Chapter 13- RNA and Protein Synthesis

Mr. Bragg2013-2014

•Describe how RNA differs from DNA•Explain the functions of RNA•Describe how cells synthesize RNA

I. 13.1 RNA

A. The Role of RNA1. How does RNA differ

from DNA?a. RNA- is a nucleic acid

that consists of long chains of nucleotides

b. Made up of a 5 carbon sugar, a phosphate group, and nitrogenous bases

c. Comparison- key differencesRNA

•Sugar is ribose

•Is single stranded

•Contains uracil instead of thymine

DNA• Sugar is

deoxyribose• Is double

stranded• Contains

thymine

2. Role Analogya. Builders use plans to build a home• DNA = the master plan – too valuable to

bring to the job site• RNA = blueprint- inexpensive and

disposable• The master plan is used to prepare the

blueprints

3. Functions of RNA

a. Major job- protein synthesisb. Controls the assembly of amino acids

into proteins

c. The 3 different types of RNA

•Messenger “m” RNA

•Ribosomal “r” RNA

•Transfer “t” RNA

d. Roles of the 3 Types of RNA

•mRNA- carry information from DNA to other parts of the cell

•rRNA- make up the subunits of the ribosome; worktable for making protein

•tRNA- carries amino acids to the ribosome and matches them to the coded mRNA message

B. RNA Synthesis1. How does the cell make mRNA?a. Transcription – segments of DNA serve as

templates to produce complimentary RNA molecules

• The base sequences of transcribed RNA complement the base sequences of the DNA template

•In prokaryotes, transcription takes place in the cytoplasm

•In eukaryotes, it takes place in the nucleus

•Transcription requires an enzyme- RNA polymerase

•RNA polymerase binds to DNA and separates the 2 strands

•Next, it uses one strand of DNA to make a template in RNA

•One gene can produce 100’s-1,000’s of RNA molecules

MIT – Lego Transcription

b. Promoters•RNA polymerase

bonds only to promoters

•This tells RNA polymerase where to start and stop transcription

•Promoters are signals in the DNA molecule’s sequence

Transcription animation

c. RNA editing•RNA molecules require some editing

before being read•Introns- the pieces that are edited out of

the pre-mRNA molecule•Exons- the remaining pieces after the pre-

mRNA molecule has been edited

•Describe how RNA differs from DNA•Explain the functions of RNA•Describe how cells synthesize RNA

•Describe how the genetic code is read•Explain the role of ribosomes in assembling

proteins•Describe the “central dogma” of molecular biology

II. 13.2 Ribosomes and Protein SynthesisA. The Genetic Code1. What is the genetic

code, and how is it read?

a. First step- transcribe the base sequences from DNA to RNA

b. RNA then must be translated into polypeptides, which form proteins

c. Polypeptides- a long chain of amino acids that makes protein

•There are 20 amino acids commonly found in proteins

•The properties of a protein depend on the order in which the amino acids are assembled

•The 4 bases (A, U, C, G) in RNA form a “language”

•These bases form the genetic code

•The sequence of bases is read a codon at a time

•Codons contain 3 bases•Each codon specifies a

single amino acid to be added to the polypeptide chain

a. There are 4 different bases in RNA, which means there are 64 possible codons

• 4 x 4 x 4 = 64 possibilities

2. How to Read Codons

• Most amino acids can be specified by more than one codon

• Ex: Leucine can be coded for in 6 ways: UUA, UUG, CUU, CUC, CUA, and CUG

• Genetic code tables are used to decode codons

3. Start and Stop Codons

a. AKA the “punctuation marks” of the genetic code

b. The methionine codon AUG = start

c. mRNA is then read 3 bases at a time until it reaches a “stop” codon.

d. Once stopped the polypeptide is complete

B. Translation (p. 368 – 369)1. What role does the

ribosomes play in assembling proteins?

a. Analogy: putting together a complex toy

• Need to read the directions AND put the parts together

• Ribosomes carry out these tasks in a cell

Lego Translation

Translation animation

•Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains

•Translation – the decoding of the mRNA message into protein

2. Steps in Translation

a. Transcribed mRNA from the nucleus moves into the cytoplasm to attach to a ribosome

b. Codons pass through the ribosome and tRNAs bring the proper amino acids into the ribosome

c. Each tRNA molecule carries just one kind of amino acid

• tRNA has 3 unpaired nitrogen bases = anticodon

•So, each tRNA anticodon is complimentary to one mRNA codon

•Ex: tRNA anticodon is UAC, the mRNA codon is AUG

d. Like an assembly-line worker that attaches one part to another, the ribosome helps form a peptide bond between the amino acids

e. The chain continues to grow with each tRNA coming in until it reaches the “stop” codon on mRNA

f. The ribosome then releases the protein and the mRNA

3. The Roles of tRNA and rRNA in Translation

a. All 3 major forms of RNA work together to make protein synthesis occur

b. mRNA brings the DNA message out of the nucleus

c. The tRNA deliver the amino acid called for by the mRNA

d. The rRNA are part of the ribosome and help locate the “start” code of the mRNA message

C. The Molecular Basis of Heredity1. What is the central dogma of molecular

biology?

a. Information is transferred from DNA to RNA to protein

Summary

b. Protein has everything to do with how genes are expressed

• Ex: A gene that codes for an enzyme to produce pigment can control the color of a flower

c. Gene expression – the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells

d. It’s universal!

•Describe how the genetic code is read•Explain the role of ribosomes in assembling

proteins•Describe the “central dogma” of molecular biology

•Describe what a mutation is•Explain how mutations affect genes

III. 13.3 MutationsA. Types of Mutations1. What are mutations?a. From Latin “mutare”-

meaning to changeb. Defined: any heritable

change in the genetic information

c. Two types • gene• chromosome

2. Gene Mutations

a. Point mutations – mutation where a single or very few nucleotides are changed

•Include substitutions, deletions, and insertions

•Usually occur during replication of DNA

b. Substitutions•One base is changed to a different base•Usually affect no more than a single

amino acid•Sometimes have no effect at all

Analogy:

OriginalThe fat cat ate the wee rat.

Point Mutation The fat hat ate the wee rat.

c. Insertions and deletions•One base is either inserted or removed

from the DNA sequence•Effects can be dramatic•Can lead to frameshift mutations

Analogy 1:OriginalThe fat cat ate the wee rat.

Frame Shift The fat caa tet hew eer at.

Analogy 2:OriginalThe fat cat ate the wee rat.

Insertion The fat cat xlw ate the wee rat.

3. Chromosomal Mutations

a. Involve changes in the number or structure of chromosomes

• Can change the location of genes or even the number of chromosomes

b. Different types• Deletion, duplication,

inversion, translocation

Animation link

B. Effects of Mutations

1. How do mutations affect genes?a. Genetic information can be altered by

natural events or by artificial means• Fact: Incorrect bases are routinely

copied during DNA replication at a rate of 1/10,000,000

2. Mutagens

a. defined: chemical or physical agents in the environment

• Chemical examples: pesticides, plant alkaloids, tobacco smoke, pollutants

• Physical examples: radiation like X-rays and UV light

3. Harmful and Helpful Mutationsa. The effects of mutations on genes vary

widely.• Some have little effect• Some are beneficial• Some negatively disrupt gene function• Most have little or no effectb. Effect is situationalc. Can generate variability within a species

d. Harmful effects•The most harmful effects come when the

structure of a protein is dramatically changed

•These can disrupt normal body routines•Can result in genetic disorders•Ex: Cystic Fibrosis

e. Beneficial Effects•Sometimes mutations can produce

proteins with new or altered functions that can be useful to an organism in a changing environment

•Ex: mutations have allowed certain insects to be resistant to pesticides – like mosquitoes

•Plant and animal breeders make good use of mutations

•Non-disjunction in plants during meiosis can be a beneficial mutation

•Leads to the formation of polyploid plants

•Plants can be triploid or tetraploid

•Ex: bananas, citrus, sugarcane, grains, etc…

Strawberries are octoploid 8N = 562N = 7

Mutations in Mammals

Siamese cat- the protein that produces fur color is dependent on heat. This is caused by a mutation in one gene White Appaloosa horse- color

is caused by a mutation in one gene

Mutations in “Herps”

Leucistic alligator, three legged frog

tomatoes

Seedless grapes

•Describe what a mutation is•Explain how mutations affect genes

•Explain how genes are regulated in prokaryotic and eukaryotic organisms

•Describe the controls placed on the development of tissues

IV. 13.4: Gene Regulation and Expression

A. How are genes regulated?

1. In prokaryotes DNA binding proteins regulate genes by controlling transcription

2. Genes are organized into operons: groups of genes that are regulated together

a. On the operon are regulatory regions

• Promoters – site where RNA polymerase binds for transcription

• Operators – site where a DNA binding protein can attach; analogy: car boot

Gene expression animation

B. The Promise of RNAi Technology1. RNA interference has an important role

in defending cells against parasitic genes – viruses

2. Also is important in directing development as well as gene expression in general.

3. Looks like a promising new method to treat various diseases like HIV and cancer

Step through RNAi modelRNA I short film

RNAi short film 2

•Explain how genes are regulated in prokaryotic and eukaryotic organisms

•Describe the controls placed on the development of tissues