biochemistry (telomere and telomerase enzyme)

telomeres

• non coding repeated stretch of eukaryotic DNA complexes with protein at the end of chromosomes, shorten with every cell division

• composed of DNA sequences plus protein located at the end of linear chromosomes

• rich in T and G nucleotides and also do not contain genes

• no telomere in prokaryotes because it has circular ds-DNA

• shortening is recognized and is a part of normal aging process

• telomeres present because eukaryotes have linear ds-DNA that cause special problem during replication

• following the removal of RNA primers from the peripheral 5' end of the lagging strand, there is no way to fill the remaining gap

• this cause shortening of the new strand than the parent strand, causing it to become single and exposed to attacks of exonucleases after each cycle of DNA replication

• telomeres are added to the ends of chromosomes in stem and germ cells to protect the genes

structure of telomeres

• consists of several thousand repeats of a noncoding hexameric sequence, 5'-AGGGTT 3', base-paired to a complementary region of Cs and As

• GT-rich strand which is longer, leaving ssDNA a few hundred nucleotides in length at the 3'-end

• the single-stranded region is thought to fold back on itself, forming a loop structure that is stabilized by protein

function of telomeres

• maintain the structural integrity of the chromosome and protects the organism's genes from being eroded

• preventing attacks by nucleases

• allow repair systems to distinguish a true end from a break in dsDNA

• aids in preventing separate chromosomes from fusing

telomerase

• enzyme responsible for synthesis of DNA telomere

• RNA dependent DNA polymerase which adds TTAGGG repeats to the ends of chromosomes

• reverse transcriptase, synthesizing DNA from an RNA template

• consists of RNA template and protein component that provides the catalytic action

• it adds a series of DNA repeats to the leading strand that allows the lagging strand to be completed by DNA polymerase

• telomerase is active in germ cell and adult stem cells but inactive in adult somatic cells

• it does not protect the DNA of adult somatic cells and continually shorten as they undergo rounds of cell division

mechanism of action of telomerase

• the reverse transcriptase use the RNA template to synthesis DNA in the usual (5'-3' direction) extending from the already longer 3' end of the leading strand

• telomerase translocates to the newly synthesized end and the process is repeated

• primase can use it as a template to synthesis an RNA primer once the GT rich strand has been lengthened

• the RNA primer is extended by DNA polymerase and the primer is removed and replaced by DNA

reverse transcriptase (RT)

• used to generate complementary DNA (cDNA) from an RNA template

• considered as RNA-directed DNA polymerases

• function as replication of retroviruses as human immunodeficiency virus HIV virus that carries their genome in the form of single-strand RNA (ssRNA) molecules

• RT uses the viral RNA as a template for the 5'-3' synthesis of viral DNA, then becomes integrated into host chromosomes

• RT inhibitors used as antiviral drugs

• replication of chromosomal ends (telomerase)

• RT activity is seen with transposons, DNA elements that can move about the genome

• RT also used in molecular cloning, RNA sequencing and PCR

biochemical activities of RT

• RNA-dependent DNA polymerase activity

• ribonuclease H

• DNA-dependent DNA polymerase activity

• all these activities are used to convert single-stranded genomic RNA into double-stranded cDNA which can integrate into the host genome