11. dna replication and telomeres

Which 4 properties make DNA the perfect molecule for carrying genetic material?

1. storage of information: 4^(1000) different arrangements → humans= 4^(3 billion)

2. replication

3. expression of information: transcription and translation

4. variation (by mutation)

in which stage of the cell cycle is DNA synthesized

interphase S phase

Why is it biologically important for DNA to be replicated during every cell cycle?

to maintain genetic continuity and proper cell function (essential property)

How precise is DNA replication? What is the error rate?

very precise → 1 error per 1 million bases

semiconservative with many enzymes and proteins involved

What are the 4 properties of the DNA molecule that allow the replication process to
occur?

1. complementarity

2. DNA strands are antiparallel

3. formation of hydrogen bonds between A-T and G-C

4. DNA property to denature

Name the three theoretical models of DNA replication. Are they all found in nature

1. conservative

2. semiconservative

3. dispersive

Explain the Meselson-Stahl experiments and how they ruled out the conservative and
dispersive replication models in bacteria

experiment used heavy nitrogen (¹⁵N) to label bacterial DNA, then switched to light nitrogen (¹⁴N), and used density-gradient centrifugation to separate DNA by weight, revealing how it replicated. After one generation, they found a single hybrid band (¹⁵N/¹⁴N), ruling out the conservative model (which predicted one heavy and one light band) and supporting the semi-conservative model (one old, one new strand). After two generations, they observed a hybrid band and a lighter ¹⁴N/¹⁴N band, which ruled out the dispersive model (predicted a progressively lighter, single band) and confirmed semiconservative replication

True or false? In eukaryotes, the exchange of genetic material between sister chromatids
produces allelic variation, as well as recombination

false, they are genetically identical and exchange doesn’t produce new cominations

Regarding DNA replication in prokaryotes, explain the following terms:
- Origin of replication
- Replication fork
- Circular chromosome

1. initiation site; where unwinding begins

2. y shaped helicase where strands separate, moving bidirectionally

3. prokaryotes have one circular continuous closed loop, starting and ending at same origin point

What are the two requirements for DNA replication to initiate in prokaryotes?

1. presence of all 4 deoxyribonucleoside triphosphates ( ATP, CTP, GTP, TTP)

2. template DNA (providing sequence information)

Briefly describe the process of adding new nucleotides to DNA: 1) how many nucleotides
at a time? 2) direction of replication.

one nucleotide at a time

new always moves in the 5’ to 3’ direction

Which enzymes conduct DNA replication? How many such enzymes are in prokaryotes
and eukaryotes

DNA polymerase, I-V

Name the five steps of DNA replication in their correct order and know the basics for each
step.

1. unwinding and stabilizing of DNA double helix

2. initiation of DNA synthesis, synthesis of RNA primers

3. continuous DNA synthesis- leading strand

4. discontinuous DNA synthesis- lagging strand

5. proofreading and correction of errors

What is RNA priming

universal feature of initiation of DNA replication both in prokaryotes and eukaryotes

DNA polymerase III requires a primer with a free 3’ OH end → RNA serves as a primer to initiate DNA synthesis

What are Okazaki fragments and why are they transient

lagging strand forms ozaki fragments composed of RNA primer and 1000 bases of DNA

they are transient because they are quickly joined into a continuous strand by DNA ligases

DNA synthesis is both CONTINUOUS and DISCONTINUOUS. Explain what this
means.

continuous on one strand and discontinuous on another, one serves as the new leading strand and the other as the lagging strand

While DNA polymerases synthesize DNA in 5’-3’ direction, all DNA polymerases also
work in 3’-5’ direction. What is this process?

proofreading and repair: 3’-5’ exonuclease activity → detects mismatches, cut out wrong nucleotides, then proceeds in 5’-3’ direction

What are the similarities and main differences in DNA replication between prokaryotes
and eukaryotes

similarities: DNA is unwound at origins of replication, formation of replication forks, bidirectional DNA synthesis, leading and lagging strand replicated simultaneously

differences: more DNA in eukaryotic cells, DNA is linear, special features of chromosome ends, DNA is in complex with multiple proteins

Why is the replication process different in telomeres? What causes this difference

they are the end part (in eukaryotes/linear), don’t contain genes, they contain special repeats

function: cap eukaryotic chromosomes for integrity and stability, protect chromosomes from end-to-end fusion, prevent gradual loss associated with RNA-primed DNA replication, protect chromosomes from exonuclease and degradation

What is TELOMERASE and how it works?

enzyme that builds telomeres: adds copies of telomeric repeat sequences to the end of chromosomes

1. telomerase adds telomeric repeat sequence to the end of lagging template

2. repeats fold back and form a hairpin loop with unorthodox G=G bonds and provide free 3’OH

3. polymerase can now fill the gap

4. hairpin in cleaved and removed

5. telomerase can synthesize new telomeric sequences on broken chromosome ends and existing telomeres

True or false? Telomerases are ribonucleoproteins that synthesize DNA from a small
stretch of RNA

true

What is the difference in the activity levels of telomerase between adult somatic cells and
undifferentiated embryonic stem cells

most adult somatic cells have have low or no telomerase activity while undifferentiated cells have high telomerase activity

What is the difference in telomerase activity between normal somatic cells and cancer
cells? What attribute does this difference give to cancer cells?

normal somatic cells have little too telomerase activity leading to it shortening with each division

cancer cells reactivate telomerase which prevents the shortening and leading to unlimited replicative potential

What is Hayflick limit

replicative senescence: human fetal cells have finite replicative potential of 50-60 doublings (hay flick 1965) which prevents from genome instability and cancer

What is common between the telomerase and reverse transcriptase

are fundamentally similar because telomerase is a type of reverse transcriptase, an RNA-dependent DNA polymerase that synthesizes DNA from an RNA template, but telomerase uniquely uses its internal RNA component (TERC) to add repetitive DNA sequences (telomeres) to the ends of chromosomes

Why are chromosomes shortening with every cell cycle in adult somatic cells but not in
stem cells or cancer cells

because telomerase is not active in adult somatic cells but is in cancer and stem cells

What is the connection between DNA replication, telomeres, and ageing

DNA replication causes telomeres (protective caps on chromosome ends) to shorten with each cell division, acting as a cellular clock; when they get too short, cells stop dividing (senescence) or die, contributing to tissue aging, disease, and reduced function, with telomere length serving as a biomarker for biological age

HeLa cells are probably the most famous cells in the world. Why

the first human cells continuously grown in culture for 70 years- immortalized

high telomerase activity

importance: workhorse for multiple outstanding scientific experiments and discoveries

What is ‘HeLa” standing for

Henrietta Lacks

HeLa (immortal) cells derived from cervucal cells of a young cancer patient in 1951

How is the “telomere problem” solved during bacterial (prokaryote) DNA replication

circular DNA, no telomere, no problem

How is the “telomere problem” solved during mitochondrial DNA replication?

circular DNA, no telomere, no problem