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What are nucleic acids made of?

All nucleic acids are polymers with nucleotides as monomers.


What are nucleotides made of?

Nucleotides are always made from three components:
1. A phosphate group.
2. A pentose sugar molecule.
3. A nitrogenous base.
These three subunits are joined together by covalent bonds.


What are the nitrogenous bases?

There are 5 nitrogenous bases in nucleic acids and they are divided into 2 categories:
Purines - These are Adenine and Guanine. Purines have a double ringed structure and are the bigger of the bases.
Pyrimidines - These are Thymine, Cytosine and Uracil. Pyrimidines have a single ring based structure and are smaller than purines.


How does a single strand of nucleic acid form?

Through a condensation reaction, the phosphate group of one nucleotide is joined to the sugar molecule of another by a covalent (di-ester) bond. This process repeats and hundreds of nucleotides are joined together in a chain to form a single strand of nucleic acid with a sugar-phosphate backbone and extrusions of organic bases.


What is the structure of DNA (Deoxyribose Nucleic Acid)?

A DNA molecule is usually found containing two strands of nucleotides joined together by hydrogen bonds between the nitrogenous bases. However, the bases do not just join together randomly; they join together in accordance to the complementary base pairing rule, whereby Adenine always joins to Thymine and Cytosine always joins to Guanine. The reason why a purine always joins onto a pyrimidine is so that the chains are always equidistance apart.

The chains are joined together so that the sugar-phosphate groups run antiparallel to each other. This results in the chains twisting to give DNA's distinctive double helix structure.


What makes DNA good for carrying out its functions?

1. Because DNA is double stranded, 2 new DNA molecules can be easily created from just one molecule through semi-conservative replication.
2. Hydrogen bonds between the two strands of a DNA molecule are weak so can break easily, allowing the DNA molecule is able to unzip easily during replication and transcription.
3. Double helix is a stable structure, which means DNA cannot be damaged easily.
4. DNA molecules are long, so they are able to carry lots of coded information.
5. DNA molecules are also compact, so lots can be stored in a small volume.


What are the stages of DNA replication?

1. The double helix of the whole DNA molecule unwinds and the hydrogen bonds between the nitrogenous bases break with the aid of enzymes like DNA-helicase, leaving 2 single strands of DNA.
2. Along each strand of DNA, free floating DNA nucleotides in the nucleus line up by pairing up in their complementary base pairs; adenine with thymine and cytosine with guanine.
3. Hydrogen bonds form between the nitrogenous bases; 2 between adenine and thymine, 3 between cytosine and guanine.
4. Nucleotides are joined together by phosphate group on one joining to deoxyribose of another by covalent (di-ester) bonds formed through condensation reactions with the aid of enzymes like DNA-polymerase to form the new sugar-phosphate backbone
5. Process continues until a brand new DNA chain is created.
6. New DNA winds again into the double helix shape.
7. This process happens along both original chains and creates 2 new, identical DNA molecules.


What is meant by semi-conservative replication?

DNA undergoes semi-conservative replication because 2 daughter DNA molecules are formed from 1 parent molecule, both of which containing 1 old strand from the parent DNA and 1 newly formed strand.


How can we show that DNA undergoes semi-conservative replication?

1. Get a batch of bacteria and grow them in a medium containing N15, which will be used to make the nitrogenous bases. After a few generations, nearly all of the bacteria's DNA will contain N15.
2. Separate out the DNA molecules by differential centrifugation and observe position.
3. Transfer a batch to N14 medium and grow for one generation.
4. Separate out DNA of first generation N14 bacteria and compare position with N14 only DNA and N15 only DNA.
5. Theoretically, the first generation N14 DNA should sit between N14 DNA and N15 DNA as it contains one strand of both through conservative replication.


What are the differences between DNA and RNA?

1. DNA nucleotides contain the pentose sugar deoxyribose whereas RNA nucleotides contain the pentose sugar ribose.
2. DNA contains the nitrogenous bases adenine, thymine, cytosine and guanine whereas in RNA, thymine is replaced with uracil.
3. DNA is usually double stranded whereas RNA is usually single stranded.
4. DNA molecules are usually larger than RNA molecules.
5. There are 3 forms of RNA whereas there's only one form of DNA.


What are the functions of the different RNA molecules?

mRNA: A complementary strand derived from a section of transcribed DNA and is used to carry the coded information in DNA out of the nucleus and to the ribosomes in the cytoplasm where proteins are synthesised.
rRNA: They make up the structure of ribosomes.
tRNA: Bonds onto specific amino acids in the cytoplasm and transports them to the ribosome where they are put together into a polypeptide.


What are the stages of DNA transcription and protein synthesis?

- A gene is a length of DNA that codes for a specific polypeptide.
- Each sequence of 3 nitrogenous bases along a DNA molecule codes for a particular amino acid. This is called a codon.
1. A section of DNA unwinds and the hydrogen bonds break between the bases.
2. RNA nucleotides line up along one strand of DNA according to the complementary base paring rule; adenine to uracil, thymine to adenine, cytosine to guanine.
3. RNA nucleotides are bonded together as covalent (di-ester) bonds form between phosphate groups and ribose molecules via condensation reaction with the aid of enzymes to form sugar-phosphate backbone.
4. Single strand of mRNA molecule detaches from DNA and leaves nucleus via nuclear pores.
5. mRNA molecule attaches to ribosome and tRNA bring amino acids in order according to the triplet base sequence on mRNA molecule by attaching via anticodon.
6. Peptide bonds form between the amino acids to create the polypeptide.