DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group and one of four types of nitrogen bases. To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating.
The four types of nitrogen bases found in nucleotides are: adenine A , thymine T , guanine G and cytosine C. The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. The complete DNA instruction book, or genome, for a human contains about 3 billion bases and about 20, genes on 23 pairs of chromosomes. DNA contains the instructions needed for an organism to develop, survive and reproduce.
To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies. Each DNA sequence that contains instructions to make a protein is known as a gene.
The size of a gene may vary greatly, ranging from about 1, bases to 1 million bases in humans. Genes only make up about 1 percent of the DNA sequence. DNA sequences outside this 1 percent are involved in regulating when, how and how much of a protein is made.
DNA's instructions are used to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA. Individual DNA molecules are extremely long, consisting of millions of base pairs matched nucleotides each.
How do cells store such large and potentially unwieldy molecules? Chromatin consists of all the DNA in the nucleus, as well as its associated proteins. There are three basic layers of chromatin scaffolding that results in a condensed DNA molecule.
The double helix shaped DNA molecule that makes up each chromosome is first coiled around clusters of histone proteins. A unit of around DNA base pairs wound around eight histone proteins makes up the smallest unit of DNA-packing structure, a nucleosome.
The nucleosomes and the linker DNA that connects them, like beads on a string, loop to form more tightly-packed nm solenoid fibers. Then, the nm fibers are coiled further and folded into loops that are tightly packed together. This last stage of scaffolding produces the chromosomes observable in metaphase of mitosis or meiosis. The process that creates the 30nm fibers is called supercoiling. Supercoiling uses the application of tension to twist a DNA molecule, so it wraps around itself, creating loops.
Even with a microscope, individual chromosomes are clearly visible in the nucleus only during the process of cell division either mitosis or meiosis.
This is because the chromatin that makes up the chromosomes is hundreds, or even thousands, of times less condensed during interphase than it is when the cell is actively dividing. It is during mitosis or meiosis that the X-shaped structures we usually picture when thinking of chromosomes can be observed.
Each of these X-shaped chromosomes consists of two identical sister chromatids. Basically, you can think of a chromatid as one copy of a chromosome. The challenges associated with energy generation limit the size of prokaryotes. As these cells grow larger in volume, their energy needs increase proportionally.
However, as they increase in size, their surface area — and thus their ability to both take in nutrients and transport electrons — does not increase to the same degree as their volume. As a result, prokaryotic cells tend to be small so that they can effectively manage the balancing act between energy supply and demand Figure 6.
Figure 6: The relationship between the radius, surface area, and volume of a cell Note that as the radius of a cell increases from 1x to 3x left , the surface area increases from 1x to 9x, and the volume increases from 1x to 27x. This page appears in the following eBook. Aa Aa Aa. Eukaryotic Cells. Figure 1: A mitochondrion. Figure 2: A chloroplast. What Defines an Organelle?
Why Is the Nucleus So Important? Why Are Mitochondria and Chloroplasts Special? Figure 4: The origin of mitochondria and chloroplasts. Mitochondria and chloroplasts likely evolved from engulfed bacteria that once lived as independent organisms.
Figure 5: Typical prokaryotic left and eukaryotic right cells. In prokaryotes, the DNA chromosome is in contact with the cellular cytoplasm and is not in a housed membrane-bound nucleus. Figure 6: The relationship between the radius, surface area, and volume of a cell. Note that as the radius of a cell increases from 1x to 3x left , the surface area increases from 1x to 9x, and the volume increases from 1x to 27x.
Organelles serve specific functions within eukaryotes, such as energy production, photosynthesis, and membrane construction. Most are membrane-bound structures that are the sites of specific types of biochemical reactions. The nucleus is particularly important among eukaryotic organelles because it is the location of a cell's DNA. Two other critical organelles are mitochondria and chloroplasts, which play important roles in energy conversion and are thought to have their evolutionary origins as simple single-celled organisms.
Cell Biology for Seminars, Unit 1. Topic rooms within Cell Biology Close. No topic rooms are there. Or Browse Visually.
Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Eukaryotic DNA is packed into bundles of chromosomes, each consisting of a linear DNA molecule coiled around basic alkaline proteins called histones, which wind the DNA into a more compact form.
Prokaryotic DNA is found in circular, non-chromosomal form. In addition, prokaryotes have plasmids, which are smaller pieces of circular DNA that can replicate separately from prokaryotic genomic DNA. Because of the linear nature of eukaryotic DNA, repeating non-coding DNA sequences called telomeres are present on either end of the chromosomes as protection from deterioration. Mitosis, a process of nuclear division wherein replicated chromosomes are divided and separated using elements of the cytoskeleton, is universally present in eukaryotes.
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