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Replication in Eukaryotes

Replication in Eukaryotes

Position-effect Variegation

Position-effect Variegation

In 1928, a German botanist Emil Heitz observed the moss nuclei with a DNA binding dye. He observed that while some chromatin regions decondense and spread out in the interphase nucleus, others do not. He termed them euchromatin and heterochromatin, respectively. He proposed that the heterochromatin regions reflect a functionally inactive state of the genome. It was later confirmed that heterochromatin is transcriptionally repressed, and euchromatin is transcriptionally active chromatin.

Lampbrush Chromosomes

Lampbrush Chromosomes

In 1882, Flemming observed lampbrush chromosomes (LBC) in salamander eggs. Later in 1892, Rückert observed LBCs in shark egg cells and coined the term "lampbrush chromosomes" because they looked like brushes used to clean kerosene lamps.
LBCs are made up of two pairs of conjugating homologous chromatids. Each chromatid consists of alternatively positioned regions of condensed-inactive chromatin and loosely placed-active side loops, which can be contracted and extended. The loops resemble the...

Polytene Chromosomes

Polytene Chromosomes

Polytene chromosomes are giant interphase chromosomes with several DNA strands placed side by side. They were discovered in the year 1881 by Balbiani in salivary glands, intestine, muscles, malpighian tubules, and hypoderm of larvae Chironomus plumosus. Hence, these are also called "Salivary gland chromosomes." These are found in insects of the order Diptera and Collembola; in certain organs of mammals; and synergids, antipodes of flowering plants. Polytene chromosomes are also regularly...

Telomeres and Telomerase

Telomeres and Telomerase

In eukaryotic DNA replication, a single-stranded DNA fragment remains at the end of a chromosome after the removal of the final primer. This section of DNA cannot be replicated in the same manner as the rest of the strand because there is no 3’ end to which the newly synthesized DNA can attach. This non-replicated fragment results in gradual loss of the chromosomal DNA during each cell duplication. Additionally, it can induce a DNA damage response by enzymes that recognize single-stranded DNA.

Replication in Eukaryotes

Replication in Eukaryotes

In eukaryotic cells, DNA replication is highly conserved and tightly regulated. Multiple linear chromosomes must be duplicated with high fidelity before cell division, so there are many proteins that fulfill specialized roles in the replication process. Replication occurs in three phases: initiation, elongation, and termination, and ends with two complete sets of chromosomes in the nucleus.
Many Proteins Orchestrate Replication at the Origin
Eukaryotic replication follows many of the same...

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Related Experiment Video

Updated: Jul 27, 2026

Chromatin Immunoprecipitation (ChIP) using Drosophila tissue

Chromatin Immunoprecipitation (ChIP) using Drosophila tissue

Published on: March 23, 2012

Chromosome ends in Drosophila without telomeric DNA sequences.

H Biessmann¹, S B Carter, J M Mason

¹Developmental Biology Center, University of California, Irvine 92717.

Proceedings of the National Academy of Sciences of the United States of America

|March 1, 1990

Summary

Drosophila X chromosomes with receding tips (RT) deletions lose DNA from their ends each generation. This DNA loss rate suggests RNA primer removal during DNA replication, not telomere shortening.

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Chromatin Immunoprecipitation (ChIP) using Drosophila tissue

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Immunofluorescent Staining for Visualization of Heterochromatin Associated Proteins in Drosophila Salivary Glands

Immunofluorescent Staining for Visualization of Heterochromatin Associated Proteins in Drosophila Salivary Glands

Published on: August 21, 2021

Area of Science:

Molecular Biology
Genetics
Drosophila melanogaster research

Background:

Chromosome ends are typically protected by telomeres to prevent degradation and fusion.
The mechanisms maintaining chromosome stability at broken ends in the absence of telomeres are not fully understood.

Purpose of the Study:

To investigate the nature of chromosome end deletions in Drosophila.
To determine the rate and mechanism of DNA loss from broken chromosome ends.

Main Methods:

Isolation and molecular cloning of terminal deletions (Df(1)RT) from Drosophila X chromosomes.
DNA sequencing of cloned terminal fragments to analyze end structures and identify sequence loss.
Quantification of DNA sequence loss over generations.

Main Results:

Recovered terminal deletions lacking telomeric sequences, yet chromatids did not fuse.
Identified heterogeneous DNA fragment lengths at the deletion breakpoints.
Demonstrated a consistent DNA sequence loss of 70-75 base pairs per generation from distal ends.

Conclusions:

Broken chromosome ends in Drosophila can be stable without telomeric sequences.
The observed DNA loss rate is consistent with the removal of an octanucleotide RNA primer during germline DNA replication.