12-3 RNA and Protein Synthesis
Genes are coded DNA instructions that control the production of proteins.
Genetic messages can be decoded by copying part of the nucleotide sequence from DNA into
RNA.
RNA contains coded information for making proteins.
The Structure of RNA
There are three main differences between RNA and DNA:
 The sugar in RNA is ribose instead of deoxyribose.
 RNA is generally single-stranded.
 RNA contains uracil in place of thymine.
Types of RNA
There are three main types of RNA:
 messenger RNA
 ribosomal RNA
 transfer RNA
 Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into
proteins.
 Ribosomes are made up of proteins and ribosomal RNA (rRNA).
 During protein construction, transfer RNA (tRNA) transfers each amino acid to the
ribosome.
Protein Synthesis
Transcription
DNA is copied in the form of RNA
This first process is called transcription.
The process begins at a section of DNA called a promoter.
During transcription, RNA polymerase uses one strand of
DNA as a template to assemble nucleotides into a strand
of RNA.
RNA Editing
Some DNA within a gene is not needed to produce a protein.
Introns ― The DNA sequences that DO NOT code for
proteins.
 Sometimes called junk DNA
Exons ― The DNA sequences that code for proteins.
The introns are cut out of RNA molecules.
The exons are the spliced together to form mRNA.
A cap and tail are added to form the final mRNA
molecule.
The Genetic Code
The genetic code is the “language” of mRNA
instructions.
The code is written using four “letters” (the bases:
A, U, C, and G).
A codon consists of three consecutive nucleotides
on mRNA that specify a particular amino acid.
The genetic code shows the amino acid to
which each of the 64 possible codons
corresponds. To decode a codon, start at the
middle of the circle and move outward.
Translation
Translation is the decoding of an mRNA message into a polypeptide chain (protein).
Translation takes place on ribosomes.
During translation, the cell uses information from messenger RNA to produce proteins.
During translation, or protein synthesis, the cell uses information from messenger RNA to
produce proteins. The cell uses all three main forms of RNA during this process.
 messenger RNA
 ribosomal RNA
 transfer RNA
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The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA.
The process continues until the ribosome reaches a stop codon.
12–4 Mutations
Mutations are changes in the genetic material.
Kinds of Mutations
Mutations that produce changes in a single gene are known as gene mutations.
Mutations that produce changes in whole chromosomes are known as chromosomal mutations.
Gene Mutations
Gene mutations involving a change in one or a few nucleotides are known as point mutations
because they occur at a single point in the DNA sequence.
Point mutations include substitutions, insertions, and deletions.
Substitutions usually affect no more than a single amino acid.
 One base replaces another.
The effects of insertions or deletions are more dramatic.
The addition or deletion of a nucleotide causes a shift in the grouping of codons.
Changes like these are called frameshift mutations.
In an insertion, an extra base is inserted into a base sequence.
In a deletion, the loss of a single base is deleted and the reading frame is shifted.
Chromosomal Mutations
Chromosomal mutations involve changes in the number or structure of chromosomes.
Chromosomal mutations include deletions, duplications, inversions, and translocations.
Chromosomal mutations involve changes in whole chromosomes.
Deletions involve the loss of all or part of a chromosome.
Duplications produce extra copies of parts of a chromosome.
Inversions reverse the direction of parts of chromosomes.
Translocations occurs when part of one chromosome breaks off and attaches to another.
Significance of Mutations
Many mutations have little or no effect on gene expression.
Some mutations are the cause of genetic disorders.
Polyploidy is the condition in which an organism has extra sets of chromosomes.