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  3. The Nucleus and DNA Replication – Anatomy and Physiology

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NCBI Bookshelf. Cooper GM. The Cell: A Molecular Approach.

Intracellular Organelles- The Nucleus

Sunderland MA : Sinauer Associates; The presence of a nucleus is the principal feature that distinguishes eukaryotic from prokaryotic cells. By housing the cell's genome, the nucleus serves both as the repository of genetic information and as the cell's control center.

DNA replication, transcription , and RNA processing all take place within the nucleus, with only the final stage of gene expression translation localized to the cytoplasm. By separating the genome from the cytoplasm, the nuclear envelope allows gene expression to be regulated by mechanisms that are unique to eukaryotes.

Whereas prokaryotic mRNAs are translated while their transcription is still in process, eukaryotic mRNAs undergo posttranscriptional processing e. Chromatin describes the material that makes up chromosomes, which are structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures.

The Atomic Nucleus

For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. In order to organize the large amount of DNA within the nucleus, proteins called histones are attached to chromosomes; the DNA is wrapped around these histones to form a structure resembling beads on a string. These protein-chromosome complexes are called chromatin. Along the chromatin threads, unwound protein-chromosome complexes, we find DNA wrapped around a set of histone proteins.

The Nucleus

The nucleus stores the hereditary material of the cell : The nucleus is the control center of the cell. We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Unless otherwise noted, images on this page are licensed under CC-BY 4. Text adapted from: OpenStax , Concepts of Biology.

The Nucleus and DNA Replication – Anatomy and Physiology

OpenStax CNX. Skip to content Increase Font Size. Figure 1 The outermost boundary of the nucleus is the nuclear envelope. Notice that the nuclear envelope consists of two phospholipid bilayers membranes —an outer membrane and an inner membrane—in contrast to the plasma membrane, which consists of only one phospholipid bilayer.

Figure 2 This illustration shows the double membrane structure surrounding the nucleus.

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Notice that both membranes are composed of a phospholipid bilayer. Credit Boumphreyfr; Wikimedia Chromatin The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm Figure 2. Figure 3 This image shows paired chromosomes.