Name: chargaff’s rules and dna pairing
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 27 s
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DNA Base Pairing

דנ"א דומה לסולם מפותל. ושלבי סולם הדנ"א הם זוגות משלימים של בסיסים חנקניים. על פי כללי זיווג בסיס, אדנין, שהוא פורין, נקשר לתימין, שהוא פירימידין, עם שני קשרי מימן.וגואנין, שהוא פורין, מופיע עם ציטוזין, שהוא פירימידין, עם שלושה קשרי מימן. אך מדוע פורינים תמיד נקשרים לפירימידינים? בגלל אילוצים סטריים, שהם מגבלות מרחביות המוטלות על ידי עמוד השדרה של סוכר הפוספט של הדנ"א, רק 10.85 אנגסטרוםנחלל זמין לזוגות בסיס בסליל דנ"א כפול.לפורינים יש שני מבנה טבעתי כפול. לכן, שני פורינים יחד יהיו גדולים מכדי להיכנס למרחב זה. מצד שני, אם נחבר בין שני פירימידינים, הכוללים רק טבעת אחת, המרחק ביניהם יהיה גדול מכדי ליצור קשרי מימן, שהם באורך של כשני אנגסטרומים.יחד עם זאת, אם נקשר פורין ופירימידין, הם יתאימו באופן מושלם לתוך סליל הדנ"א ויהיו די קרובים זה לזה כדי ליצור קשרי מימן. קשרי מימן יכולים להיווצר כשאטום מימן הוא במרחק של כשני אנסטגרומים מאטום בעל אלקטרושליליות כגון חמצן או חנקן. לאדנין יש אטום מימן אחד הקרוב לחמצן ולתימין.ולתימין יש מימן אחד קרוב לחנקן ולאדנין. זה מוביל להיווצרות שני קשרי מימן. אדנין לא יכול ליצור קשרי מימן עם ציטוזין מפני שלציטוזין יש אטום מימן במקום שבו יימצאו החמצן והתימין.ואטום המימן הנמצא בתימין נעדר מציטוזין. תופעה דומה מתרחשת בזוג הבסיסים גואנין וציטוזין, שבו החמצן בגואנין וחמצן וחנקן בציטוזין ממוקמים כל אחד מול מימן, מה שמוביל להיווצרות של שלושה קשרי מימן, מה שלא קורה בזיווג בסיסים של גואנין ותימין. הספציפיות הגבוהה של זיווג בסיסים, וכן העזרה של אנזימי שכפול דנ"א, הם הסיבה לכך שאדנין נקשר תמיד לתימין וגואנין נקשר תמיד לציטוזין.

זיווג בסיסי dna

<div style="direction:rtl">הכללים של ארווין צ'רגף על שקילות DNA סללו את הדרך לגילוי של זיווג בסיסים ב-DNA. הכללים של צ'רגף קובעים שבמולקולת DNA דו-גדילית, <ol style="padding-right: 35px; margin: 0;"> <li style="text-align: right;">כמות האדנין (A) שווה לכמות הטימין (T);</li> <li style="text-align: right;">כמות הגואנין (G) שווה לכמות הציטוזין (C); ו</li> <li style="text-align: right;">סכום הפורינים, A ו-G, שווה לסכום הפירימידינים, C ו-T (כלומר, A+G = C+T).</li> </ol> עבודה מאוחרת יותר של ווטסון וקריק גילתה שב-DNA דו-גדילי, A תמיד יוצר שני קשרי מימן עם T, ו-G תמיד יוצר שלושה קשרי מימן עם C. זיווג בסיסים זה שומר על רוחב קבוע של הסליל הכפול של DNA, כמו גם A-T וגם זוגות C-G הם באורך 10.85Å ומשתלבים בצורה מסודרת בין שני עמודי השדרה של הסוכר-פוספט. זיווגי בסיסים גורמים לבסיסים החנקניים להיות בלתי נגישים למולקולות אחרות עד שקשרי המימן נפרדים. עם זאת, אנזימים ספציפיים יכולים לשבור בקלות את קשרי המימן הללו כדי לבצע תהליכים נחוצים בתא, כגון שכפול ותעתוק DNA. מכיוון שלזוג G-C יש יותר קשרי מימן מאשר זוג A-T, DNA עם אחוז גבוה של זוגות G-C יזדקק לאנרגיה הגבוהה יותר להפרדה של שני גדילי DNA מאשר אחד עם אחוז דומה של זוגות A-T. אנלוגים בסיס כרפואה זיווג בסיסים נכון חיוני לשכפול הנאמן של ה-DNA. אנלוג בסיס הם מולקולות שיכולות להחליף את בסיסי ה-DNA הסטנדרטיים במהלך שכפול ה-DNA. אנלוגים אלה הם אנטי-ויראליים ואנטי-סרטניים יעילים נגד מחלות כמו הפטיטיס, הרפס ולוקמיה. אציקלוביר, הידוע גם בשם אציקלוגואנוזין, הוא אנלוג בסיס של גואנין והוא נפוץ בטיפול בנגיף ההרפס סימפלקס. חלק הגואנוזין של אציקלוביר מזדווג עם אדנין כרגיל במהלך שכפול ה-DNA; עם זאת, מכיוון שאין לו קצה 3' של הנוקלאוטיד, DNA פולימראז לא יכול להמשיך ליצור זוגות בסיסים, והשכפול מסתיים.</div>

זיווג בסיסי DNA

חוקי שקילות הדנ"א של ארווין צ'רגאף סללו את הדרך לגילוי זיווג בסיסים בדנ"א. הכללים של Chargaff קובעים כי ב ...

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chargaff’s rules and dna pairing

Chargaff’s rules and DNA base pairing explain how DNA strands match up. In double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Chargaff also found that A equals T, G equals C, and the total amount of purines (A and G) equals the total amount of pyrimidines (C and T).

Watson and Crick later showed why these matches matter. A forms two hydrogen bonds with T, while G forms three hydrogen bonds with C. These pairings keep the DNA double helix a constant width because both A-T and C-G pairs fit neatly between the two sugar-phosphate backbones.

Base pairing also keeps the nitrogenous bases hidden from other molecules until the hydrogen bonds separate. Specific enzymes can break these bonds when the cell needs to copy DNA or make RNA during transcription. DNA with more G-C pairs usually needs more energy to separate than DNA with more A-T pairs, because G-C pairs have more hydrogen bonds.

Correct pairing is also important in medicine. Base analogs are molecules that can replace normal DNA bases during replication, and they can act as antiviral or anticancer drugs against diseases such as hepatitis, herpes, and leukemia. Acyclovir, also called acycloguanosine, is a guanine analog used to treat herpes simplex virus. It pairs with adenine, but it lacks a 3' end, so DNA polymerase cannot keep building the strand and replication stops.

Chargaff’s Rules and DNA Pairing

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dna base pairing

Chargaff’s rules and DNA base pairing explain how DNA strands match up. In double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Chargaff also found that A equals T, G equals C, and the total amount of purines (A and G) equals the total amount of pyrimidines (C and T). Watson and Crick later showed why these matches matter. A forms two hydrogen bonds with T, while G forms three hydrogen bonds with C. These pairings keep the DNA double helix a...

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Polymers are long chains of molecules made from repeating units called monomers. The word polymer comes from the Greek words "poly," meaning many, and "mer," meaning parts. Polymers can be found in nature, such as DNA and proteins, or made synthetically, such as plastics. A polymer’s shape helps define its properties. Some polymers form linear chains, while others are branched or arranged in complex networks. Functional groups on a polymer also affect how it behaves. The repeating units in a...

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Learn how Chargaff’s rules, hydrogen bonding, and base analogs like acyclovir help explain DNA pairing, replication, and antiviral treatment.

Chargaff’s Rules and DNA Pairing | JoVE K12

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Nucleic Acid Structure

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Overview

Deoxyribonucleic acid, or DNA, is the genetic material responsible for passing traits from generation to generation in all organisms and most ...

DNA Packaging

Overview

Eukaryotes have large genomes compared to prokaryotes. In order to fit their genomes into a cell, eukaryotes must pack their DNA tightly inside ...

RNA Structure

Overview
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Translation

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Regulation of Metabolism

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Cells Coordinate Growth and Proliferation

Use a model to illustrate the role of the cell cycle and differentiation in producing and maintaining complex organisms. Emphasis is on conceptual understanding that mitosis passes on genetically identical materials via replication, not on the details of each phase in mitosis.

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Chargaff’s Rules and DNA Pairing

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