These revision notes provide an in depth summary of this specific chapter of AQA Biology in the A Level Specification -
Very comprehensive notes and enabled me to achieve an A* in my A Level Biology exams.
Structure of RNA and DNA
- Are ribonucleic acid and deoxyribonucleic acid
- Double helix structure of DNA makes it immediately
recognisable
- DNA carries genetic information
- DNA is made up of nucleotides that have three basic
components
Nucleotide structure
Mono
- A pentose sugar (deoxyribose in DNA and ribose in
RNA)
- A phosphate group Di
- A nitrogenous base – A nitrogen-containing organic
base
o C – Cytosine
o G – Guanine
Poly
o T – Thymine
o A – Adenine
o U – Uracil
- Joined by a condensation reaction and make up the
DNA nucleotide (mononucleotide)
- Two mononucleotides may be joined in a
condensation reaction between the deoxyribose sugar
of one mononucleotide and the phosphate group of another
o Bond formed between them is known as a phosphodiester bond
- The new structure is called a dinucleotide
- A continuation of this linking forms a new structure known as a polynucleotide
Ribonucleic acid structure
- RNA is a polymer made up of nucleotides
- It’s a single, relatively short, polynucleotide chain in which the
pentose sugar is always ribose and the organic bases are A, G, C
and U
- One type of RNA transfers genetic info from DNA to the
ribosomes
- The ribosomes themselves are made of proteins and another type
of RNA
- A third type of RNA is involved in protein synthesis
DNA structure
- In 1953, James Watson and Francis Crick worked out the
structure of DNA after using the work of Rosalind Franklin
- In DNA the pentose sugar is deoxyribose and the organic bases are A, T, C and G
, - DNA is made up of two strands of nucleotides (polynucleotides)
- Each of the two strands is very long, and is joined together by H bonds formed between
certain bases
- DNA can be thought of as a ladder in which phosphate and deoxyribose molecules
alternate to form the uprights and the organic bases pair together to form the rungs
DNA and RNA
DNA RNA
Sugar – Deoxyribose Sugar – Ribose
Bases – Adenine, Thymine, Guanine and Bases – Adenine, Uracil, Cytosine and
Cytosine Guanine
Specific base pair – A-T C-G Specific base pair – A-U C-G
Base pairing
- The bases on the two strands of DNA attach to each other
by H bonds
o Its these H bonds that hold the two strands together
- The base pairing is specific
o Adenine bonds with Thymine
o Cytosine bonds with Guanine
- A is complimentary to T and G is complimentary to C
- Because of this complimentary pairing, the amount of A
and T is always equal and same is for G and C however
ratio of AT:CG is not always the same
The double helix
- The uprights of phosphate and deoxyribose wind around one another to form a double
helix
- They form the structural backbone of the DNA molecule
, The stability of DNA
- DNA is a stable molecule because:
o The phosphodiester backbone protects the more chemically reactive organic
bases inside the double helix
o H bonds link organic base pairs forming bridges (rungs) between the
phosphodiester uprights
As there a 3 H bonds between C and G the higher the proportion of C-G
pairings, the more stable the DNA molecule
- There are other interactive forces between the base pairs that hold the molecule
together
Function of DNA
- DNA is the hereditary material responsible for passing genetic information from cell to
cell and generation to generation
- Due to large number of base pairs in DNA, there is an almost infinite variety of
sequences of bases along the length of a DNA molecule
o It’s this variety that provides genetic diversity within living organisms
- The DNA molecule is adapted to carry out its function in a number of ways:
o Stable molecules which normally passes from generation to generation without
change – Only rarely does it mutate
o Separates easily during DNA replication and protein synthesis as weak H bonds
o It’s an extremely large molecule and therefore carries an immense amount of GI
o By having base pairs within helical cylinder of deoxyribose-phosphate backbone,
GI is protected from being corrupted by outside chemical and physical forces
o Base pairing leads to DNA being able to replicate and transfer info to mRNA
- The function of DNA depends on the sequence of base pairs that it possesses
- This sequence is important to everything it does and to life itself
DNA replication
- Cells that make up organisms are always derived from existing cells by the process of
division
- Cell division occurs in two main stages:
o Nuclear division – process by which the nucleus divides. Two types of nuclear
division – Mitosis and meiosis
o Cytokinesis – nuclear division and is the process by which the whole cell divides
- Before a nucleus divides its DNA must be replicated
- This is to ensure that all the daughter cells are more or less genetically identical to the
original one
- Semi-conservative model is universally accepted as how DNA replication takes place
Semi-conservative replication
- For semi-conservative replication to take place there are 4 requirements:
o Four types of nucleotide, each with their bases of A, C, G, T are present
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller callumgallagher. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £3.49. You're not tied to anything after your purchase.
