Download DNA - Bio by Aguayo

Nucleic Acids and Protein Synthesis
Cell  Nucleus
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Chromosomes  Genes
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DNA
Proteins
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Organic molecules (macromolecules) made by cells
Make up a large part of your body
Used for growth, repair, enzymes, etc.
Composed of long chains of small units called amino acids bonded together by peptide bonds
Twenty amino acids exist
DNA
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Deoxyribonucleic acid
Contains the instructions for making proteins from 20 different amino acids
Structure discovered by Watson & Crick in 1953
Sides made of pentose (5-sided) sugars attached to phosphate groups by phosphodiester bonds
Pentose sugar called Deoxyribose
Steps or rungs of DNA made of 4 nitrogen-containing bases held together by weak hydrogen bonds
Purines (double carbon-nitrogen rings) include adenine (A) and guanine (G)
Pyrimidines (single carbon-nitrogen rings) include thymine (T) and cytosine (C)
Base pairing means a purine bonds to a pyrimidine (Example: A --- T and C --- G)
Coiled, double stranded molecule known as double helix
Make up chromosomes in the nucleus
Subunits of DNA called nucleotides
Nucleotides contain a phosphate, a Deoxyribose sugar, and one nitrogen base (A,T,C, or G)
Free nucleotides also exist in nucleus
Most DNA is coiled or twisted to the right
Left twisted DNA is called southpaw or Z-DNA
Hot spots which can result in mutations occur where right & left twisted DNA meet
History of DNA discovery
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Freidrich Miescher (1868) found nuclear material to be ½ protein & ½ unknown substance
1890’s, unknown nuclear substance named DNA
Walter Sutton (1902) discovered DNA in chromosomes
Fredrick Griffith (1928) working with Streptococcus pneumoniae conducted transformation
experiments of virulent & nonvirulent bacterial strains
Levene (1920’s) determined 3 parts of a nucleotide
Hershey & Chase (1952) used bacteriophages (viruses) to show that DNA, not protein, was the cell’s
hereditary material
Rosalind Franklin (early 1950’s) used x-rays to photograph DNA crystals
Erwin Chargraff (1950’s) determined that the amount of A=T and amount of C=G in DNA; called
Chargaff’s Rule
Watson & Crick discovered double helix shape of DNA & built the 1st model
DNA Replication
 Process by which DNA makes a copy of itself
  Occurs during S phase of interphase before cell division
 Extremely rapid and accurate (only 1 in a billion are incorrectly paired)
 Requires many enzymes & ATP (energy)
 Begins at special sites along DNA called origins of replication where 2 strands open & separate
making a replication fork
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Nucleotides added & new strand forms at replication forks
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DNA helicase (enzyme) uncoils & breaks the weak hydrogen bonds between complementary bases
(strands separate)
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DNA polymerase adds new nucleotides to the exposed bases in the 5’ to 3’ direction
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Leading strand (built toward replication fork) completed in one piece
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Lagging strand (built moving away from the replication fork) is made in sections called Okazaki
fragments
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DNA ligase helps join Okazaki segments together
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DNA polymerase proofreads the new DNA checking for errors & repairing them; called excision
repair
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Helicase recoils the two, new identical DNA molecules
RNA
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Ribonucleic acid
Single stranded molecule
Found in nucleus & cytoplasm
Contains ribose sugar
Contains the nitrogen base uracil (U) instead of thymine so A pairs with U
Base pairings are A-U and C-G
Three types of RNA exist (mRNA, TRNA, & rRNA)
mRNA
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Messenger RNA
Single, uncoiled, straight strand of nucleic acid
Found in the nucleus & cytoplasm
Copies DNA’s instructions & carries them to the ribosomes where proteins can be made
mRNA’s base sequence is translated into the amino acid sequence of a protein
Three consecutive bases on mRNA called a codon (e.g. UAA, CGC, AGU)
Reusable
tRNA
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Transfer RNA
Single stranded molecule containing 80 nucleotides in the shape of a cloverleaf
Carries amino acids in the cytoplasm to ribosomes for protein assembly
Three bases on tRNA that are complementary to a codon on mRNA are called anticodons (e.g. codonUUA; anticodon- AAU)
Amino Acid attachment site across from anticodon site on tRNA
Enters a ribosome & reads mRNA codons and links together correct sequence of amino acids to
make a protein
Reusable
rRNA
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Ribosomal RNA
Globular shape
Helps make up the structure of the ribosomes
Aids in moving ribosomes along the mRNA strand as amino acids are linked together to make a
protein
Amino Acids
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20 exist
Linked together in a process called protein synthesis in the cytoplasm to make polypeptides (subunits
of proteins)
DNA contains the instructions for making proteins but is too large to leave the nucleus
Three consecutive bases on DNA called a triplet (e.g. TCG, ATG, ATT)
mRNA codon table tells what 3 bases on mRNA code for each amino acid (64 combinations of 3 bases)
Methionine (AUG) on mRNA is called the start codon because it triggers the linking of amino acids
UAA, UGA, & UAG on mRNA signal ribosomes to stop linking amino acids together
DNA
Codon
mRNA
Codon
tRNA
Anticodon
Amino
Acid
GCU
TAC
AUU
UUU
TCA
UCU
CTT
ACU
ACU
Protein Synthesis
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Consists of 2 parts --- Transcription & Translation
Begins in the nucleus with mRNA copying DNA’s instructions for proteins (transcription)
Completed in the cytoplasm when tRNA enters ribosomes to read mRNA codons and link together
amino acids (translation)
Steps in Transcription
1. DNA helicase (enzyme) uncoils the DNA molecule
2. RNA polymerase (enzyme) binds to a region of DNA called the promoter which has the start codon
TAC to code for the amino acid methionine
3. Promoters mark the beginning of a DNA chain in prokaryotes, but mark the beginning of 1 to
several related genes in eukaryotes
4. The 2 DNA strands separate, but only one will serve as the template & be copied
5. Free nucleotides are joined to the template by RNA polymerase in the 5’ to 3’ direction to form the
mRNA strand
6. mRNA sequence is built until the enzyme reaches an area on DNA called the termination signal
7. RNA polymerase breaks loose from DNA and the newly made mRNA
8. Eukaryotic mRNA is modified (unneeded sections snipped out by enzymes & rejoined) before leaving
the nucleus through nuclear pores, but prokaryotic RNA isn’t
9. All 3 types of RNA called transcripts are produced by this method
Steps in Translation
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mRNA brings the copied DNA code from the nucleus to the cytoplasm
mRNA attaches to one end of a ribosome; called initiation
tRNA’s attach the correct amino acid floating in the cytoplasm to themselves
tRNA with its attached amino acid have 2 binding sites where they join the ribosome
The tRNA anticodon “reads” & temporarily attaches to the mRNA codon in the ribosome
Two amino acids at a time are linked together by peptide bonds to make polypeptide -chains (protein
subunits); called elongation
7. Ribosome(s) move along the mRNA strand until they reach a stop codon (UAA, UGA, or UAG);
called termination
8. tRNA’s break loose from amino acid, leave the ribosome, & return to cytoplasm to pick up another
amino acid