DNA Revision

Everything you need to know for a good grade on DNA. Made for the old sylabus but still contains lots of useful information which is still relevant. Includes transcription, translation, mitosis, chromosome structure and number... etc.

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  • Created by: Hope
  • Created on: 19-11-08 14:52
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Nucleic Acids
DNA (deoxyribose nucleic acid) and RNA (ribose nucleic acid) are
nucleic acids.
Nucleic acids made up of nucleotides.
Nucleotides made up of 3 parts
Phosphate or phosphoric acid
5carbon pentose sugar
Organic base (nitrogenous)
Formation of nucleotides-
Nucleotides are formed by two condensation reactions.
These happen between the sugar and the phosphate and the sugar
and the base.
The bonds formed are phosphoester bonds.
5-carbon pentose sugar
DNA deoxyribose sugar (1 less oxygen on carbon 2)
RNA ­ ribose sugar
Organic bases
Bases in DNA
Adenine (purine) Thymine (pyrimidine)
Cytosine (pyrimidine) Guanine (purine)
In RNA thymine is replaced by uracil (pyrimidine)

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Formation of DNA
Polynucleotides join together by condensation reactions to form a
polynucleotide chain.
Phosphodiester bonds form between the phosphate of one nucleotide
and the 4carbon of another. This is a covalent bond.
In DNA two polynucleotide chains twist around each other to form a
double helix.
The phosphate and 5carbon sugar form the phosphate backbone of
the helix.
Base pairs on inside of helix pair up.
Bases joined by hydrogen bonds.…read more

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Only 20 different amino acids in the body
Different enzymes are made by different arrangements of amino acids.
1 amino acid is coded for by 3 DNA bases
for example ­ GGA codes for Glycine
Why 3 bases?
3 bases codes for all the different amino acids found in the body.…read more

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Protein synthesis uses 3 types of RNA
Messenger RNA _ mRNA
Single polynucleotide chain
Formed from DNA, uses DNA as template
Uracil not Thymine
Formed in nucleus during transcription.
Carries instructions from DNA to ribosomes.
Transfer RNA tRNA
Single polynucleotide chain folded into a clover shape
Has a binding site for an amino acid
Has 3 unpaired bases at the bottom (anticodon)
Kept in shape by hydrogen bonds
Present in cytoplasm
Transports amino acids to ribosomes for assembly into polypeptides.…read more

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The polyA tail acts as a signal for the mRNA to leave the nucleus. It provides
some protection from the enzyme action.
Translation is the process in which the information carried on the mRNA is
used in the synthesis of polypeptide chain. The base sequence of the mRNA
determines the sequence of amino acids in the polypeptide. It occurs in the
1. Introns are removed from the mRNA strand by enzyme action.
2.…read more

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Notes Peptide bonds between the amino acids forms a polypeptide chain
The polypeptide chain doesn't become and enzyme or a protein until it
is folded.
Chromosome Number
And Structure
Chromosones occur in pairs, this is that there are two identical sets of
chromosomes in each cell.
Diploid (2n) 46 chromosones 23 pairs
Haploid (n) 23 chromosones
DNA is only in chromosomes when the cell is dividing
When cell is not dividing DNA is in form of chromatin, like a mass of
thread.…read more

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The Cell Cycle
Cell cycle is the events from when a cell is formed to when it divides. It
contains three stages
1. Interphase
2. Mitosis
3. Cytokinesis
Interphase is the longest stage in the cell cycle (more than ¾)
Split into three parts
G1 ­ growth phase ­ new organelles are made, lots of protein
S ­ DNA synthesis phase ­ new DNA made, DNA replication, the
amount of DNA doubles to 92 chromosomes.…read more

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Four stages
Prophase (DNA present as mass of threads)
DNA present as chromatin
Threads of chromatin thicken and shorten due to spiralisation.
Now two chromatids held together by centromere, this is a
Centrioles move to opposite ends of the cell.
Microtubules form the spindle
Nuclear envelope breaks down.
Chromosomes attach to spindle fibres
Centomere attaches to the spindle
Chromosomes pulled apart when spindle fibres get shorter.…read more

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Use 5mm of root tip.
Cover with 9 drops of aceitic orcein and 1 drop of hydrochloric acid.
The hydrochloric acid breaks it up.
Heat gently.
Cut away 1mm of root tip.
Add acetic acid to stain the cells.
Use a cover slip and gently squash the root tip.…read more


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