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Histones: Small chromosomal proteins (approx 12-20 kD) possessing an open, unfolded structure and attached to the DNA in cell nuclei by ionic linkages. Classification into the various types (designated histone I, histone Ii, etc.) is based on the relative amounts of arginine and lysine in each.

DNA Packaging

JoVE 10785

Eukaryotes have large genomes compared to prokaryotes. In order to fit their genomes into a cell, eukaryotes must pack their DNA tightly inside the nucleus. To do so, DNA is wound around proteins called histones to form nucleosomes, the main unit of DNA packaging. Nucleosomes then coil into compact fibers known as chromatin.

Most cells in the human body contain about 3 billion base pairs of DNA packaged into 23 pairs of chromosomes. It is hard to imagine exactly how much DNA these numbers represent. So how much packing has to happen to fit the genome into a cell? We can gain some insight by expressing the genome in terms of length. If we were to arrange the DNA of a single human cell, like a skin cell, into a straight line, it would be two meters long–over 6.5 feet. The human body contains around 50 trillion human cells. This means that each person has a total of about 100 trillion meters of DNA. In other words, each person has enough DNA to stretch from the Earth to the Sun 300 times! And humans do not have particularly large genomes–those of many fish, amphibians, and flowering plants are much larger. For example, the genome of the flowering plant Paris japonica is 25 times larger than the human diploid genome. These figures emphasize the astonishing task that eukaryotes must accomplish to pack their DNA inside cells.

 Core: Biology

Genomic DNA in Eukaryotes

JoVE 10760

Eukaryotes have large genomes compared to prokaryotes. To fit their genomes into a cell, eukaryotic DNA is packaged extraordinarily tightly inside the nucleus. To achieve this, DNA is tightly wound around proteins called histones, which are packaged into nucleosomes that are joined by linker DNA and coil into chromatin fibers. Additional fibrous proteins further compact the chromatin, which is recognizable as chromosomes during certain phases of cell division. Most cells in the human body contain about 6 billion base pairs of DNA packaged into 23 pairs of chromosomes. It is hard to imagine exactly how much DNA these numbers represent, and therefore it is difficult to grasp how densely packed DNA must be to fit into a cell. We can gain some insight by expressing the genome in terms of length. If we were to arrange the DNA of a single diploid cell into a straight line, it would be about two meters long! Note that humans do not have unusually large genomes. Many fish, amphibians, and flowering plants have much larger genomes than humans. For example, the haploid genome of the Japanese flowering plant Paris japonica contains about 50 times more DNA than the human haploid genome. These figures emphasize the astonishing work that histones and other chromatin remodeling proteins must do to package DNA.

 Core: Biology

DNA Isolation and Restriction Enzyme Analysis- Concept

JoVE 10628

The revelation of DNA as the hereditary molecule in all organisms has led to enormous scientific and medical breakthroughs and significantly enhanced our understanding of ourselves and other organisms. DNA isolation and profiling have been the fundamental first steps for many of the advancements in the past century; from identification of gene function, to revolutions of agriculture and…

 Lab Bio

Chromatin Immunoprecipitation

JoVE 5551

Histones are proteins that help organize DNA in eukaryotic nuclei by serving as “scaffolds” around which DNA can be wrapped, forming a complex called “chromatin”. These proteins can be modified through the addition of chemical groups, and these changes affect gene expression. Researchers use a technique called chromatin immunoprecipitation (ChIP) to …

 Genetics

Types of RNA

JoVE 10800

Three main types of RNA are involved in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). These RNAs perform diverse functions and can be broadly classified as protein-coding or non-coding RNA. Non-coding RNAs play important roles in the regulation of gene expression in response to developmental and environmental changes. Non-coding RNAs in prokaryotes can be manipulated to develop more effective antibacterial drugs for human or animal use. The central dogma of molecular biology states that DNA contains the information that encodes proteins and RNA uses this information to direct protein synthesis. Different types of RNA are involved in protein synthesis. Based on whether or not they encode proteins, RNA is broadly classified as protein-coding or non-coding RNA. Messenger RNA (mRNA) is the protein-coding RNA. It consists of codons—sequences of three nucleotides that encode a specific amino acid. Transfer RNA (tRNA) and ribosomal RNA (rRNA) are non-coding RNA. tRNA acts as an adaptor molecule that reads the mRNA sequence and places amino acids in the correct order in the growing polypeptide chain. rRNA and other proteins make up the ribosome—the seat of protein synthesis in the cell. During translation, ribosomes move along an mRNA strand where they stabilize the binding of tRNA molecules and catalyze the for

 Core: Biology

The Nucleus

JoVE 10691

The nucleus is a membrane-bound organelle that contains a eukaryotic organism’s genetic instructions in the form of chromosomal DNA. This is distinct from the DNA in mitochondria or chloroplasts that carry out functions specific to those organelles. While some cells—such as red blood cells—do not have a nucleus, and others—such as skeletal muscle cells—have multiple nuclei, most eukaryotic cells have a single nucleus. The DNA in the nucleus is wrapped around proteins such as histones, creating a DNA-protein complex called chromatin. When cells are not dividing—that is, when they are in the interphase part of their cell cycle—the chromatin is organized diffusely. This allows easy access to the DNA during the transcription process when messenger RNA (mRNA) is synthesized based on the DNA code. When a eukaryotic cell is about to divide, the chromatin condenses tightly into distinct, linear chromosomes. Humans have 46 chromosomes in total. Chromatin is particularly concentrated in a region of the nucleus called the nucleolus. The nucleolus is important for the production of ribosomes, which translate mRNA into protein. In the nucleolus, ribosomal RNA is synthesized and combined with proteins to create ribosomal subunits, which later form functioning ribosomes in the cytoplasm of the cell. The interior of t

 Core: Biology

An Overview of Epigenetics

JoVE 5549

Since the early days of genetics research, scientists have noted certain heritable phenotypic differences that are not due to differences in the nucleotide sequence of DNA. Current evidence suggests that these “epigenetic” phenomena might be controlled by a number of mechanisms, including the modification of DNA cytosine bases with methyl groups, the addition…

 Genetics

Cell Division- Concept

JoVE 10571

Cell division is fundamental to all living organisms and required for growth and development. As an essential means of reproduction for all living things, cell division allows organisms to transfer their genetic material to their offspring. For a unicellular organism, cellular division generates a completely new organism. For multicellular organisms, cellular division produces new cells for…

 Lab Bio
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