Chromosome how many molecules of dna

In DNA the base thymine always bonds to adenine, while cytosine always bonds to guanine because of their complementary chemical structure and "fit". As a result of this complementary structure, if the base sequence of one strand is known, then the structure of the other strand can be deduced.

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Create your free OpenLearn profile. In contrast to eukaryotes, the DNA in prokaryotic cells is generally present in a single circular chromosome that is located in the cytoplasm. Recall that prokaryotic cells do not possess a nucleus.

Prokaryotic chromosomes are less condensed than their eukaryotic counterparts and don't have easily identified features when viewed under a light microscope. Figure 2: A the appearance of DNA during interphase versus mitosis. During interphase, the cell's DNA is not condensed and is loosely distributed. A stain for heterochromatin which indicates the position of chromosomes shows this broad distribution of chromatin in a mouse cell upper left.

The same stain also shows the organized, aligned structure of the chromosomes during mitosis. HP1 and the dynamics of heterochromatin maintenance. Nature Reviews Molecular Cell Biology 5, All rights reserved.

Figure Detail. Figure 3 Eukaryotic chromosomes consist of repeated units of chromatin called nucleosomes , which were discovered by chemically digesting cellular nuclei and stripping away as much of the outer protein packaging from the DNA as possible. The chromatin that resisted digestion had the appearance of "beads on a string" in electron micrographs — with the "beads" being nucleosomes positioned at intervals along the length of the DNA molecule Figure 3.

Nucleosomes are made up of double-stranded DNA that has complexed with small proteins called histones. The core particle of each nucleosome consists of eight histone molecules, two each of four different histone types: H2A, H2B, H3, and H4.

The structure of histones has been strongly conserved across evolution, suggesting that their DNA packaging function is crucially important to all eukaryotic cells Figure 4. Histones carry positive charges and bind negatively charged DNA in a specific conformation. In particular, a segment of the DNA double helix wraps around each histone core particle a little less than twice. The exact length of the DNA segment associated with each histone core varies from species to species, but most such segments are approximately base pairs in length.

Furthermore, each histone molecule within the core particle has one end that sticks out from the particle. These ends are called N-terminal tails , and they play an important role in higher-order chromatin structure and gene expression.

Figure 4: The nucleosome structure within chromatin. Each nucleosome contains eight histone proteins blue , and DNA wraps around these histone structures to achieve a more condensed coiled form. Figure 5: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes.

Although nucleosomes may look like extended "beads on a string" under an electron microscope, they appear differently in living cells. In such cells, nucleosomes stack up against one another in organized arrays with multiple levels of packing. The first level of packing is thought to produce a fiber about 30 nanometers nm wide. These 30 nm fibers then form a series of loops, which fold back on themselves for additional compacting Figure 5.

The multiple levels of packing that exist within eukaryotic chromosomes not only permit a large amount of DNA to occupy a very small space, but they also serve several functional roles. For example, the looping of nucleosome-containing fibers brings specific regions of chromatin together, thereby influencing gene expression.

Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid DNA. Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

The term chromosome comes from the Greek words for color chroma and body soma. Scientists gave this name to chromosomes because they are cell structures, or bodies, that are strongly stained by some colorful dyes used in research.

The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones. Without such packaging, DNA molecules would be too long to fit inside cells. For example, if all of the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch 6 feet.

For an organism to grow and function properly, cells must constantly divide to produce new cells to replace old, worn-out cells. During cell division, it is essential that DNA remains intact and evenly distributed among cells.

Chromosomes are a key part of the process that ensures DNA is accurately copied and distributed in the vast majority of cell divisions. Still, mistakes do occur on rare occasions. Changes in the number or structure of chromosomes in new cells may lead to serious problems.

For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes. It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure.

If not, the resulting offspring may fail to develop properly. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people. Chromosomes vary in number and shape among living things.