DNA STRUCTURE AND REPLICATION
Nitrogenous bases joined together through hydrogen bonds.
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Adenine
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Thymine
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Cytosine
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Guanine
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PURINES (2 rings)
PYRAMIDINES (3 rings)
ADENINE
THYMINE
GUANINE
CYTOSINE
CHARGAFF'S RULE - A2T G3C
- Adenine and Thymine joined with 2 hydrogen bonds.
- Guanine and Cytosine joined with 3 hydrogen bonds
Purine + Pyramidine -> Base Pair
G3C is more stable than A2T becuase it has more hydrogen bonds.
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COMPLEMENTARY BASE PAIRING
DNA - Deoxyribonucleic Acid
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Double helix (two strands)
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Each strand is made up of nucleotide units.
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One nucleotide: Sugar + Phosphate + Nitrogenous Base
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Backbone
PHOSPHATE
SUGAR
(ribose)
*very strong backbone due to covalent bonds
BASE
TRIPLET CODONE - 3 nucleotides
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Conveys information
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Make protein and enzymes
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Switch genes on/off
phospho-diester bond
*Uracil is complementary to adenine in RNA (Thymine is not found in RNA)
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*Nucleotides are joined together through covalent bonds (permanent and very strong)
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*Base pairs inside DNA are joined together through hydrogen bonds
C5
C5
5' (5 prime)
C1
C2
C3
C4
3' (3 prime)
C2
C3
C1
C4
C5
C1
C4
C3
C2
C2
C3
C4
C1
C5
- The two strands in DNA are anti-parallel to each other (run in the opposite directions).
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- Genes are heritable factors that control specific characteristics.
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- Sugar-phosphate backbone is hydrophilic.
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- Nitrogenous bases are very reactive, thus found on the inside of DNA.
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- Nucleosomes hold the DNA together to form the frame of the chromosomes.
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- DNA wraps twice around each nucleosome before moving to the next one
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- H1 Linker protein involved to keep the nucleosome in place.
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- Supercoiled sections of genes cannot be expressed as they cannot be opened up for transcription.
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- Supercoiling allows control over which genes are expressed.
3' (3 prime)
5' (5 prime)
During transcription, the mRNA molecule removes the introns and attaches all the exons together in the process of splicing
DNA is made up of single copy genes and highly repetitive sequences.
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SINGLE COPY GENES
- 1.5% of genome makes polypeptides
- Of which 3% code for on/off gene switches (TAA/TAG/TGA)
- Each codon (3 nucleotides) codes for one amino acid
HIGHLY REPETITIVE SEQUENCES
- 5-45% of genome
-Satellite DNA (can be 5-200 base pairs)
- Do not affect phenotype (exons)
EXONS - Coding regions
INTRONS - Non-coding regions
Use this table to figure out which amino acid or on/off switch different codons code for.
DNA REPLICATION DURING SYNTHESIS ('S' phase in interphase)
ENYZMES INVOLVED:
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HELICASE - Unwinds and unzips DNA and breaks H-H bonds between base pairs
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RNA PRIMASE - Adds a small sequence of RNA molecules to DNA called 'RNA primers'.
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DNA POLYMERASE - Attaches in a 3' to 5' direction on DNA strands and creates a complementary strand for both DNA strands. Cannot attach to nucleotides on its own, needs RNA primers
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DNA POLYMERASE I - Removes the RNA strand after DNA Polymerase III bonds and makes complementary pairs.
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DNA LIGASE - Covers the area of RNA molecules that was removed by DNA Polymerase I. Attaches okazaki fragments together.
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RNA PRIMERS - Initiation sites for DNA Polymerase III on the lag strand
OKAZAKI FRAGMENTS - Sections of new DNA formed on the lag strand
PARENT STRAND - Runs in a 3' to 5' direction
LAG STRAND - Runs in a 5' to 3' direction (anti-parallel to parent strand)
- The parent strand acts as a template for the new (complementary) strands.
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- The base sequence is conserved so DNA is identical to the new copied DNA.
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- DNA replication is semi-conservative. In the two new DNA strands formed, one is the parent strand, and one is a newly formed one.
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DNA Replication is initiated at many points in eukaryotes. This makes DNA replication faster and more efficient.
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Points are known as: Origins of Initiation
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Proteins (Origin recognition complexes) will bind here, allowing DNA Helicase to attach.
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In the diagram above, the parent strand automatically gets a complementary strand as DNA Polymerase III once attached works in the same direction as the DNA helicase, forming a complementary strand in one go.
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The strand opposite the parent strand is called the lag strand. DNA replication for this strand is slightly more complicated.
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RNA Primase leaves RNA primers along the lag strand. These markers are used as initiation points for DNA Polymerase III.
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When DNA polymerase III reaches another RNA primer on the strand, it detaches and attaches to the next RNA primer following DNA Helicase.
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DNA Polymerase I moves along the replication fork removing RNA Primers.
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DNA Ligase attaches the okazaki fragments into a continuous strand of DNA (this uses ATP). ​
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