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Eukaryotic Chromosome Structure
The principal feature that distinguishes a eukaryotic cell from a prokaryotic cell is the presence of a membrane-bound nucleus. The nuclear membrane , or nuclear envelope , contains channels called pores that regulate the movement of molecules in and out of the nucleus.
The DNA inside the nucleus is organized into chromosomes. At the most basic level, a chromosome is a molecule of DNA that is tightly coiled around proteins called histones.
Eukaryotic cells have multiple chromosomes that are linear in shape. 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 sister chromatids are connected to each other by a region of the chromosome called the centromere. During mitosis, spindle fibers attach to this region, and they eventually pull the sister chromatids apart to form two separate chromosomes, one for each daughter cell.
The centromere also divides each chromatid into two regions: the p arm and the q arm. The p arm is the shorter arm, and the q arm is the longer arm. They are diploid cells, which means that those 46 chromosomes are organized into 23 pairs. Diploid is sometimes abbreviated as 2n where n is the number of different chromosomes. In humans, 22 out of the 23 chromosome pairs are autosomes, or non-sex chromosomes. These are referred to as homologous pairs because the two chromosomes within the pair are the same size and shape and contain the same genes with potentially different alleles, which are alternate versions of a gene.
The 23rd pair is made up of the sex chromosomes. Typically, people who are biologically male have an XY genotype an X chromosome and a Y chromosome , and people who are biologically female have an XX genotype two X chromosomes. The XX pair, but not the XY pair, is considered to be a homologous pair.
The sex cells, or gametes, of sexually-reproducing eukaryotic organisms are haploid 1n , meaning they only have 23 unpaired chromosomes. When a sperm cell fertilizes an egg, the resulting zygote is diploid; the combination of the two haploid sex cells is what results in the zygote having the full 46 chromosomes. Telomeres are special regions, located on each end of the chromosome, that consist of repetitive sequences of base pairs.
They are typically described as being like the plastic caps on the ends of shoelaces. This is because they protect the ends of chromosomes and keep them from sticking to other chromosomes. However, the most important function of telomeres is that they allow cells to divide without losing their genetic material.
If there were no telomeres, some base pairs would be lost every time DNA replication takes place. Fortunately, it is the telomeres, and not the genes, that get shorter with every cell division. For example, the telomeres in human cells generally lose between 30 and base pairs every time a cell divides. Normally, a cell in the human body can divide around times before it becomes senescent inactive, no longer dividing or dies.
As humans age, the telomeres on the ends of the chromosomes gradually become shorter. A special enzyme called telomerase adds telomeres to the ends of replicated chromosomes, meaning that cells with an increased level of this enzyme—like sperm and egg cells, or certain cancer cells—can divide more frequently. An article on chromosomes from Khan Academy. An article on eukaryotic chromosome structure from North Dakota State University. DNA Overview.
Prokaryotic Chromosomes. Eukaryotic vs. DNA Structure. When you select "Subscribe" you will start receiving our email newsletter. Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe.
The somatic cells of sexually-reproducing eukaryotic organisms are diploid, meaning that they have 2 sets of homologous chromosomes. Telomeres protect the ends of chromosomes, and they get shorter every time a cell divides. Eukaryotic and prokaryotic chromosomes differ in their shape, size, number, and location within the cell. External Sources An article on chromosomes from Khan Academy.
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Chromosome , the microscopic threadlike part of the cell that carries hereditary information in the form of genes. A defining feature of any chromosome is its compactness. The structure and location of chromosomes are among the chief differences between viruses, prokaryotes , and eukaryotes. The nonliving viruses have chromosomes consisting of either DNA deoxyribonucleic acid or RNA ribonucleic acid ; this material is very tightly packed into the viral head. Among organisms with prokaryotic cells i. The single chromosome of a prokaryotic cell is not enclosed within a nuclear membrane.
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.
What is a chromosome?
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins , bind to and condense the DNA molecule to maintain its integrity. Chromosomes are normally visible under a light microscope only during the metaphase of cell division where all chromosomes are aligned in the center of the cell in their condensed form. The joined copies are now called sister chromatids.
How the same DNA sequences can function in the three-dimensional architecture of interphase nucleus, fold in the very compact structure of metaphase chromosomes and go precisely back to the original interphase architecture in the following cell cycle remains an unresolved question to this day. The strategy used to address this issue was to analyze the correlations between chromosome architecture and the compositional patterns of DNA sequences spanning a size range from a few hundreds to a few thousands Kilobases. This is a critical range that encompasses isochores, interphase chromatin domains and boundaries, and chromosomal bands.
Topics in the Cells and DNA chapter
Registration is open as usual for many events, with back-up plans in place to move further courses and conferences online as necessary. This interdisciplinary symposium will highlight exciting new insights into the molecular principles that govern the functional framework of genomes in space and time. The meeting program will cover all levels of organisational complexity, from DNA to chromosomes, and in model systems ranging from bacteria to humans. This symposium will bring together scientists from different disciplines working at the forefront of chromosome biology to discuss the most recent advances in the field. A particular highlight will be the integration of discoveries made in different disciplines, including cell and molecular biology, biophysics, modelling, structural biology and biochemistry. By combining efforts from biology, physics and chemistry, the programme will open new horizons for future research in this exciting area of the life sciences. Principles of Chromosome Structure and Function.
Внизу фреон протекал сквозь дымящийся ТРАНСТЕКСТ, как обогащенная кислородом кровь. Стратмор знал, что охладителю потребуется несколько минут, чтобы достичь нижней части корпуса и не дать воспламениться расположенным там процессорам. Он был уверен, что все сделал вовремя, и усмехнулся.