Name: origins of replication in chromosomes
Uploaded: 2020-11-23T11:47:35.000Z
Duration: 2 min 31 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Life Science

Inheritance and Variation of Traits

3.1.9-12.P

Chromosome Replication

During the S phase of the cell cycle, the process of chromosome replication begins simultaneously in each of a cell&#8217;s chromosomes at numerous origins of replication.&#160;
An origin of replication is a nucleotide sequence that interacts with specialized initiator proteins, called the origin recognition complex, at sites where DNA replication begins.&#160;
Defining these origins of replication is difficult. Given the large size of the human genome, for example, an estimated tens of thousands of origins would be necessary to replicate the entire genome in a timely fashion.&#160;
In humans, no clear consensus on specific origin sequences has been reached. Instead, it is likely that origins are defined by a combination of nucleotide sequences and chromatin structure.&#160;
The length of DNA that spans an origin of replication and its two respective termini of replication, where adjacent replication forks eventually fuse, is called a replicon.&#160;
Replication does not occur simultaneously in all of the replicons, however. Different cell types have their own specific timing for initiation of replication, typically in clusters of origins.&#160;
DNA replication progresses in this fashion throughout the chromosome unt

chromosome replication

Before a cell can divide, it must accurately replicate all of its chromosomes, including the DNA and its associated histone and non-histone proteins.&#160; This process begins at numerous origins of replication during the S phase of the cell cycle in each of a cell&#8217;s chromosomes simultaneously. Certain nucleotides can act as origins of replication, but these sequences are not well defined - especially in complex, multi-cellular, eukaryotic species. The length of DNA that spans an origin of replication and its two respective termini of replication, where adjacent replication forks will eventually fuse, is called a replicon. DNA replication progresses in this fashion, cluster of origins at a time, until it reaches the telomeres, which have their own specialized replication process. At this point, when the cell is in M phase and ready to divide, the cell&#8217;s chromosomal mass has effectively doubled.&#160;
While DNA replication progresses, new histone proteins are synthesized and new histone core particles are formed. These proteins are just as important to chromosome function as DNA since histones are critical to the physical structure of the chromosome.&#160;
Origins of Replication
DNA replication begins at certain nucleotide sequences called origins of replication. These sites interact with specialized initiator proteins that begin the process of DNA separation and replication. Defining and quantifying these sequences has proven difficult, especially in complex, multi-cellular, eukaryotic species. For example, the large size of the human genome would necessitate tens of thousands of origins of replication throughout all of the chromosomes. However, counting these sites is difficult because there is no clear consensus on specific origin sequences. It is likely that origins of replication are defined by a combination of nucleotide sequences, various proteins, and chromatin structure.&#160;
Replication of histones
Histones are proteins that are responsible for packaging DNA into chromatin and then into chromosomes. Thus, histones are critical to the physical structure and function of the eukaryotic chromosome. During chromosome replication, new histones must also be synthesized in order to package the new DNA into nucleosomes. As the replication fork moves forward, the new and old histones re-assemble randomly on the daughter cells.&#160;

Before a cell can divide, it must accurately replicate all of its chromosomes, including the DNA and its associated histone and non-histone ...

Education

JoVE Core

Molecular Biology

Unit 1

DNA and Chromosome Structure

KEY TERMS AND CONCEPTS

origins of replication in chromosomes

Chromosome replication starts at many origins of replication during the S phase of the cell cycle. Each chromosome copies its DNA at the same time. The process also includes the histone and non-histone proteins attached to the DNA. By the time the cell reaches M phase and is ready to divide, its chromosomal mass has effectively doubled.

Replication begins at nucleotide sequences called origins of replication. These sites work with specialized initiator proteins that start DNA separation and copying. In eukaryotic cells, especially in complex multicellular species, the exact origin sequences are hard to define. The human genome would need tens of thousands of these sites, but they are difficult to count because there is no clear consensus sequence.

The DNA region from one origin of replication to the two termini of replication is called a replicon. Replication moves through the chromosome a cluster of origins at a time. Adjacent replication forks eventually fuse. The process continues until it reaches the telomeres, which use a specialized replication process of their own.

Histone replication is also part of chromosome replication. Histones are proteins that package DNA into chromatin and then into chromosomes. New histone proteins are made while DNA is being copied, and new histone core particles are formed. As the replication fork moves forward, new and old histones reassemble randomly in the daughter cells.

Origins of Replication in Chromosomes

dna packaging

DNA Packaging in Chromosomes

DNA packaging in chromosomes helps cells fit long DNA molecules into a compact space. The DNA must be organized so it can be stored and used by the cell. This packaging also supports chromosome structure. In eukaryotic cells, DNA wraps around histone proteins. Histones help form chromatin, which is the DNA-protein material that makes up chromosomes. This arrangement packs DNA more tightly while still keeping it organized. The way DNA is packaged affects how a cell manages its genetic material.

dna as a genetic template

DNA Replication Proved by Meselson-Stahl

DNA replication was confirmed by the Meselson and Stahl experiment, which tested how DNA copies itself. The experiment also connects to the Watson-Crick model of DNA, first proposed in 1952. That model was built from X-ray crystallography work by Rosalind Franklin and Maurice Wilkins, and Watson, Crick, and Wilkins later shared the 1962 Nobel Prize in Physiology or Medicine. Franklin was not included, and that decision remains debated. The Watson-Crick model describes DNA as two strands of...

organization of genes

Genes on Chromosomes: DNA Organization

Genes are organized within DNA and packed into chromosomes. This structure helps keep genetic information arranged and readable inside the cell. A gene is a segment of DNA that carries instructions for a specific trait or product. In many genes, the coding sequence is broken into exons, which are the parts that remain in the final message, and introns, which are removed before the message is used. The order of these parts is part of how genes are organized. Genes also include regulatory...

chromosome structure

Chromosome Ends, Centers, and Replication

A functional eukaryotic chromosome needs three key parts: a centromere, telomeres, and many origins of replication. These regions help the chromosome stay organized, copy its DNA, and move correctly during cell division. The centromere is a DNA sequence that holds sister chromatids together. After the chromosome is replicated, this region is where kinetochores are built. Kinetochores are protein complexes that bind spindle microtubules, which help move chromosomes within the cell during...

Chromosome replication starts at many origins of replication during the S phase of the cell cycle. Each chromosome copies its DNA at the same time. The process also includes the histone and non-histone proteins attached to the DNA. By the time the cell reaches M phase and is ready to divide, its chromosomal mass has effectively doubled. Replication begins at nucleotide sequences called origins of replication. These sites work with specialized initiator proteins that start DNA separation and...

the nucleosome

How DNA Is Packed into Nucleosomes

DNA in a human cell is almost 2 m long, yet it fits inside a nucleus that is only a few microns wide. To make this possible, DNA is packed into several higher levels of compaction. The most compact form is a chromosome, which can be seen under a microscope in a dividing cell. A key step in this packing process is the nucleosome. In a nucleosome, DNA winds twice around a histone core made of 8 histone proteins. This DNA and histone protein complex is the fundamental and functional unit of DNA...

the nucleosome core particle

Histone Tail Modifications in Nucleosomes

Nucleosomes are DNA wrapped around a histone core. The core is an octamer, or eight-protein complex, made of two copies each of H2A, H2B, H3, and H4. This DNA-histone structure helps package DNA while still keeping it organized inside the cell. Nucleosomes have to balance two opposite jobs. They protect delicate DNA from physical damage and help create a higher level of compaction. At the same time, they must let polymerase enzymes reach the DNA for replication and transcription. Nucleosomes...

nucleosome remodeling

How Chromatin Opens for DNA Access

Chromatin must open at the right time so proteins can reach DNA. Nucleosomes, the basic units of chromatin compaction, wrap DNA tightly around a histone core. This packing makes DNA less accessible to proteins such as DNA polymerase and RNA polymerase. Eukaryotic cells use ATP-dependent nucleosome remodeling enzymes to solve this problem. These enzymes bind to histones and the wrapped DNA. They can shift nucleosomes along DNA, a process called nucleosome sliding, or replace part or all of the...

chromatin packaging

DNA Packing Inside the Nucleus

DNA packing inside the nucleus depends on chromatin and histones. Each human somatic cell contains about 6 billion base-pairs of DNA. Because each base-pair is 0.34 nm long, a diploid cell holds about 2 meters of DNA. That long strand must fit inside a nucleus that is only 10 to 20 microns wide. Chromatin is the compact DNA-protein complex formed when the DNA double helix combines with specialized DNA-binding proteins called histones. Chromatin is then packed even further into higher-order...

karyotyping

Chromosome Pairing and Karyotypes

Chromosome pairing and karyotypes help scientists study the number and structure of chromosomes in a cell. A karyotype is a visual display of chromosomes arranged in matching pairs. It can reveal large-scale changes that affect chromosome shape or count. To make a karyotype, chromosomes are first collected when they are most condensed and easiest to see. They are then stained and photographed so the pairs can be organized. The finished image gives a clear view of the full chromosome set in one...

position-effect variegation

Chromatin Spreading and Gene Silencing

Chromatin spreading can silence a gene when it moves near heterochromatin. This effect is called position-effect variegation, or PEV. It happens because the gene’s location changes, not because the gene itself changes. Emil Heitz first noticed that some chromatin in moss nuclei spread out during interphase, while other regions stayed compact. He named the open form euchromatin and the compact form heterochromatin. He suggested that heterochromatin marked a part of the genome that was...

histone modification

Histone Marks and Gene Control

Histone marks help control how tightly DNA is packed and how genes are turned on or off. Histone proteins have a flexible N-terminal tail that extends out from the nucleosome. These tails can be modified after the protein is made. Common modifications include acetylation, methylation, phosphorylation, and ubiquitination. Different combinations of these changes create a histone code. This code influences chromatin folding and tissue-specific gene expression. In other words, the same DNA can...

spreading of chromatin modifications

Histone Marks and Chromatin Boundaries

Histone marks help control how tightly DNA is packed in chromatin. These post-translational modifications, or PTMs, happen on histone proteins in nucleosomes and can increase or decrease access to DNA. Common PTMs include methylation, acetylation, phosphorylation, and ubiquitination of lysine amino acids in the histone H3 tail region. These histone marks carry specific meaning for the cell. For that reason, they are often called the histone code. The protein complex that helps place and read...

lampbrush chromosomes

Lampbrush Chromosomes in Oocyte Meiosis

Lampbrush chromosomes are large chromosomes found in oocytes during meiosis. They are best known from lower vertebrates, invertebrates, and birds. Their unusual shape and size make them a useful model for studying chromosome structure and gene activity. Flemming first observed lampbrush chromosomes in salamander eggs in 1882. Later, in 1892, Rückert saw them in shark egg cells and gave them their name. He chose the term because they looked like the brushes used to clean kerosene lamps. These...

polytene chromosomes

Polytene Chromosomes in Salivary Glands

Polytene chromosomes are giant interphase chromosomes made of many DNA strands aligned side by side. They were first described by Balbiani in 1881. They are often called salivary gland chromosomes because they are regularly seen in salivary gland cells, especially in Drosophila. These chromosomes are found in insects of the order Diptera and Collembola. They also appear in certain organs of mammals and in synergids and antipodes of flowering plants. In larvae of Chironomus plumosus, they were...

histone variants at the centromere

CENP-A and Centromere Identity

CENP-A is a specialized histone H3 variant found in centromeric chromatin. It has about 60% similarity to canonical histone H3. This histone variant helps build the kinetochore, the protein structure that links chromosomes to microtubules during cell division. CENP-A is also thought to act as an epigenetic mark. An epigenetic mark is a signal that helps cells keep centromere identity without changing the DNA sequence. The centromere-targeting process depends on the CENP-A targeting domain, or...

inheritance of chromatin structures

Epigenetic Memory in Chromatin

Epigenetic memory in chromatin helps cells keep their identity without changing the DNA sequence. Epigenetics is the study of inherited changes in cell phenotype that do not alter DNA. This kind of memory helps maintain cell lineage, position-effect variegation, dosage compensation, and chromatin structures such as telomeres and centromeres. Centromeres are a clear example of this process. Their structure and location on chromosomes can be inherited epigenetically. Centromere function is not...

euchromatin

Euchromatin and DNA Packaging

Euchromatin is the less compact form of chromatin. Chromatin can be studied by staining DNA with specific dyes, because dense regions take up more dye and appear darker under the microscope. Lighter-staining areas are less compact, and euchromatin is one of these regions. This open chromatin state is linked to histone changes. Histone H3 in euchromatin is extensively acetylated on lysine 9. In promoter regions, histones can also carry methylated lysine 4 and phosphorylation at position 10.

chromatin position affects gene expression

chromatin-position-affects-gene-expression

Chromatin Folding Shapes Gene Activity

Chromatin folding inside the nucleus shapes gene activity in eukaryotic cells. Chromatin is the large complex of DNA and proteins packed into the nucleus. How it is arranged affects how easily genetic information can be reached. In general, the nucleus periphery is linked with lower transcription, while the cell interior is linked with more active transcription. Gene promoters are often kept physically separate from regulatory DNA elements such as enhancers. To turn on gene expression, these...

heterochromatin

Heterochromatin Staining and Chromatin Packing

Heterochromatin is the tightly packed form of chromatin, and it can be seen by staining DNA with specific dyes. Under the microscope, these dense regions take up more dye than less compact chromatin. Heterochromatin is usually divided into constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin is a highly compact part of chromatin. It is mostly found at centromeres and telomeres. In this form, the amino acid at position 9 on the histone H3 tail is di- or...

key terms and concepts

Learn how chromosome replication begins at origins of replication, forms replicons, and copies histones during S phase before cell division.

Origins of Replication in Chromosomes | JoVE K12

molecular biology

JoVE Core Molecular Biology showcases the molecular processes of the cell through clear and concise animated video lessons. Additionally, in scientist-in-action videos, these concepts' application is demonstrated through real experiments in modern laboratories.

Louisiana Student Standards for Science

High School

Life Sciences

from-molecules-to-organisms-structures-and-processes

From Molecules To Organisms: Structures And Processes

HS-LS1-1

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance And Variation Of Traits

HS-LS3-1

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

Heredity: Inheritance and Variation of Traits

Alabama Course of Study Science

Biology

9-12.B.3.12.b

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

From Molecules to Organisms: Structures and Processes

B-LS1-1.

B-LS3-1.

What is the Cell Cycle?

The cell cycle refers to the sequence of events occurring throughout a typical cell&#8217;s life. In eukaryotic cells, the somatic cell cycle has two ...

Interphase

The cell cycle occurs over approximately 24 hours (in a typical human cell) and in two distinct stages: interphase, which includes three phases of the ...

The Cell Cycle Control System

Positive Regulator Molecules

Inhibition of CDK Activity

Mitosis and Cytokinesis

In eukaryotes, the cell division cycle is divided into distinct, coordinated cellular processes that include cell growth, DNA replication/chromosome ...

Chromosome Duplication

Cohesins

Condensins

The Mitotic Spindle

Centrosome Duplication

Spindle Assembly

Attachment of Sister Chromatids

Forces Acting on Chromosomes

Separation of Sister Chromatids

The Spindle Assembly Checkpoint

Anaphase A and B

Anaphase Promoting Complex

The Contractile Ring

Determining the Plane of Cell Division

The Phragmoplast

Distribution of Cytoplasmic Content

An Introduction to Cell Division

Cell division is the process by which a parent cell divides and gives rise to two or more daughter cells. It is a means of reproduction for single-cell ...

Cell Cycle Analysis

Cell cycle refers to the set of events through which a cell grows, replicates its genome, and ultimately divides into two daughter cells through the ...

Live Cell Imaging of Mitosis

Mitosis is a form of cell division in which a cell&#8217;s genetic material is divided equally between two daughter cells. Mitosis can be broken down into ...

Fertilization

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Gastrulation

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Neurulation

Neurulation is the embryological process which forms the precursors of the central nervous system and occurs after gastrulation has established the three ...

Cell Migration

Cell migration, the process by which cells move from one location to another, is essential for the proper development and viability of organisms ...

an-introduction-to-developmental-genetics

An Introduction to Developmental Genetics

Development is the complex process through which a single-celled embryo transforms into a multicellular organism. Developmental processes are guided by ...

An Introduction to Organogenesis

Organogenesis is the process by which organs arise from one of three germ layers during the later stages of embryonic development. Researchers studying ...

Fate Mapping

Fate mapping is a technique used to understand how embryonic cells divide, differentiate, and migrate during development. In classic fate mapping ...

Students who demonstrate understanding can use a model to illustrate the role of cellular division (mitosis) and differentiation in producing and maintaining complex organisms.  Assessment Boundary: Assessment does not include specific gene control mechanisms or rote memorization of the steps of mitosis.

3.1.9-12.D

DNA Packaging

DNA as a Genetic Template

Two structural features of the DNA molecule provide a basis for the mechanisms of heredity: the four nucleotide bases and its double-stranded nature. The ...

Organization of Genes

Structure of a Gene

A gene is the fundamental unit of heredity. Every individual has two copies of each gene, one inherited from each parent. Although most people contain the ...

Chromosome Structure

A functional eukaryotic chromosome must contain three elements: a centromere, telomeres, and numerous origins of replication.
The centromere is a DNA ...

The Nucleosome

DNA in a human cell is almost 2m long and it is packed inside a tiny nucleus that is only a few microns in diameter. The level of compaction of DNA inside ...

The Nucleosome Core Particle

Nucleosomes are the DNA-histone complex, where the DNA strand is wound around the histone core. The histone core is an octamer containing two copies of ...

Nucleosome Remodeling

Nucleosomes are the basic units of chromatin compaction. Each nucleosome consists of the DNA bound tightly around a histone core, which makes the DNA ...

Chromatin Packaging

Each human somatic cell contains 6 billion base-pairs of DNA. Each base-pair is 0.34 nm long, which means that each diploid cell contains a staggering 2 ...

Karyotyping

Proteins: From Genes to Degradation

The DNA Helix

Deoxyribonucleic acid, or DNA, is the genetic material responsible for passing traits from generation to generation in all organisms and most viruses. DNA ...

The Central Dogma

The central dogma explains the flow of genetic information from DNA nucleotides to the amino acid sequence of proteins.
RNA is the Missing Link Between ...

Mutations

Mutations are changes in the sequence of DNA. These changes can occur spontaneously or they can be induced by exposure to environmental factors. Mutations ...

genome-size-and-the-evolution-of-new-genes

Genome Size and the Evolution of New Genes

Gene Families

Students who demonstrate understanding can ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring. Assessment Boundary: Assessment does not include the phases of meiosis or the biochemical mechanism of specific steps in the process.

Genomic DNA in Prokaryotes

The genome of most prokaryotic organisms consists of double-stranded DNA organized into one circular chromosome in a region of cytoplasm called the ...

Binary Fission

Fission is the division of a single entity into two or more parts, which regenerate into separate entities that resemble the original. Organisms in the ...

Genomic DNA in Eukaryotes

Eukaryotes have large genomes compared to prokaryotes. To fit their genomes into a cell, eukaryotic DNA is packaged extraordinarily tightly inside the ...

What is Meiosis?

Meiosis is the process by which diploid cells divide to produce haploid daughter cells. In humans, each diploid cell contains 46 chromosomes, half from ...

Meiosis I

Meiosis is a carefully orchestrated set of cell divisions, the goal of which&#x2014;in humans&#x2014;is to produce haploid sperm or eggs, each containing ...

Meiosis II

Meiosis II is the second and final stage of meiosis. It relies on the haploid cells produced during meiosis I, each of which contain only 23 ...

Crossing Over

Unlike mitosis, meiosis aims for genetic diversity in its creation of haploid gametes. Dividing germ cells first begin this process in prophase I, where ...

Nondisjunction

During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed ...

Base-pairing and DNA Repair

The DNA Replication Fork

Lagging Strand Synthesis

Students who demonstrate understanding can make and defend a claim based on evidence that inheritable genetic variations may result from (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors. Assessment Boundary: Assessment does not include the phases of meiosis or the biochemical mechanism of specific steps in the process.

3.1.9-12.Q

Origins of Replication in Chromosomes

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