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Biology 12: DNA and Protein SynthesisText:

Vocabulary:

1. Genetic Material Vocabulary:

-Complementary base pairing, DNA helicase, DNA polymerase, nucleotides, recombinant DNA, replication, semi-conservative replication

2. Protein Vocabulary:

-Amino acid, anti-codon, codon, DNA sequence (genetic code), elongation, environmental mutagen, genetic disorder, initiation, messenger RNA (mRNA), mutation, polypeptide chain, ribosomes, termination, transcription, transfer RNA (tRNA), translation

Goals:

*You should already know the following from the Biochemistry Unit!

1. List the four main functions of nucleic acids (RNA and DNA)

2. Name the four nitrogenous bases in RNA and describe the structure of RNA using the following terms: Nucleotide, linear/single stranded, sugar-phosphate backbone.

3. Name the four nitrogenous bases in DNA and describe the structure of DNA using the following terms: Nucleotide, complementary base pairing, double helix, hydrogen bonding and sugar-phosphate backbone.

4. Compare the general structural composition of DNA and RNA.

*The following are the new goals for this unit:

B5 Describe DNA replication

B5.1 Describe the three steps in the semi–conservative replication of DNA:

– “unzipping” (DNA helicase), complementary base pairing (DNA polymerase), joining of adjacent nucleotides (DNA polymerase).

B5.2 Describe the purpose of DNA replication

B5.3 Identify the site of DNA replication within the cell

B6 Describe recombinant DNA

B6.1 Define recombinant DNA

B6.2 Describe a minimum of three uses for recombinant DNA

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B7 Demonstrate an understanding of the process of protein synthesis

B7.1 Identify the roles of DNA, messenger RNA (mRNA), transfer RNA (tRNA), and ribosomes in the processes of transcription and translation, including initiation, elongation, and termination

B7.2 Determine the sequence of amino acids coded for by a specific DNA sequence (genetic code), given a table of mRNA codons

B7.3 Identify the complementary nature of the mRNA codon and the tRNA anti-codon

B8 Explain how mutations in DNA affect protein synthesis

B8.1 Give examples of two environmental mutagens that can cause mutations in humans

B8.2 Use examples to explain how mutations in DNA change the sequence of amino acids in a polypeptide chain, and as a result may lead to genetic disorders

Part A: Nucleic Acid Review

-At this point you should be able to recognize what the building blocks of nucleic acids are and how DNA and RNA differ.

-Notice in the diagrams that the two sides of a DNA ladder run in opposite directions. Directionality is determined from where the OH and Phosphate groups are.

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Part B: DNA Replication

1. Describe the Purpose of DNA Replication

-DNA replication is the production of exact copies of DNA in the nucleus.

-Why does DNA have to replicate?

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-Through what processes is DNA replicated?

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-Examine the division occurring in the onion cells below. Can you identify what stages of division are occurring?

2. Describe the Three Steps in the Semi-Conservative Replication of DNA

*On masteringbiology view Bioflix DNA Replication and try Activity: DNA Replication.

-The three stages of DNA replication are:

a) Unzipping the Parental DNA

-This is done by DNA helicase and forms Replication Bubbles.

-This step occurs at many spots along the DNA strand at the same time.

b) Complementary Base Pairing

-Daughter DNA strands are formed through DNA Polymerase assisting nitrogenous bases to complementary base pair with the parental DNA strand.

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-DNA Polymerase can only run in the 5’ to 3’ direction.

-What type of bond holds nitrogenous bases together (A-T, C-G)?

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-Why is DNA replication called semi-conservative?

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c) Joining of Adjacent Nucleotides

-Because DNA strands run in opposite directions and DNA polymerase can only run in the 5’ to 3’ direction, two strands of DNA are created during replication:

i) Leading Strand

-Runs 5’ to 3’ in the direction that DNA Helicase is expanding the Replication Bubble.

-This strand is continuous.

ii) Lagging Strand

-Runs 3’ to 5’ in the direction that DNA Helicase is expanding the Replication Bubble.

-Why is this strand called “discontinuous”?

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-Okazaki fragments must be joined together by DNA Ligase to form the complete daughter DNA strand.

Part C: Recombinant DNA

-The diagram right illustrates the idea of Recombinant DNA. What do you think is meant by this term?

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-What are some uses for recombinant DNA that you know?

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*See how this process works by viewing the masteringbiology Activity: Cloning a Gene in Bacteria Lab

-Examine the diagram right. What is a restriction enzyme?

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-Inside bacteria, restriction enzymes provide protection from viral genetic material.

-Researchers can use restriction enzymes to cut DNA molecules in reproducible ways.

-View How Insulin is made using bacteria from the DNA Learning Centre.

*Explore this idea in relation to insulin production through the Recombinant DNA Lab.

Part D: Protein Synthesis

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-The diagrams below and right illustrate the general steps involved in protein synthesis.

-What is Transcription and why is it necessary?

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-What is Translation?

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-Below we will have a look at these processes in more detail:

1. Transcription

-Transcription occurs in three distinct steps:

a) Initiation

-RNA polymerase attaches to the promotor (TATA box is 25 nucleotides upstream from transcription start point) and begins RNA synthesis.

b) Elongation

-RNA strand grows as synthesis continues.

c) Termination

-RNA polymerase reaches the terminator DNA and stops copying and detaches from the DNA strand.

-In Eukaryotes, this is the polyadenylation signal (AAUAAA) and 10-35 nucleotides downstream from it the RNA is released from the polymerase.

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-mRNA must be further processed before it is functional:

a) The 5’ end gets a 5’ Cap consisting of a modified guanine.

b) The 3’ end gets a poly(A) tail of 50-250 adenine nitrogenous bases.

-Steps a and b help to protect the mRNA from enzymes, to export it out of the nucleus and to attach to the ribosome.

c) Remove non-coding regions (introns-intervening sequences) so that coding regions (exons-expressed sequences) are joined together.

-Having introns and exons means that alternate RNA splicing can occur, where many proteins can be coded by the same gene depending on what sections are treated as exons.

-If there are twenty different types of amino acids, how many nitrogenous bases are required to code for them all?

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-If the code on the DNA template strand is TACGTTAGCGATCTGATC, what will the sequence of nitrogenous bases be on the mRNA?

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-What will the sequence of amino acids be in the polypeptide that it produced from the mRNA?

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-What would happen if the DNA template strand was altered to read TACGTTAGCCATCTGATC?

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2. Translation

-Translation is the synthesis of a polypeptide from the mRNA and involves some new types of RNA:

a) tRNA

-Examine the diagram right. What does tRNA do in protein synthesis?

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-Remember, RNA is single stranded but can bond with itself to make a stable structure. This can be represented in a couple of different ways:

b) rRNa

-Examine the image at the top of the page. What does rRNA do in protein synthesis?

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-The ribosome is made of two subunits which connect together around the mRNA at the start codon. The subunits will detach when the stop codon is reached.

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-Translation occurs in three steps:

a) Initiation

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How does the tRNA know where to bond on the mRNA strand?

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-Ribosomes have three different sites used by tRNA:

i) P site: Holds the tRNA carrying the polypeptide chain.

ii) A site: Holds the tRNA carrying the next aa to be added.

iii) E site: Holds tRNA that has passed off its aa.b) Elongation

-Elongation is the process by which aa are added to the polypeptide until the stop codon is reached.

-What reaction joins amino acids to the polypeptide chain?

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c) Termination

-How are the Start and Stop codons different from one another?

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*Translation occurs many times on the same mRNA strand at the same time through the use of polyribosomes!

-Try the concept map on protein synthesis below:

Part E: DNA Mutations and Protein Synthesis

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-Sickle-Celled Anemia is an inherited disorder and is the consequence of DNA mutation.

-Why does the change in one DNA nucleotide matter?

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-Sickle Celled Anemia is a recessive trait and, although it causes numerous serious health issues, it also provides some resistance to malaria.

-Sickle Celled Anemia is the product of a Point Mutation, of which there are two types (see image right)

-Why are these types of mutations called point mutations?

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-Which type of point mutation would cause more problems, substitution or deletion?

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-What is a spontaneous mutation?

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Part F: Environmental Mutagens Affecting Humans

-Mutagens are physical or chemical agents that alter DNA.

-What are some environmental mutagens that can affect humans?

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