Name: dna replication proved by meselson-stahl
Uploaded: 2020-11-23T11:47:34.000Z
Duration: 2 min 5 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Life Science

Inheritance and Variation of Traits

3.1.9-12.P

DNA as a Genetic Template

The Watson-Crick model, proposed in 1953, elucidated two structural features of the DNA molecule that provide a basis for heredity: its double-stranded nature and the four nucleotide bases.&#160;
This model proposed that DNA is made up of two strands of nucleotides that twist around each other to form a right-handed helix.
These strands are anti-parallel in nature - meaning the 3&#8217; end of one strand faces the 5&#8217; end of the other strand.
Each strand of DNA contains a sequence of nucleotides that is exactly complementary to its partner strand - enabling each strand to act as a template for its partner.
Thus, a cell can replicate its DNA before cell division by separating two strands of DNA and producing two new strands of DNA that are exactly complementary to each other.&#160;
Additionally, the Watson-Crick model posited that base-pairing takes place between a purine - either adenine or guanine - and a pyrimidine - either cytosine or thymine.&#160;
Adenine, guanine, cytosine, and thymine comprise the four-letter nucleotide &#8216;alphabet.&#8217;&#160;
We now know that when these nucleotide letters are strung together into a three-letter codon during translation, they can code for one of the 20 amino acids - much li

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 Watson-Crick model of double-helical DNA structure, proposed in 1952, drew heavily upon the X-ray crystallography work of researchers Rosalind Franklin and Maurice Wilkins. Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine for their work in 1962. Franklin was, controversially, excluded from the prize for reasons that are still debated.
The Watson-Crick model of DNA, very simply, proposed that DNA is made up of two strands of nucleotides that twist around each other to form a right-handed helix and that nucleotide pairing takes place between a purine and a pyrimidine.
The two DNA strands are antiparallel, meaning that the 3&#8217; end of one strand faces the 5&#8217; end of the other. This allows each strand to act as a template for its partner during DNA replication, producing two new strands of DNA that are exactly complementary to each other. However, whether or not DNA replication occurred in this fashion was not clear.
The Meselson and Stahl Experiment
Meselson and Stahl grew E. coli for several generations in a medium containing a &#8220;heavy&#8221; isotope of Nitrogen, 15N. Over time, the heavy Nitrogen was incorporated into the nitrogenous nucleotide bases and, thus, into the DNA. After this, the E. coli was placed into a medium containing a different isotope of Nitrogen, 14N, and grown for several more generations. After each generation, a DNA sample was isolated from some of the cells, loaded into a gradient, and centrifuged at high speeds. In a gradient, the DNA will separate according to its buoyant density (i.e. the density within the gradient where the DNA will float).
In the 15N medium, a single band of high density was observed in the lower portion of the centrifuge tube. Immediately after the bacteria were transferred to the &#8220;lighter&#8221; media, this single band shifted upwards in the column, indicating a lower density. However, subsequent generations resulted in two bands: one corresponding to the 14N density and another in an intermediate location. This result could only be explained by a semi-conservative mode of replication, thus validating the Watson-Crick model.

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 ...

Education

JoVE Core

Molecular Biology

Unit 1

DNA and Chromosome Structure

KEY TERMS AND CONCEPTS

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 nucleotides twisted into a right-handed double helix. It also explains that nucleotide pairing happens between a purine and a pyrimidine. The two strands are antiparallel, which means the 3' end of one strand faces the 5' end of the other. Because of this structure, each strand can serve as a template for the other during replication.

Meselson and Stahl tested whether DNA copy making was semi-conservative. They grew E. coli for several generations in a medium containing heavy nitrogen, 15N. The heavy nitrogen became part of the nitrogenous bases and then part of the DNA. The bacteria were then moved to a medium containing lighter 14N and grown for several more generations.

After each generation, the scientists isolated DNA, loaded it into a density gradient, and spun it at high speed. In a gradient, DNA separates by buoyant density, which is the point where it floats. DNA from the 15N culture first formed one heavy band low in the tube. After transfer to 14N, that band shifted upward, showing lower density. Later generations produced two bands, one at 14N density and one in an intermediate position. This pattern supported semi-conservative replication and confirmed the Watson-Crick model.

DNA Replication Proved by Meselson-Stahl

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 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

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...

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

Explore how the Meselson-Stahl experiment used heavy and light nitrogen to show semi-conservative DNA replication and support the Watson-Crick model.

DNA Replication Proved by Meselson-Stahl | 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.

Structure and Function

3.1.9-12.A

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

Minnesota Academic Standards in Science

1.1.1 9L.1.1.1.1

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New Jersey Student Learning Standards for Science

Heredity: Inheritance and Variation of Traits

Texas Essential Knowledge and Skills

Biology

BIO.7

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South Carolina College and Career Ready Science Standards

From Molecules to Organisms: Structures and Processes

B-LS1-1.

B-LS3-1.

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3.1.9-12.D

DNA Packaging

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3.1.9-12.Q

DNA Replication Proved by Meselson-Stahl

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