Structure of nucleotides and the DNA molecule

Most DNA molecules consist of two polymer chains, made from four different monomers called nucleotides, connected together in the form of a double helix.

Unlike man-made poly(ethene), from the monomer ethene etc. DNA is a naturally occurring polymer - long molecular chains of joined up monomer (single) molecules.

The nucleotide is the small basic molecular unit - the monomer from which the polymer is formed.

Nucleotides form the building blocks of DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). An individual nucleotide consists of three molecular bits combined together - the same phosphate group, a variable base (adenine, cytosine, guanine or thymine), and the same pentose sugar (pentose just means having a ring of 5 atoms). The phosphate group and base are attached to the sugar (see left diagram of a single nucleotide).

The DNA (and RNA) polymer chain is formed by a large number of phosphate-sugar linkages. The base is a sort 'branch' off the main chain, but this helps it to intermolecular bond with a base of another opposite strand of DNA.

The result is full DNA molecule consists of two 'molecular' strands coiled together to form a double helix, but how is this helix held together?

The two polymer strands of DNA are cross-linked by a series of complementary base pairs joined together by weak intermolecular bonds called hydrogen bonds - cross links (base-pairing bonds shown here as on the diagram):

There are four bases in DNA holding the structure together and the same two bases are always paired together - known as complementary base pairing.

This is shown on the right diagram, holding the two strands of DNA together.

Key to names of A, T, C and G

Adenine (A) with thymine (T)  AT,

and cytosine (C) with guanine (G) CG.

Whererepresents the weak (but crucial) intermolecular attractive hydrogen bonding force between the pairs of bases.

This weaker intermolecular bond is actually called a hydrogen bond, but you might not need to know any more detail at GCSE/IGCSE level.

These cross linking complementary base pair bonds hold the DNA molecules tightly together giving it the necessary stability to perform their genetic roles - but not to tightly, that they cannot be 'unzipped' - a necessary process in cell replication!

Here complementary means 'matching pairs'. A with T and C with G are the linked complimentary base pairs.

The double helix structure is shown in the diagram above on the right, illustrating how the DNA is held together by the cross-linking hydrogen bonds between the bases to hold together the double helix together.

A short section of DNA is illustrated in more detail below.

A more detailed diagram of a very short section of a double-helix DNA molecule showing the two different base pairings holding the two molecular strands together.

It is the order of the bases in the DNA strands of a gene that decides the order of amino acids in a protein.

---

Key points on the structure of DNA and its significance

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

(For more details on 2. and 3. see other pages from the DNA index)

The Structure of DNA and Its Importance in Understanding Genetics

---

1. The Structure of DNA

DNA (deoxyribonucleic acid) is the molecule that carries genetic information in living organisms. It is found in the nucleus of cells, packed into structures called chromosomes.

Key Features of DNA Structure

• Double Helix: DNA has a twisted ladder-like shape, known as a double helix, first described by James Watson and Francis Crick in 1953, using data from Rosalind Franklin.

• Nucleotides: DNA is made up of repeating units called nucleotides. Each nucleotide consists of:

  • A phosphate group

  • A sugar molecule (deoxyribose)

  • A nitrogenous base

• Bases and Complementary Pairing:

  • There are four bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

  • Bases pair up following complementary base pairing:

    • A pairs with T

    • C pairs with G

  • The bases are held together by hydrogen bonds.

---

2. Importance of DNA in Genetics

DNA plays a crucial role in inheritance, protein synthesis, and evolution.

Genetic Information and Inheritance

• DNA contains genes, which are specific sequences of bases that code for proteins.

• Offspring inherit DNA from their parents via sexual reproduction.

• Variations in DNA lead to genetic differences among individuals.

Protein Synthesis

DNA carries instructions to make proteins through two key processes:

1. Transcription:

   • DNA is copied into mRNA (messenger RNA) in the nucleus.

   • mRNA carries the genetic code to the ribosomes.

2. Translation:

   • At the ribosome, tRNA (transfer RNA) helps assemble amino acids into proteins.

Mutations and Genetic Disorders

• Changes in DNA sequence (mutations) can lead to genetic disorders, such as cystic fibrosis.

• Some mutations can be beneficial and contribute to evolution by creating new traits.

---

3. The Role of DNA in Scientific Research

• Understanding DNA has led to advancements in genetic engineering, such as GM crops and medical treatments.

• DNA sequencing allows scientists to study genetic diseases and trace human ancestry.

• DNA fingerprinting is used in forensics to solve crimes.

---

Summary of learning objectives and key words or phrases

• Know the structure of DNA, including its components.

• Understand complementary base pairing and how it ensures accurate DNA replication.

• Explain protein synthesis step by step.

• Be familiar with mutations and their effects.

• Give examples of genetic diseases and modern applications of DNA research.

• Be able to interpret simple diagrams of the molecular structure of nucleotides (sugar, phosphate and base components) and deoxyribonucleic acid (DNA).

• Know the base pair pairings that hold the double helix together (A-T and C-G).

---

WHAT NEXT?

TOP OF PAGE

INDEX of biology notes on DNA, RNA, synthesis of proteins and functions of proteins

Big website and use [SEARCH BOX] below, maybe quicker than the indexes

INDEX of all my BIOLOGY NOTES

HOME PAGE of Doc Brown's Science website

UK KS3 Science Quizzes for KS3 science students aged ~11-14, ~US grades 6, 7 and 8

Biology * Chemistry * Physics UK GCSE level students aged ~14-16, ~US grades 9-10

Advanced Level Chemistry for pre-university age ~16-18 ~US grades 11-12, K12 Honors

Find your GCSE/IGCSE science course for more help links to all science revision notes

email doc brown - comments - query?

---

Doc Brown's school biology revision notes: GCSE biology, IGCSE  biology, O level biology,  ~US biology science grade 8, grade 9 and grade 10 school science courses or equivalent for ~14-16 year old students of biology IGCSE AQA GCSE Biology Edexcel GCSE Biology IGCSE OCR Gateway Science Biology OCR 21st Century Science Biology  Some of these biology revision notes might be suitable for UK KS3 Science-Biology courses for ages 12-14 (~US biology science grades 6, grade 7 and grade 8), Based on the syllabus-specifications for students taking the IGCSE/GCSE level biology examinations summary revision notes and key points on the nucleotide structure of DNA for students studying the AQA igcse/gcse biology notes on the nucleotide structure of DNA, Edexcel gcse biology notes on the nucleotide structure of DNA,  OCR 21st century GCSE biology notes on the nucleotide structure of DNA, OCR gateway GCSE biology notes on the nucleotide structure of DNA, WJEC gcse biology notes on the nucleotide structure of DNA, CCEA gcse biology notes on the nucleotide structure of DNA for students taking & studying CIE Cambridge igcse biology, or any other GCSE or IGCSE level biology exams notes on the nucleotide structure of DNA, useful for US grade 9-10 students taking biology courses, Explaining the importance of explaining the complimentary base pairs in DNA in GCSE level biology, What you need to know about explaining the complimentary base pairs in DNA for GCSE level biology, Explaining the use of explaining the complimentary base pairs in DNA knowledge in GCSE level biology, Examples of explaining the complimentary base pairs in DNA explained when studying GCSE level biology, What is the significance of explaining the complimentary base pairs in DNA in GCSE level biology, describing and explaining the theory of explaining the complimentary base pairs in DNA when studying GCSE level biology, revision notes for explaining the complimentary base pairs in DNA in exams, online help for understanding explaining the complimentary base pairs in DNA for GCSE biology, exam revision notes for explaining the complimentary base pairs in DNA, what do I need to learn about explaining the complimentary base pairs in DNA? revision summary for explaining the complimentary base pairs in DNA in GCSE biology exams, learning notes for explaining the complimentary base pairs in DNA, help to pass the explaining the complimentary base pairs in DNA topic in an exam question, how to prepare for questions on explaining the complimentary base pairs in DNA in a GCSE biology examination?

---

SITEMAP Website content © Dr Phil Brown 2000+. All copyrights reserved on Doc Brown's biology revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial.