Etty Widayanti, SSi. MBiotech.Bagian Anatomi Sub Bagian Biologi

Fak. Kedokteran Univ. YARSI

Nov 2009

Aging is a biological processAging not disease

Aging is a PROCESS that converts a healthy, fit organism (for its

environment) into one that is less healthy and fit

What IS Aging?

Aging occurs at multiple levels

• molecules• cells• tissues• organ systems

Cells = molecules + response-----> tissue, organ system effects

Cellular “aging” = response to damage or stress

Cell death(apoptosis)

Arrested cell growth(cell senescence)

Cellular “aging” responses:

Good news!(prevents cancer)

Bad news!(promotes aging)

Cellular senescence(cellular aging)

'Young'Presenescent

'Aged'Senescent

Senescent human fibroblasts

(1)Chromatid. One of the two identical copy of the chromosome

(2) Centromere. The point where the two chromatids touch.

During mitosis, the centromere must divide so that the chromatids can migrate to opposite poles of the cell. Telomeres are the region of DNA at the end of the

linear eukaryotic chromosome (from Greek telos - end, mere - unit) that are required for the (1) stability and . (2)replication of the chromosome.

Eukaryotic Chromosome Structure

Definition

• The long repetitive sequences of a DNA at the ends of a chromosome, that usually consists of one

hundred to fifteen hundred copies of a single DNA sequence.

• During DNA replication, small parts of the telomere are lost with each cycle. This loss may be related to

the aging process.

Every time linear eukaryotic chromosomes are replicated, the DNA polymerase complex stops several hundred bases before the end; if it were not for telomeres, this would quickly result in the loss of useful genetic information.

telomere

Are Telomers necessary for prokaryotes?

No, in prokaryotes, chromosomes are circular and thus do not have ends to suffer replication termination.

Only eukaryotes possess or require telomeres.

Why?

Linear chromosomes cannot normally replicate without telomeres at each end.

Why?

Let us remember the replication process

• DNA helicase opens up the helix

• Single stranded binding protein prevents strands from reannealing before they are replicatedAfter DNA helicase opens up the helix

DNA replication begins with the formation of short RNA primers,(green) synthesized by Primase. This is because DNA polymerases can not add nucleotides unless there is a pre-existing 3' end to add

nucleotides to.

Once the primer is in place, DNA polymerase can elongate it from 5’ to 3’

How the replication from 3’-to-5’ is going?

The End Replication Problem:Telomeres shorten with each S phase

OriDNA replication is bidirectional

Polymerases move 5' to 3'

Requires a labile primer

3'5'

3'5'

5'

5' 3'3' 5'

Each round of DNAreplication leaves

50-200 bp DNA unreplicatedat the 3' end

If there were no mechanism to solve this problem,

the single-stranded ends would likely be destroyed.

Thus each time DNA replication occurred

the linear chromosomes would keep getting shorter by

several hundred bases before the end;

In humans, the telomere sequence is a repeating string of TTAGGG, between 3 and 20 kilobases in length.

if there were no long sequence of a DNA at the ends of a chromosome - telomeres, this would quickly result in the loss of

useful genetic information

• There is a DNA fragment at the end of chromosome which have no genetic information - telomere

• Because it is not possible for DNA polymerise to replicate the end of a linear DNA, in each replication cycle a telomere itself became shorter.

Thus in several replication cycle will not be a telomere sequence at the end of chromosome.

So, there is the problem: how to keep safely telomere sequence at the end of chromosome?

This problem is solved by an enzyme telomerase

Telomerase recognizes an existing telomere sequence and elongate it in the 5’- to 3’ direction.

Question: . this telomere lost during replication 3’________----------------------- 5’

5’----------- 5’ 3’ elongationHow is possible to synthesize in 5’- to 3’ direction in the absence of the complementary DNA strand?

Answer: Telomerase is a ribonucleoprotein: one subunit is RNA fragment acts as a template for the synthesis of telomeric DNA, while a protein subunit is DNA polymerase itself.

. RNA template 3’________----------------------- 5’

5’----------- 5’ 3’ elongation

Telomerase – unusual enzyme, contains all information used to mantain the telomeric sequence.

The processes that shorten (replication) and restore (telomerase extends) are only approximately balances, each chromosome end contains a variable number of repeats.

Telomeres and ageing

At the cellular level, an important recent finding has been that the lifespans of cells in the human body are determined by telomeres.

Each time a cell divides, the telomere becomes shorter; the senescence and death of the cell is triggered when the telomere is reduced to a certain critical length.

senescence?

In biology, senescence is the state or process of ageing.

Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture.

Telomeres, ageing

Is the number of replication unlimited?

Single-celled organisms must maintain their telomeres and they pass them on intact to their daughter cells.

Multi-cellular organisms in somatic tissue have to divide for a limited number of times before the tissue is fully developed. Therefore, their telomeres do not have to be maintained at the same length.

Somatic cells that are grown in culture will stop growing after 50-100 generations (senescence).

Thus, the organisms appear to have a "mitotic clock"

This clock tells them when they are old, and so to stop dividing.

In the body this mechanism would guard against proliferation of very old cells, that might have acquired harmful mutations over the course of their lifetime.

The “clock” mechanism is thought to guard against cancer. As the probability of the genetic mutation at each stage is low (about 1 in a million) it could take at least 80 cell divisions for the original cell to become a tumour,

Happily it is longer than the normal lifespan of a somatic cell.

The diagram shows the multi-stage progression of a typical

tumour

Telomere length and telomerase enzyme activity were measured in a number of cell types

 Cell typeTelomere

length Telomerase

activity

Cultured "mortal"

shortens 0

Cultured "immortal"

stable +

Normal testis stable +

Normal somatic

shortens 0

Tumours (breast,

ovary, colon, prostate etc)

stable or shortens

+ in most cases

Telomerase is more likely to

be active in late-stage tumours, and may indicate

a poor prognosis.

In most somatic tissues, telomerase is expressed at verylow levels or not at all -- as cells divide, telomeres shorten

Telomerase and Senescence

Short telomeres signal cells to senesce (stop dividing)

Telomerase and Cancer

The presence of telomerase in cancer cells allows them tomaintain telomere length while they proliferate

Sensed as DNAdamage by the p53 checkpoint

Stops Cell Division

Short telomeres cause growth arrest via a checkpoint

If the cells of a particular tumor still have p53, this works

Sensed as DNAdamage by the p53 checkpoint

Cell division continueswithout telomeresleading to chromosomalrearrangements

Tumor cells often lose p53

This genomic instability can promote tumorigenesis

“Menjadi tua itu pasti, menjadi dewasa itu pilihan”

ReferencesCampbell, N.A., Reece, J.B. and Mitchell, L.G. 2004.

Biologi. Jilid ke-1. Ed ke-5. Penerbit Erlangga, Jakarta.

Hyde, D. 2009. Introduction to genetic principles. McGraw-Hill, Boston.

Starr, C., Taggart, R., Evers, C. and Starr, L. 2009. Cell biology and genetics. Biology: The unity and diversity of life. 12th ed. Brooks/Cole, Belmont.