built-in template, allowing telomerase to add DNA onto
the end of a chromosome and prevent it from getting shorter.
1. Unreplicated segment of the telomere at the 3’ end of the lagging strand forms a
single strand overhang
2. Telomerase binds to the overhanging section of single stranded parent DNA. Once
the enzyme has bound, it begins catalyzing the addition of deoxyribonucleotides in
the 5’ to 3’ direction, that’re complimentary to its built-in RNA template
3. Telomerase moves in 5’ to 3’ direction and continues to catalyze the addition of
deoxyribonucleotides
4. Once the single stranded overhang on the lagging strand is lengthened in this way,
the normal machinery of DNA synthesis resumes synthesis of the lagging strand in
the 5’ to 3’ direction. The lagging strand becomes slightly longer than it was
originally
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Document Summary
Telomere region at the end of a linear chromosome. The single stranded dna that remains at the end is eventually degraded, which results in the shortening of the chromosome. If this process were to continue unabated, every chromosome would shorten by 50-100 deoxyribonucleotides on average each time dna replication occurred. Over time, linear chromosomes would be expected disappear completely. How do eukaryotes maintain the integrity of linear chromosomes: enzyme telomerase is involved in replication telomeres. Telomerase catalyzes the synthesis of dna from an rna template. Once the enzyme has bound, it begins catalyzing the addition of deoxyribonucleotides in originally single strand overhang deoxyribonucleotides. Telomere shortening has a role in limiting the amount of time cells remain in an actively growing state. As dna polymerase goes along a parent dna template, h bonding occurs between incoming deoxyribonucleotides and the ones on the template strand.
