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LAMC Bio 3 Lecture Notes: Chapter 10 - Molecular Biology of the Gene
Reading Assignment: Chapter
10
Learning Outcomes:
By the end of this lecture you should be able to:
- describe the structure of DNA and RNA, and the differences between them
- explain the process of transcription and translation, where in the cell they
occur and the steps in each process
I. Structure of DNA and RNA
Experiment by Hershey and Chase showed that DNA is the
genetic material; it is a double-stranded helix.
RNA is single-stranded and functions in protein synthesis, (and is the genetic
material of some viruses).
DNA and RNA are composed of macromolecules called nucleic acids, which are long
chains of nucleotides.
The nucleotides found in DNA are adenine (A), guanine (G), thymine (T), and
cytosine (C). RNA contains uracil
(U) instead of thymine (T).
II. DNA Replication (Transcription):
DNA replication depends on specific base pairing: (A-T and
G-C). In RNA, uracil (U) pairs with
adenine (A).
The DNA genotype is expressed as proteins, which provide the molecular basis for
phenotypic traits.
Transcription: the transfer of genetic information from DNA into an RNA
molecule.
Translation: the transfer of information in the RNA into a protein.
The flow of genetic information in a cell: DNA (nucleus) ŕ
RNA (nucleus) ŕ
protein (cytoplasm).
Triplet code: the genetic instructions for the amino acid sequences of a protein
are written in DNA and RNA as a series of three-base words, called
codons.
Each codon specifies only one amino acid, although several codons may code
for the same amino acid (redundancy).
The steps of transcription:
1) Initiation: the attachment of the RNA polymerase to a
“promoter” sequence, and the start of RNA synthesis
2) Elongation of the RNA: as RNA synthesis continues, the RNA strand peels away
from its DNA template, allowing the two separate DNA strands to come back
together in the region already transcribed
3) Termination: the synthesis of RNA ends when the RNA polymerase reaches a
“terminator” sequence
Eukaryotic RNA is processed before leaving the nucleus:
1) a cap and a tail section are added to facilitate the transport of the RNA
from the nucleus
2) The introns are removed (spliced); only the exons remain.
Messenger RNA (mRNA): conveys genetic information from the
DNA to the protein synthesis machinery of the cell.
Transfer RNAhe
DNA to the protein synthesis machinery of the cell.
Transfer RNA (tRNA): works as an interpreter between the codons encoded in the
mRNA and the amino acids floating around in the cell.
Protein synthesis involves the construction of a specific sequence of
amino acids, and this cannot occur without the tRNA.
III. Protein Synthesis (Translation):
Ribosomes build polymers of amino acid sequences, which
make up proteins.
A ribosome consists of two subunits, each made up of proteins and ribosomal RNA
(rRNA).
The steps of translation:
1) Initiation: an mRNA binds to a small ribosomal subunit;
an initiator tRNA binds to a “start” codon.
Then, a large ribosomal subunit binds to the small one, creating a
functional ribosome.
2) Codon recognition: The anticodon of a new tRNA molecule, carrying its amino
acid, pairs with the mRNA codon.
3) Peptide bond formation: the bond between the tRNA and its amino acid
separates; a new bond between the two amino acids forms.
A chain of amino acids (protein molecules) is formed in this way.
4) Termination: the elongation of amino acids ends when a “Stop” codon is
reached. The protein is released
from the tRNA and exits the ribosome.
Mutation: any change in the nucleotide sequence of DNA;
mutations can involve large regions of a chromosome, or a single nucleotide
pair, and they can change the meaning of genes.
Mutations can occur due to errors during DNA replication, or they can be caused
by a physical or chemical agent.
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