BIO MOLECULES Pt 6 (DNA etc)
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What does DNA and RNA stand for?
- DNA - deoxyribonucleic acid
- RNA - ribonucleic acid
Name the 4 bases that form part of nucleotides.
- Adenine & thymine, cytosine & guanine
- In RNA, uracil replaces thymine.
What are the monomers of nucleic acids?
What are the 3 subunits that make up a single nucleotide?
- Phosphate group
- Sugar molecule (5 carbon sugar, either deoxyribose (in DNA) or ribose (in RNA)
- Organic nitrogenous base
- (The 3 subunits are joined by covalent bonds in condensation reactions to form single nucleotide molecule)
How are nucleotides joined together to form nucleic acids?
- A condensation reaction between phosphate group of one nucleotide and sugar of another nucleotide. Repeating gives long chain of nucleotides.
- The "backbone" of long chain molecule consists of repeating sugar-phosphate chain.
How can nucleic acids be either described as DNA or RNA?
Because only nucleotides carrying the same sugar molecule bind together.
How can the five organic nitrogenous bases be grouped?
- Into purines and pyrimidines. Purines are larger.
- Purines - Adenine and Guanine
- Pyrimidines - Thymine, Uracil, Cytosine
What does too much nucleic acid in the body cause?
- Where too much uric acid is in the blood, and insoluble crystals form and deposited in joints in the extremities, such as the toes. (Uric acid is produced when excess purines are broken down in liver - excreted in urine).
DNA is a what?
A double-stranded polynucleotide
Why is DNA stable and why is it vital that it is?
- Because of the hydrogen bonds between the bases from the two antiparallel strands of polynucleotides, strengthening the structure.
- Vital as it carries instructions to make an organism - if it were unstable, instructions would go wrong too easily.
Why is the term "antiparallel" used to describe the two strands in DNA?
Because the strands run in opposite directions to each other in a parallel fashion, their nitrogenous bases projecting inwards to hydrogen bond.
The chains are always the same distance apart - parallel. Why?
- Because the bases pair up in a specific way - where a pyrimidine appears on one side, a purine appears on the other. Thus always the same length.
- Furthermore, A is complementary to T, G is complementary to C.
How is the double helix shape produced?
The antiparallel chains twist, to form the final structure which is known as a double helix.
In what stage of the cell cycle does DNA replication take place?
Why is the process of DNA replication described as semi-conservative replication?
Because each new DNA molecule produced consists of one conserved strand plus one newly built strand.
Outline the steps involved in semi-conservative replication of DNA.
- Double helix untwisted
- Hydrogen bonds broken apart to "unzip" DNA - exposes bases.
- Free DNA nucleotides are hydrogen-bonded onto exposed bases according to base-pairing rules. Joined together by enzyme DNA polymerase.
- Covalent bonds are formed between phosphate and sugar molecules to seal the backbone.
- Two new identical DNA molecules are produced.
Why is DNA in a double-helix structure?
To give the molecule stability.
Explain how the structure of DNA is suited for its function.
- Sequence of bases can store information.
- Molecules are long, so large amount of information can be stored.
- Base-pairing rules allow for complimentary strands to be replicated.
- Double helix structure gives molecule stability.
- Hydrogen bonds allow easy unzipping for copying & reading information.
List 4 ways in which RNA is different from DNA.
- Three forms of RNA molecule exist.
- RNA polypeptide chain is usually single-stranded
- Sugar molecule that makes up the nucleotides is ribose
- Nitrogenous base uracil is found instead of thymine.
What is the process by which RNA copies genetic code of DNA?
List the 3 forms RNA molecules exist in.
- messenger RNA (mRNA) made as a strand complementary to one strand of DNA molecule
- ribosomal RNA (rRNA) is found in ribosomes
- transfer RNA (tRNA) carries amino acids to the ribosomes, where they are bonded together to form polypeptides.
What is a gene?
- A length of DNA (part of DNA molecule) that codes for one (or more) polypeptides. Each gene occupies a specific place (locus) on a chromosome.
- Different versions of the same gene are called alleles.
Why is RNA less stable than DNA, and explain why this is important.
- RNA single stranded - less stable because bases are exposed and not paired. Also contains uracil which may contribute to lower stability of molecule.
- As mRNA results in production of proteins, if the cell is to control protein production, the disintegration of mRNA stops too much of a certain chemical being made. Allows for regulation of protein levels in a cell.
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