B 4a prokaryotic and eukaryotic cells answers

Study Guide Chapter 17 From Gene to Protein-ANSWERS

17.1 Transcription and Translation____________________________________________

(Start at sub-section Basic Principles of Transcription and Translation)

1. Name at least 3 differences between DNA and RNA.

1. DNA- double stranded; RNA is single stranded

2. DNA has Thymine nitrogenous bases, no Uracil; RNA has Uracil nitrogenous bases, no Thymine

3. DNA has a Deoxyribose sugar, RNA has a Ribose sugar

2. Making proteins from DNA requires what two major processes (list them in order)?

1.Transcription (make mRNA from DNA)

2. Translation (assemble amino acids according to the mRNA sequence)

3A. Where does transcription occur in prokaryote cells?

cytoplasm

B. Where does transcription occur in eukaryote cells?

Nucleus

C. Define and outline the process of transcription. Include the following terms: chromosome, gene, DNA, mRNA, RNA polymerase

DNA double helix of certain gene on a chromosome is opened/unwound by RNA polymerase.

RNA polymerase binds with promoter on one strand of the DNA molecule

RNA polymerase adds RNA nucleotides to the DNA template at the 3’ end of the growing mRNA molecule,

RNA polymerase falls off the DNA template when it encounters a stop codon in the DNA

mRNA molecule is now ready for translation in prokaryotes, modification in eukaryotes then translation.

4A. Where does translation occur in prokaryotic cells?

Cytoplasm

B. Where does translation occur in eukaryotes cells?

Cytoplasm

C. Define and outline the process of translation. Include the following terms: mRNA, ribosome polypeptide, and protein.

-mRNA molecule binds to small ribosome subunit, initiator tRNA bonds with start codon of mRNA molecule, large subunit binds, initiator tRNA is in P site of large ribosome subunit = initiation complex

-New tRNA molecule bearing correct anticodon to match the codon of the mRNA in the A site brings new amino acid.

-peptide bond formed between amino acid chain in P site and amino acid in A site

-mRNA molecule moves down 3 nucleotides (1 codon) which causes translocation of the tRNA in A site to the P site and the tRNA in the P site moves to the E site.

-Release factor binds to stop codon in mRNA = polypeptide released and ribosome subunits and mRNA disassociate.

5. What is the main difference between prokaryotic and eukaryotic transcription initiation?

Eukaryotic transcription initiation required transcription factors to bind to the TATA box located within the promoter before the RNA polymerase can bind

6. How many mRNA nucleotide bases (codon) correspond to one amino acid? 3

7. In the process of transcription, only one of the two DNA strands is transcribed into mRNA. What is this strand called?

Template strand

8A. If the nucleotide sequence along the DNA molecule is 3’-ACG-5’ what triplet code would be found in the transcribed mRNA molecule? 5’-UGC-3’

8B. mRNA base triplets are called ___codons__________________. Each one of these specifies which one of the 20 amino acids will be incorporated into the ____polypeptide/protein_______________.

9. Most of the mRNA codons code for an amino acid. What is the function of the other codons? 1 start codon (tells the ribosome where to start translation), 3 stop codons (tell the ribosome when to stop translation)

10. What amino acid sequence would you expect if the following mRNA transcript were being translated? (List the order of the amino acids).

MRNA: 5’-AUGGUUACUAAUAACGGU-3’

Amino acids: Met-Val-Thr-Asn-Asn-Gly

17.2 Transcription DNA mRNA____________________________________________

11. Define the following:

RNA polymerase- Pulls apart DNA molecule, adds RNA nucleotides to the 3’ end of a growing mRNA molecule

Promoter- sequence of nucleotides that RNA polymerase recognizes and binds to directly in prokaryotes, and transcription factors bind to in eukaryotes which then allows RNA polymerase to bind

Terminator- Sequence that signals the end of transcription in prokaryotes

Transcription unit- The stretch of DNA that is transcribed

Transcription factors- a collection of proteins that bind to the TATA box located within the promoter and facilitate the binding of the RNA polymerase

Transcription initiation complex- The completed assembly of transcription factors and RNA polymerase bound to the promoter

TATA box- nucleotides sequence located within the promoter to which transcription factors bind

12. Name the three stages of transcription (in order).

1.Initiation

2. elongation

3. termination

Transcription Initiation

13A. How does RNA polymerase “know” where to start transcribing a gene in PROKARYOTES?

Prokaryotes- binds to nucleotide sequence (TATA box) located within promoter

13B. How does RNA polymerase “know” where to start transcribing a gene in EUKARYOTES?

Eukaryotes- transcriptions factors must first bind to TATA box located within promoter for the RNA pol to bind to the promoter

Prokaryotes- no transcription factors needed

Transcription Elongation

14A.During the elongation stage of transcription, RNA polymerase moves along the DNA and __untwists__ the double helix exposing about ___10-20__(#) DNA bases at a time.

14B.The RNA polymerase enzyme adds RNA nucleotides to the __3’__(3’ or 5’) end of the growing RNA molecule. The new RNA molecule then peels away from its DNA template and the DNA double helix re-forms.

Transcription Termination

15. Describe how transcription termination is different in prokaryotes and eukaryotes. (Include this term when describing eukaryote termination: Polyadenylation signal)

Prokaryotes- RNA pol associates with the terminator and falls off DNA

Eukaryotes- process still unknown, but the polyadenylation signal (hundreds of A nucleotides at the end of the mRNA transcript) is thought to play a role.

17.3 RNA modification after transcription______________________________________

16A. Name the three main modifications (processing events) eukaryotic mRNAs undergo in the nucleus before they are exported to the cytoplasm.

1. 5’cap

2. 3’ polyA tail (polyadenylation)

3. Splice out introns, ligate exons together

16B. Identify the purpose of these mRNA modifications.

-5’cap & 3’ polyA tail (polyadenylation)= To protect the ends of the mRNA molecule for degradation

-Splicing= many different mRNA molecules can be made from a single mRNA molecule. Each of these mRNAs usually codes for a distinct protein. Therefore, many different proteins can be generated from a single DNA sequence!

17. Describe RNA splicing. Include the terms intron, exon & spliceosome

Some mRNA transcript introns are spliced out of the mRNA by a spliceosome and then the exons are ligated together. The same mRNA molecule can be spliced many different ways. Therefore, many different proteins can be made from a single DNA sequence.

18. What is alternative splicing? How can this process generate a greater number of polypeptides than there are genes?

The same mRNA molecule can be spliced many different ways. Therefore, many different proteins can be made from a single DNA sequence Sometimes exons are spliced out with introns or introns are not spliced out of the mRNA transcripts = more than one transcript and subsequently more than one protein can be made using 1 gene.

17.4 Translation mRNAProtein____________________________________________

19. Define the following terms:

Transfer RNA (tRNA)-

RNA molecule that has an anticodon on one end of the molecule (that binds with the mRNA codon during translation) and a place for attachment of an amino acid (that will correspond to the mRNA codon the anticodon binds with) on the other end of the molecule

Anticodon-

part of the tRNA molecule that binds with the codon of mRNA being translated

20A. As a tRNA molecule arrives at a ribosome, it bears a specific __amino acid____at one end. At the other end of the tRNA is a nucleotide triplet called an anticodon, which base pairs with a complementary condon on the_____mRNA__________ being translated.

20B. Provide an example of a tRNA molecule that would pair with the mRNA codon UUU. (What amino acid and anticodon would the tRNA have?)

AAA-lsy

21A.Ribosomes are made up of two subunits called the _large ___and __small__.

B. These subunits are made of __rRNA___and proteins.

C. The only time the two come together is _during translation, to associate with mRNA_.

22. Ribosome structure:

Binding site for tRNA on the large ribosome subunit are listed below. What does each capital letter for each site name represent?

A site- Amino acid enter site

P site-Peptide bond formation site (peptide bond forms between the amino acid attached to the tRNA in the P site and the new amino acid that was brought into the A site)

E site- Exit site, tRNAs without an amino acid exit the ribosome

23. Translation Initiation includes the following major players: mRNA, initiator tRNA, two subunits of ribosome, initiation factors and initiation complex. Describe the translation initiation process using these terms.

mRNA molecule binds to small subunit

Initiator tRNA binds to start codon on mRNA

Large subunit binds, initiator tRNA is in P site

24. Translation Elongation of the polypeptide chain includes the following steps:

1. codon recognition

2. peptide bond formation

3. translocation

Describe the major events that take place during each step

1. Codon recognition-

Anticodon of tRNA binds to mRNA codon in the A site. On the other end of this tRNA molecule, the amino acid corresponding to the mRNA codon in the A site is bound.

2. Peptide bond formation

Peptide bond forms between the amino acid bound to the tRNA molecule in the P site and new amino acid in the A site and the entire polypeptide chain is transferred to tRNA in A site. At the same time, the bond the amino acid had to the tRNA in the P site is broken.

3. Translocation

tRNA in A site then moves to P site, while tRNA that was in P site moves to the E site and exits the ribosome

25. Translation Termination is the final stage of translation. Describe this process.

How is the polypeptide chain released from the ribosome?

Release factor binds to termination codon in mRNA molecule in the A site = polypeptide release and translation assembly falls a part.

26. What is a polyribosome? Why are they beneficial for the organism?

One mRNA molecule with several ribosomes attached, each making a polypeptide.

17.5 Types RNA_______________________________________________

27. Define and describe the function of each type of RNA listed below:

mRNA-messenger RNA-RNA copy of the DNA, make in a process called transcription, used to make proteins

tRNA- transfer RNA- brings amino acids to the ribosome during translation

rRNA- ribosomal RNA-, structural part of the ribosome

17.7 Mutations___________________________________________________________

28. Define and give and example of the following:

Point mutation- change in just one base pair in DNA

Base-pair substitutions:

-Missense mutations – altered codon still codes for an amino acid, but not the right amino acid

-Nonsense mutations- altered codon causes translation to terminate (amino acid codon stop codon)

Insertions and deletions:

-frameshift mutation-occurs whenever the number of nucleotides inserted or deleted in not a multiple of 3

29. The template strand of a gene contains the sequence 3’-TACTTGTCCGATATC-5’. Draw the complementary strand of DNA and the resulting mRNA. Label all 3’ and 5’ ends and determine the amino acid sequence that results.

Complementary DNA strand 5’-ATGAACAGGCTATAG-3’

mRNA 5’-AUG AAC AGG CUA UAG-3’

a.a. sequence Met-Asn-Arg-Leu-STOP

30. How would the following point mutation in the DNA template change the amino acid sequence? Point mutation = 3’-TACTTGTCCAATATC-5’

a.a. sequence Met-Asn-Leu-Leu-STOP