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