{"id":707,"date":"2014-05-01T11:30:19","date_gmt":"2014-05-01T11:30:19","guid":{"rendered":"http:\/\/hyndland-sec-glasgow.blogs.rm.com\/CfE-Higher-Unit-1\/?page_id=112"},"modified":"2014-05-01T11:30:19","modified_gmt":"2014-05-01T11:30:19","slug":"organisation-of-dna","status":"publish","type":"page","link":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/organisation-of-dna\/","title":{"rendered":"Organisation of DNA"},"content":{"rendered":"

(b) Organisation of DNA – circular chromosomal DNA and plasmids in prokaryotes. Circular plasmids in yeast. Circular chromosome in mitochondria and chloroplasts of eukaryotes. DNA in the linear chromosomes of the nucleus of eukaryotes is tightly coiled and packaged with associated proteins. <\/span><\/p>\n\n\n\n
\"PPt\"<\/a><\/td>\n\"HwrkRevisionIcon\"<\/a><\/td>\n\"MCQIcon\"<\/a><\/td>\n\"MCQIcon\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Prokaryotic (Bacterial) cells<\/strong><\/h2>\n
abridged from\u00a0Genome Packaging in Prokaryotes: the Circular Chromosome of\u00a0E. coli<\/i>\u00a0 <\/a>By:\u00a0Ann Griswold, Ph.D.\u00a0\u00a9\u00a02008\u00a0Nature Education<\/h6>\n

Prokaryotic cells do not contain nuclei or other membrane-bound organelles. In fact, the word “prokaryote” literally means “before the nucleus.” \u00a0 The nucleoid is simply the area of a prokaryotic cell in which the chromosomal DNA is located. \u00a0 This arrangement is not as simple as it sounds, however, especially considering that the E. coli chromosome is much larger \u00a0than the cell itself. \u00a0 To enable the DNA to fit int he cell, it is “packaged”.<\/p>\n

Eukaryotic cells ( see below) \u00a0wrap their DNA around proteins called histones to make true chromosomes. \u00a0However, most prokaryotes do not have histones. \u00a0Instead, their DNA is supercoiled (see Figure opposite).\"Kavanoff_Ecoli-chromosome_FULL\"<\/a>\u00a0 \u00a0Imagine twisting a rubber band so that it forms tiny coils. Now twist it even further, so that the original coils fold over one another and form a condensed ball. \u00a0 This is supercoiling.<\/p>\n

Another difference between prokaryotes and eukaryotes is that prokaryotic cells often contain one or more plasmids (i.e., extrachromosomal DNA molecules that are \u00a0circular). \u00a0They are typically smaller than the chromsome and encode non-essential genes, such as those that aid growth in specific conditions or encode\u00a0antibiotic resistance. \u00a0 The plasmids replicate independently of the rest of the genome.<\/p>\n

\/\/ <\/p>\n

Extension (outside the scope of the syllabus)<\/a><\/p>\n

Eukaryotic (Animal, plant, fungal) cells<\/strong><\/h2>\n

(abridged from DNA Packaging: Nucleosomes and Chromatin<\/a>
\n By: Anthony T. Annunziato, Ph.D. (Biology Department, Boston College) \u00a9 2008 Nature Education<\/a>)<\/a>
\n\"EukChr\"<\/a><\/p>\n

The human genome contains approximately 6 billion base pairs of DNA packaged into 23 pairs of chromosomes. \u00a0 The DNA per cell is around 2 metres long, yet a cell is only around 10-100 micrometres.
\nThe ability to hold this length of DNA in every cell in the body is made possible because of DNA packaging.<\/p>\n

Certain proteins, called\u00a0histones\u00a0pack the chromosomal DNA into the microscopic space of the\u00a0eukaryotic\u00a0nucleus, creating a much smaller volume than DNA can fold by itself.\u00a0 \u00a0The resulting DNA-protein complex is called\u00a0chromatin<\/strong>.
\nThis protein\/ DNA complex forms a nucleosome<\/strong>, which looks like a tiny bead. \u00a0Around\u00a0166 base pairs are wrapped around each nucleosome, so each\u00a0chromosome (over 100 million base pairs of DNA on average) contains hundreds of thousands of nucleosomes. \u00a0They are linked by the DNA that runs between them (an average of about 20 base pairs) – and so appear similar to beads on a string.<\/span>
\nHistones are a family of small, positively charged proteins and as DNA is negatively charged, due to the phosphate groups in its phosphate-sugar backbone, histones bind with DNA very tightly.<\/p>\n

The packaging of DNA into nucleosomes shortens the fibre length about sevenfold. In other words, a piece of DNA that is 1 meter long will become \u00a0just 14 centimetres long. \u00a0To fit into the nucleus, which is typically only 10 to 20 microns in diameter, further coiling is required. \u00a0The diagram below illustrates this process (Click on the diagram to access an interactive presentation on\u00a0DNA packaging) .<\/p>\n

\"18847_6\"<\/a>
\nProcesses such as transcription and replication require the two strands of DNA to come apart temporarily, thus allowing polymerases access to the DNA template. \u00a0 However, the presence of nucleosomes and further folding pose barriers to the enzymes that unwind and copy DNA. \u00a0 Cells need to open the chromatin fibres and\/or temporarily remove histones to permit transcription and replication to proceed. \u00a0 These processes are reversible, so the chromatin can be returned to its compact state after transcription and\/or replication are complete.<\/p>\n

\n

Prokaryotic DNA replication occurs at a rate of 1,000 nucleotides per second, and prokaryotic transcription occurs at a rate of about 40 nucleotides per second (Lewin, 2007), so\u00a0bacteria\u00a0must have highly efficient methods of accessing their DNA strands. But how?<\/p>\n

Researchers have noted that the nucleoid usually appears as an irregularly shaped mass within the prokaryotic cell, but it becomes spherical when the cell is treated with chemicals to inhibit transcription or\u00a0translation. Moreover, during transcription, small regions of the chromosome can be seen to project from the nucleoid into the\u00a0cytoplasm\u00a0(i.e., the interior of the cell), where they unwind and associate with\u00a0ribosomes, thus allowing easy access by various transcriptional proteins (D\u00fcrrenberger\u00a0et al<\/i>., 1988). These projections are thought to explain the mysterious shape of nucleoids during active growth. When transcription is inhibited, however, the projections retreat into the nucleoid, forming the aforementioned spherical shape.<\/p>\n

Other DNA Differences Between Prokaryotes and Eukaryotes<\/h2>\n
\n
\n
\n

Most prokaryotes reproduce asexually and are\u00a0haploid, meaning that only a single copy of each\u00a0gene\u00a0is present. \u00a0This makes it relatively easy to generate mutations in the lab and study the resulting phenotypes. \u00a0By contrast, eukaryotes that reproduce sexually generally contain multiple chromosomes and are said to be\u00a0diploid, because two copies of each gene exist\u2014with one copy coming from each of an organism’s parents.<\/p>\n

\u00a0Yet another difference between prokaryotes and eukaryotes is that prokaryotic cells often contain one or more plasmids (i.e., extrachromosomal DNA molecules that are either linear or circular). These pieces of DNA differ from chromosomes in that they are typically smaller and encode nonessential genes, such as those that aid growth in specific conditions or encode\u00a0antibiotic resistance.\u00a0Borrelia<\/i>, for instance, contains more than 20 circular and linear plasmids that encode genes responsible for infecting ticks and humans (Fraser\u00a0et al<\/i>., 1997). Plasmids are often much smaller than chromosomes (i.e., less than 1,500 kilobases), and they replicate independently of the rest of the genome. However, some plasmids are capable of integrating into chromosomes or moving from cell to cell.<\/p>\n

Perhaps due to the space constraints of packing so many essential genes onto a single chromosome, prokaryotes can be highly efficient in terms of genomic organization. Very little space is left between prokaryotic genes. As a result, noncoding sequences account for an average of 12% of the prokaryotic genome, as opposed to upwards of 98% of the genetic material in eukaryotes (Ahnert\u00a0et al.<\/i>, 2008). Furthermore, unlike eukaryotic chromosomes, most prokaryotic genomes are organized into\u00a0polycistronic operons, or clusters of more than one coding region attached to a singlepromoter<\/a>, separated by only a few base pairs. The proteins encoded by each\u00a0operon\u00a0often collaborate on a single task, such as the metabolism of a sugar into by-products that can be used for energy (see Figure below).<\/p>\n<\/div>\n

\"9_1_2\"<\/a><\/p>\n

Because there is no nuclear membrane to separate prokaryotic DNA from the ribosomes within the cytoplasm,\u00a0transcription and translation occur simultaneously in these organisms<\/a>. This is strikingly different from eukaryotic chromosomes, which are confined to the membrane-bound nucleus during most of the\u00a0cell cycle. In eukaryotes, transcription must be completed in the nucleus before the newly synthesized\u00a0mRNA\u00a0molecules can be transported to the cytoplasm to undergo translation into proteins.<\/p>\n

Prokaryotic DNA replication occurs at a rate of 1,000 nucleotides per second, and prokaryotic transcription occurs at a rate of about 40 nucleotides per second (Lewin, 2007), so\u00a0bacteria\u00a0must have highly efficient methods of accessing their DNA strands. But how?<\/p>\n

Researchers have noted that the nucleoid usually appears as an irregularly shaped mass within the prokaryotic cell, but it becomes spherical when the cell is treated with chemicals to inhibit transcription or\u00a0translation. Moreover, during transcription, small regions of the chromosome can be seen to project from the nucleoid into the\u00a0cytoplasm\u00a0(i.e., the interior of the cell), where they unwind and associate with\u00a0ribosomes, thus allowing easy access by various transcriptional proteins (D\u00fcrrenberger\u00a0et al<\/i>., 1988). These projections are thought to explain the mysterious shape of nucleoids during active growth. When transcription is inhibited, however, the projections retreat into the nucleoid, forming the aforementioned spherical shape.<\/p>\n

Other DNA Differences Between Prokaryotes and Eukaryotes<\/h2>\n
\n
\n
\n

Most prokaryotes reproduce asexually and are\u00a0haploid, meaning that only a single copy of each\u00a0gene\u00a0is present. \u00a0This makes it relatively easy to generate mutations in the lab and study the resulting phenotypes. \u00a0By contrast, eukaryotes that reproduce sexually generally contain multiple chromosomes and are said to be\u00a0diploid, because two copies of each gene exist\u2014with one copy coming from each of an organism’s parents.<\/p>\n

\u00a0Yet another difference between prokaryotes and eukaryotes is that prokaryotic cells often contain one or more plasmids (i.e., extrachromosomal DNA molecules that are either linear or circular). These pieces of DNA differ from chromosomes in that they are typically smaller and encode nonessential genes, such as those that aid growth in specific conditions or encode\u00a0antibiotic resistance.\u00a0Borrelia<\/i>, for instance, contains more than 20 circular and linear plasmids that encode genes responsible for infecting ticks and humans (Fraser\u00a0et al<\/i>., 1997). Plasmids are often much smaller than chromosomes (i.e., less than 1,500 kilobases), and they replicate independently of the rest of the genome. However, some plasmids are capable of integrating into chromosomes or moving from cell to cell.<\/p>\n

Perhaps due to the space constraints of packing so many essential genes onto a single chromosome, prokaryotes can be highly efficient in terms of genomic organization. Very little space is left between prokaryotic genes. As a result, noncoding sequences account for an average of 12% of the prokaryotic genome, as opposed to upwards of 98% of the genetic material in eukaryotes (Ahnert\u00a0et al.<\/i>, 2008). Furthermore, unlike eukaryotic chromosomes, most prokaryotic genomes are organized into\u00a0polycistronic operons, or clusters of more than one coding region attached to a singlepromoter, separated by only a few base pairs. The proteins encoded by each\u00a0operon\u00a0often collaborate on a single task, such as the metabolism of a sugar into by-products that can be used for energy (see Figure below).<\/p>\n

\"9_1_2\"<\/a><\/p>\n

Because there is no nuclear membrane to separate prokaryotic DNA from the ribosomes within the cytoplasm,\u00a0transcription and translation occur simultaneously in these organisms<\/a>. This is strikingly different from eukaryotic chromosomes, which are confined to the membrane-bound nucleus during most of the\u00a0cell cycle. In eukaryotes, transcription must be completed in the nucleus before the newly synthesized\u00a0mRNA\u00a0molecules can be transported to the cytoplasm to undergo translation into proteins.<\/p>\n

<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

(b) Organisation of DNA – circular chromosomal DNA and plasmids in prokaryotes. Circular plasmids in yeast. Circular chromosome in mitochondria and chloroplasts of eukaryotes. DNA in the linear chromosomes of the nucleus of eukaryotes is tightly coiled and packaged with associated proteins. Prokaryotic (Bacterial) cells abridged from\u00a0Genome Packaging in Prokaryotes: the Circular Chromosome of\u00a0E. coli\u00a0 … Continue reading “Organisation of DNA”<\/span><\/a><\/p>\n","protected":false},"author":2454,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"footnotes":""},"class_list":["post-707","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/wp-json\/wp\/v2\/pages\/707","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/wp-json\/wp\/v2\/users\/2454"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/wp-json\/wp\/v2\/comments?post=707"}],"version-history":[{"count":0,"href":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/wp-json\/wp\/v2\/pages\/707\/revisions"}],"wp:attachment":[{"href":"https:\/\/blogs.glowscotland.org.uk\/gc\/hyndsecbiohu1\/wp-json\/wp\/v2\/media?parent=707"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}