BIO342H5 Chapter Notes - Chapter 14: Peptidyl Transferase, Ribosomal Rna, Start Codon

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CHAPTER 14: MOLECULAR GENETICS
Genes are composed of DNA which contain information coded in the sequence of its
base pairs, providing the cell with a blueprint for protein synthesis
DNA has the ability to self-replicate and is the basis of heredity
DNA
A. Structure
o The nucleotide is the basic unit of DNA
Made of a deoxyribsoe bonded to a P group and nitrogenous base
o Two types of bases: purines (double-ringed) and pyrimidines
(single-ringed)
o Purines in DNA are adenine (A) and guanine (G)
o Pyrmidines in DNA are cytosine (C) and thymine (T)
o Nucleotides bond together to form polynucleotides
The 3’ hydroxyl group of the sugar on one nucleotide is joined to the 5’
hydroxyl group of the adjacent sugar by a phosphodiester bond
o Is a double-stranded helix with a sugar-phosphate backbone
T always forms two hydrogen bonds with A
G always forms three hydrogen bonds with C
This base-pairing forms ‘rungs’ on the interior of the double helix that link
the two polynucleotide chains together
o The strands are positioned antiparallel to each other
One strand has a 5’ 3’ polarity
The complementary strand has a 3’ 5’
B. DNA replication (eukaryotic)
1. Semiconservative Replication
The helix unwinds and each strand acts as a template for complementary
base-pairing in the synthesis of two new daughter helices
The daughter helix contains the parent helix and a new synthesized helix
o DNA replication is semiconservative
2. Origin of Replication
Begins at specific sites along the DNA called origin or replication
o Proceeds in both directions simultaneously and forms a replication
fork
Rate of about 50 nucleotide additions per second (in mammals)
3. Unwinding and Initiation
DNA helix unwound by helicase
Single-strand binding (SSB) proteins prevents the strands from recoiling
DNA gyrase (topoisomerase) introduces negative supercoils to the DNA
molecule to enhance the helicase action
A primer (made of RNA) is necessary for the initiation process
o Primase synthesizes the primer
The primer binds to a segment of DNA complementary and serves as the
site for nucleotide addition
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The first nucleotide binds to the 3’ end of the primer chain
4. Synthesis
Proceeds in the 5’ 3’ direction
Catalyzed by DNA polymerases
As the helix unwinds, free nucleotides are aligned opposite the parent
strand and form phosphodiester linkages
o The bases form H-bonds with their components
One daughter strand is the leading strand that is continuously
synthesized by DNA polymerase in the 5’ 3’ direction
The other strand is the lagging strand synthesized discontinuously in the
5’ 3’ direction
o Forms a series of short segments called Okazaki fragments
o The overall growth of this strand occurs in the 3’ 5’ direction
o Each fragment is started with a primer and these RNA primers will
be removed and replaced with DNA
o The fragments are linked covalently by DNA ligase
RNA
Similar to DNA except that its sugar is ribose, contains uracil (U) instead of thymine, and
it is usually single-stranded
Can be found in both the nucleus and the cytoplasm
There are several types
A. Messenger RNA (mRNA)
o Carries the complement of a DNA sequence and transports it from the nucleus to
the ribosomes where protein synthesis occurs
o Monocistronic one mRNA strand codes for one polypeptide
B. Transfer RNA (tRNA)
o Found in the cytoplasm
o Aids in the translation of mRNA’s nucleotide code into a sequence of amino acids
o Brings amino acids to the ribosomes during protein synthesis
o There is at least one type of tRNA for each amino acid
Therefore, 40 know types of tRNA
C. Ribosomal RNA (rRNA)
o Structural component and is the most abundant of all types of RNA
o rRNA is synthesized in the nucleolus
D. Heterogeneous nuclear RNA (hnRNA)
o Large ribonucleoprotein complex that is the precursor of mRNA
Protein Synthesis
A. Transcription
o Information coded in the base sequence of DNA is transcribed into a strand of
mRNA (similar process to DNA replication)
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o DNA helix unwinds at the point of transcription, and synthesis occurs in the 5’
3’ direction, using only one DNA strand (the antisense strand) as a template
o mRNA is synthesized by the enzyme RNA polymerase
binds to sites on the DNA called promoters to being RNA synthesis
o Synthesis continues until the polymerase encounters a termination sequence
o The strand is then processed and leaves the nucleus through nuclear pores
B. Post-Transcriptional RNA Processing
o Most eukaryotic DNA does not code for proteins
There are non-coding sequences found between the coding sequences
o A typical gene consists of several coding sequences, exons, interrupted by
noncoding sequences, introns.
o The RNA initially transcribed is a precursor molecule, hnRNA, which contains
both introns and exons.
When processed, the introns are cleaved and removed
The exons are spliced to form a mRNA molecule coding for a single
polypeptide
C. The Genetic Code
o The DNA language (A, T, C, and G) must be translated into the language of the
proteins (20 amino acids)
Done through the triplet code (i.e. codons)
o The sequence of three consecutive bases codes for a particular amino acid
Universal for almost all organisms
o Because there are 64 different codons possible based on the triplet code and
there are only 20 amino acids, most amino acids have more than one codon
specifying them
Known as degeneracy or redundancy of the genetic code
D. Translation
o Process whereby mRNA codons are translated mRNA codons are translated into
sequence of amino acids
o Occurs in the cytoplasm and involves tRNA, ribosomes, mRNA, amino acids,
enzymes, and other proteins
tRNA
o Brings amino acids to the ribosomes in the correct sequence for
polypeptide synthesis
o Recognizes both the amino acid and the mRNA codon
o One end contains a three-nucleotide sequence (anti-codon) which
is complementary to one of the mRNA codons
o Other end is the site of amino acid attachment and consists of a
CCA sequence for all tRNA
o Each amino acid has its own aminoacyl-tRNA synthetase which
binds to both the amino acid and its corresponding tRNA
Forms an aminoacyl-tRNA complex
Ribosomes
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Document Summary

Genes are composed of dna which contain information coded in the sequence of its base pairs, providing the cell with a blueprint for protein synthesis. Dna has the ability to self-replicate and is the basis of heredity. Dna: structure, the nucleotide is the basic unit of dna. The 3" hydroxyl group of the sugar on one nucleotide is joined to the 5" hydroxyl group of the adjacent sugar by a phosphodiester bond. Is a double-stranded helix with a sugar-phosphate backbone. T always forms two hydrogen bonds with a. G always forms three hydrogen bonds with c. This base-pairing forms rungs" on the interior of the double helix that link the two polynucleotide chains together: the strands are positioned antiparallel to each other. One strand has a 5" 3" polarity. The complementary strand has a 3" 5": dna replication (eukaryotic, semiconservative replication.

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