Biology - Topic 2.3

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Define Nucleotide
molecule consisting of a 5 carbon sugar, a phosphate group and a nitrogenous base
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Phosphate esters of pentose sugars, where a nitrogenous base is bonded to the C1 of the pentose sugar, and a phosphate group is bonded to C3orC5 (covalently). They are the monomers of DNA+RNA. help regulate metabolic pathways, components of coenzymes
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Structure of DNA
A polymer made of nucleotides. 2 polynucleotide strands that are ANTIPARALLEL. 4 nitrogenous bases. Covalent bond between sugar+phosphate grp is phosphodiester bond.
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Have 2 rings. Adenine + Guanine
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1 ring. Cytosine, Uracil + Thymine (CUT)
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Hydrogen Bonding between antiparallel strands
Adenine+Thymine joined by 2. Guanine+Cytosine joined by 3. Purine always bonds with pyramidine. The H bonds allow the molecule to unzip.
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The Antiparallel Sugar
The opposite direction of the 2 strands refers to the direction that the 3rd+5th carbon atoms on the sugar are facing. The 5' end is where the phosphate grp is attached to the 5th C atom on the sugar. 3' end is 3rd C atom.
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How DNA is organised in Eukaryotic Cells
Majority is in the nucleus. Each molecule of DNA is wound around histone proteins into chromosomes. Also a loop of DNA, without histone proteins, inside mitochondria and chloroplasts
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How DNA is organised in Prokaryotic Cells
DNA is in a loop and is within the cytoplasm. No nucleus. No histone proteins. 'Naked DNA'
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Define DNA polymerase
an enzyme that catalyses the formation of DNA from activated deoxyribose nucleotides, using single stranded DNA as a template
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Define Helicase
an enzyme that catalyses the breaking of hydrogen bonds between the nitrogenous pairs of bases in a DNA molecule
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Define Semi-Conservative Replication
how DNA replicates, resulting in 2 new molecules, each of which contains 1 old strand and 1 new strand. 1 old strand is conserved in each molecule
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To make a new copy, each DNA molecule must.......
UNWIND - the double helix is untwisted, catalysed by a gyrase enzyme. UNZIP - hydrogen bonds between nucleotide bases are broken, catalysed by DNA helicase, resulting in 2 strands of DNA with exposed bases.
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What happens after DNA unwinds+unzips? 1
(1) free phosphorylated nucleotides are bonded to the exposed complementary bases (2) DNA polymerase catalyses the addition of new bases in 5' to 3' direction of both strands, using each strand as a template.
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Leading+Lagging Strands
The leading strand is synthesised continuously, whereas the lagging strand is done in fragments that are later joined, catalysed by ligase enzymes
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What happens after DNA unwinds+unzips? 2
(2) hydrolysis of the activated nucleotides to release the extra phosphate grps takes place, and this supplies the energy to make phosphodiester bonds between sugar of one molecule and phosphate group on the next.
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During replication, errors can occur and the wrong nucleotide could be added. There are enzymes that double check the strands. Not all mutations are harmful.
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Evidence for Semi-Conservative Replication 1
1958. 2 scientists grew E.coli bacteria for 14 generations in medium containing heavy isotope 15N (extra neutron in each nucleus). After 14 replications, all DNA is heavy. Transferred some into medium with 14N isotope, left for 1 replication.
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Evidence for Semi-Conservative Replication 2
DNA after this replication was hybrid (1 strand 14N, 1 strand 15N). Replicated once more, DNA extracted+centrifuged. Produced 2 bands of DNA, 1 hybrid and 1 light (14N). Shows semi-conservative replication.
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Define Gene
a length of DNA that codes for a polypeptide or for a length of RNA that is involved in regulating gene expression
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They are specific lengths of DNA, containing code that determines the sequence of amino acids. Within each gene is is sequence of base triplets that code for the amino acid sequence. As long as this is correct, the protein will fold into its tertiary
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The Need for mRNA
DNA cannot pass into the cytoplasm, so a copy of each gene has to be transcribed onto a length of RNA. In this form, the sequence of base triplets, called codons, can pass into the ribosomes, ensuring the instructions are translated.
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Define Transcription
the process of making mRNA from a DNA template
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Define Translation
the formation of a protein, in the ribosomes, by assembling amino acids into a particular sequence according to the coded instructions, carried by mRNA
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3 words describing the Genetic Code
UNIVERSAL - in almost all organisms the same triplets codes for the same AA. DEGENERATE - each amino acid has more than one base triplet, reducing effect of mutations. NON-OVERLAPPING - every 3 bases
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Transcription 1
(1) gene unwinds+unzips (2) hydrogen bonds between complementary bases break (3) RNA polymerase catalyses the formation of temporary H bonds between RNA nucleotides and complementary DNA bases. DNA strand is the template strand
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Transcription 2
(4) a length of RNA that is complementary to the template strand is produced. It is a copy of the other DNA strand - the coding strand. (5) the mRNA passes out of the nucleus
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Define Codon
a triplet of bases on a length of mRNA
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Define Anticodon
a triplet of bases on tRNA molecule, complementary to the mRNA codon
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tRNA molecules are made in the nucleolus and pass into the cytoplasm. Single strand polynucleotides. One end has a trio of bases that recognise and attach to a specific amino acid. At the loop of the 'hairpin' is another trio of bases, anticodon
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(1) tRNA molecules bring AAs and anticodon forms temporary hydrogen bonds to the codon on mRNA. (2) ribosome moves along mRNA, reading code, once two AAs are adjacent a peptide bond forms between them (3) ATP needed for polypeptide synthesis
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(4) AA sequence is determined by the sequence of triplets of nucleotide bases on DNA (5) after polypeptide assembled, mRNA breaks down and molecules recycled. (6) new polypeptide is helped by chaperone proteins to fold into tertiary structure
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Phosphate esters of pentose sugars, where a nitrogenous base is bonded to the C1 of the pentose sugar, and a phosphate group is bonded to C3orC5 (covalently). They are the monomers of DNA+RNA. help regulate metabolic pathways, components of coenzymes

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Structure of DNA


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