Chapter 13
RNA & Protein Synthesis
Section 13.1 RNA
Be Able To:
● Explain the difference between DNA and RNA.
● Describe the steps of transcription.
Key Terms:
● RNA, messenger RNA (mRNA), ribosomal RNA (rRNA),
transfer RNA (tRNA), transcription, RNA polymerase,
promoter, intron, exon.
The Role of RNA
● RNA is a nucleic acid composed of nucleotides that differs from DNA by:
1) The sugar of RNA is ribose instead of deoxyribose
2) RNA is generally single stranded whereas DNA is double stranded
3) RNA contains a nitrogen based called uracil (U) in place of thymine (T).
● There are three types of RNA each with a different job:
1) mRNA or messenger RNA carries the copy of a DNA gene to other parts of the cell.
2) rRNA or ribosomal RNA makes up part of a ribosome called subunits
3) tRNA or transfer RNA carries amino acids to the ribosome based on the mRNA code.
Which of the following describes RNA?
RNA Synthesis
● Transcription is the synthesis of an RNA molecule from a DNA template.
● Segments of DNA serve as templates to make complimentary copies.
● RNA polymerase is an enzyme that links together the growing chain of RNA nucleotides during transcription of the DNA template strand.
● RNA polymerase:
1) Binds to DNA and separates the strands
2) It uses one strand, template strand, to assemble nucleotides that is complementary to the DNA template or gene.
● Transcription has 3 steps:
1) Initiation at the promoter, region of DNA, that signals where to start transcription
2) Elongation RNA nucleotides added to create a copy of the DNA template or gene
3) Termination end of transcription with a specific DNA sequence
Transcription
The process by which the genetic code of DNA is
copied into a strand of RNA is called
RNA Editing
● The initial mRNA strand or pre-mRNA transcript requires editing before it is translated or read by the ribosome.
● Introns are nucleotide
sequences that are removed by enzymes from the pre-mRNA before it leaves the nucleus.
● Exons are sequences that
remain intact and are spliced
together to form the final mRNA
transcript.
The portion of the mRNA code that is cut out and
discarded are called?
Section 13.2 Ribosomes & Protein Synthesis
Be Able To:
● Explain how the genetic code is read.
● Explain the role of the ribosome in assembling proteins.
● Define the central dogma of molecular biology.
Key Terms:
● Polypeptide, genetic code, codon, anticodon, translation,
and gene expression.
The Genetic Code
● The first step in decoding a gene is transcribing a nucleotide sequence from DNA to RNA.
● The genetic code is a collection of mRNA codons that directs the building of a protein from amino acids.
● A codon is a sequence of 3 consecutive mRNA nucleotides that specify a single amino acid to be added to a polypeptide chain, long chains of amino acids used to form a protein.
● Many amino acids, 20 in total, can be specified by more than one codon.
● The amino acid methionine serves as a start codon to initiate translation, whereas a stop codon ends translation without adding an amino acid to the polypeptide chain.
In messenger RNA, each codon specifies a
particular
Translation
● The sequence of nucleotide bases in mRNA is a set of instructions that gives the order of amino acids to be joined in a polypeptide chain.
● When the polypeptide chain is complete, it folds into a final shape to become a functional protein.
● tRNA or transfer RNA holds an amino acid that complements a mRNA codon by its own anticodon.
mRNA
tRNA
Translation
● Ribosomes read the codons of mRNA to
assemble a polypeptide chain in the process of translation in 3 steps:
1) Initiation - the ribosome attaches to mRNA at the start codon AUG
2) Elongation - tRNA delivers amino acids to the ribosome which attaches them in a growing chain
3) Termination- occurs when the stop codon is reached and the ribosome releases the polypeptide.
The Process of Translation
Which mRNA sequence signifies the end of
translation?
The Molecular Basis of Heredity
● Molecular biology was born from the work of Gregor Mendel, from the famous studies that linked nucleic acids to the inheritance of traits, and from the discovery of DNA structure by Watson and Crick.
● Molecular biology seeks to explain living organisms by studying them at the level of molecules.
● The central dogma of molecular biology is that information is transferred from DNA to RNA to protein.
● Although there are exceptions, such as viruses, the central dogma helps to explain gene
expression, building proteins from the DNA genetic code.
Which of the following statements about the genetic
code is true?
Section 13.3 Mutations
Be Able To:
● Explain the scientific understanding of mutation.
● Explain how mutations affect genes.
Key Terms:
● Mutation, point mutation, frameshift mutation, mutagen,
polyploidy.
Types of Mutations
● A mutation is a change in the genetic material of the cell.
● Mutations can be heritable changes in genetic information.
● Mutations fall into two main categories:
1) Gene mutations
2) Chromosomal mutations
● Gene mutations that involve one or a few nucleotides are point mutations.
● Point mutations can be:
1) Substitutions- one nucleotide base is changed to a different nucleotide base
2) Insertion- one nucleotide is added to the DNA sequence 3) Deletion- one nucleotide is removed from the DNA
sequence
● Insertions and deletions are frameshift mutations because they alter the length of the DNA code which changes the intended amino acid sequence.
Changes in DNA sequences that affect
genetic information are known as?
Chromosomal Mutations
● Chromosomal mutations involve changes in the structure of chromosomes or in the number of chromosomes.
● Types of chromosomal mutations:
1) Deletion - loss of all or part of the chromosome
2) Duplication - produces extra copy of all or part of a chromosome
3) Inversion - reverses the direction of a DNA sequence
4) Translocation - part of the chromosome breaks off and attaches to another
“jumping gene”
A single base mutation in a region of DNA coding for mRNA could transcribe the DNA
sequence CAGTAT into
Effects of Mutations
● Mutations may or may not affect organisms.
● The effects of mutations can be beneficial or harmful.
● Mutations can occur through:
○ errors in DNA replication
○ environmental stress can increase the general mutation rate for a species
○ Mutagens are agents that cause mutations, ie chemical or physical
● The most harmful mutations affect gene activity or protein structure ie sickle cell or cancer
● Beneficial mutations can alter proteins that can be useful in changing environments.
● Polyploidy is condition that alters the number of chromosome sets an organism has (3N or 4N), ie grass carp
DNA Damage and Mutations
A substance that can cause a change in the
DNA code of an organism is called a?
Section 13.4 Gene Regulation & Expression
Be Able To:
● Explain how prokaryotic genes are regulated.
● Explain how eukaryotic genes are regulated.
● Explain what controls the development of cells and tissues in multicellular organisms.
Key Terms:
● Operons, operator, RNA interference, differentiation,
homeotic gene, homeobox gene, Hox gene.
Prokaryotic Gene Regulation
● To conserve energy, prokaryotic express only those genes necessary for survival.
● By regulating gene expression, transcription and translation, bacteria can respond to their
environment.
● An operon is a group or prokaryotic genes that share a common operator and promoter to be transcribed into a single mRNA, ie lac operon in E.
coli
● A promoter is a site on the DNA molecule for RNA polymerase to attach to initiate transcription.
● The operator is a DNA sequence for a repressor protein to bind to on the DNA molecule, either turns
“on” or “off” the operon for transcription.
lac operon
trp operon
Eukaryotic Gene Regulation
● Most eukaryotic genes have a more complex regulatory process than prokaryotes.
● TATA box is a short region of DNA, 25 to 30 bp long, that helps bind a protein that helps
position RNA polymerase in transcription.
● DNA binding proteins called transcription factors are essential in eukaryotic transcription.
● Eukaryotic promoters generally require multiple transcription factors to initiate transcription.
● This complex gene regulation mechanism in eukaryotes makes cell specialization possible.
● RNA interference is the introduction of double-stranded RNA into a cell that inhibits gene expression, done by small pieces of RNA called microRNA or miRNA
Genetic Control of Development
● Gene regulation helps cells become specialized in structure and function, known as
differentiation.
● Homeotic genes are a class of regulatory genes that determine the identity of certain body parts and regions in embryonic development.
● Homeotic genes share a very similar 130 bp sequence known as homeobox genes.
● Homeobox genes code for transcription factors that activate other genes critical in cell
development and differentiation.
● HOX genes researched heavily in flies determine the identity of each body segment.
● The environment of an organism can also play an important role in development, ie metamorphosis