Nucleusand DNA
The nucleus of a cell houses the deoxyribonucleic acid (DNA), which is the hereditary material .
DNA inhumans is linear and is made up of two complementary strands . Therefore, DNA is saidto be double stranded . The two DNAstrands are
connected to one another byweak hydrogen bonds that formbetween the nitrogenous bases—adenine,thymine, cytosine, and guanine— are on oppositestrands . This hydrogenbonding between nitrogenous bases is called complementary base pairing.
Figur1-4 shows the structure of DNA
DNA Structure: This figure shows the structure ofDNA, with a deoxyribose sugarphosphate backbone and nitrogenous bases adenine, thymine,cytosine, and guanine . Notice how adenine complementary basepairs with thymine and cytosine complementary base pairs with guanine .
As you can see in Figure 1-4, there is a sugar phosphatebackbone made up of deoxyribose sugar and phosphate . The nitrogenous bases—adenine, thymine, cytosine, and guanine—areattached
to this backbone. Adenine on one strandof DNA complementarybase pairs with thymine on the otherstrand, and guanine on one strand base pairs with cytosine on the other strand. Did you notice the twist in the DNAmacromolecule? The DNA structure forms what is known as a doublehelix, which is the twisted structure that you observe .
Within a cell nucleus, the DNA iscarefully packaged via proteins knownas histones . This packaging allows a lot of DNA to fit into a small space and
helps to protect this genetic material. The DNA isvery important and must be protected becauseit is essentially a book of recipesfor proteins . Keepin mind, however, that the cell doesnot use all the recipes . Perhaps you have a book ofrecipes, from which youmight choose only a few recipesto use to make your favorite cake or cookies. Cells work in a similar way—they only make the proteins that they need based on specific demands, and therefore, they only use the DNA recipes thatare required to make those proteins . These DNA recipesare called genes . Genes are DNA sequences that encode for aminoacids that make up proteins . To put itin simple terms, a gene is a recipeto make a protein .
DNA’s appearance changes depending on the point wherethe cell is within the cell cycle. Figure 1-5shows the cell cycle. During the interphase portionofthe cell cycle, the cell is not dividing. When
the cell is not dividing,the DNA housed in the nucleus is very spread out and is calledchromatin . The cell progresses frominterphase into mitosis . During mitosis (nuclear division) and cytokinesis (cytoplasmic division), the cell is actively dividing. The DNA is verycondensed during much of mitosis, and we call the DNA in this condensed state chromosomes .
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