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Genetics CH9 EuKs
Exam 4 Review/Gene Expression
| Question | Answer |
|---|---|
| What is an enhancer? | what controls levels of expression (sequences that help control how strongly or weakly a gene is expressed.) |
| How is RNA processed in eukaryotic cells? | (5’ cap and poly-A tail)Eukaryotic mRNAs receive “a methylated cap” at the 5′ end and “a poly-A tail” at the 3′ end |
| What is the 5' cap? | methylated guanine (G) residue |
| How is the 5' cap made? | The nucleotide at the 5′ end of a eukaryotic mRNA is a G in reverse orientation… A special capping enzyme adds it… Enzymes known as methyltransferases then add methyl groups… forming a methylated cap. |
| What is the Poly-A tail? | The 3′ end consists of 100 to 200 As (Adenines) |
| Is the poly-A tail coded for by the DNA? | no, there’s no stretch of Ts in the Dna that it is coding for , instead the As are just added to the RNA bc cleavage depends on the sequence AAUAAA then polymerase adds As. |
| What is splicing? | the process that deletes introns and joins together successive exons to form a mature mRNA consisting only of exons. |
| What are introns? | need to be spliced out (sequences found in the DNA of the gene but not in the mature mRNA) |
| What are splice donor sites? | 5′ end of intron - a type of short sequence within primary transcript that helps ensure the specificity of splicing. |
| What are splice acceptor sites? | 3′ end of intron - a type of short sequence within primary transcript that helps ensure the specificity of splicing. |
| What are branch sites? | internal A used to form the lariat - a type of short sequence within primary transcript that helps ensure the specificity of splicing. |
| What are exons? | need to be brought back together will be accomplished using a spliceosome. (sequences found in both a gene’s DNA and the mature mRNA) |
| What is a spliceosome? | a complicated intranuclear machine that is required for splicing |
| What is a ribozyme? | RNA molecules that can act as enzymes. (Some primary transcripts can function as ribozymes which can then splice themselves) |
| *What is alternative splicing? | bigger proteins have certain domains. The domains are included in exons that can be included and removed or left alone. Cell can utilize all 5 exons that are brought together, |
| What is the main function of alternative splicing? | it can tailor the nucleotide sequence of a primary transcript to produce more than one kind of polypeptide. |
| Simple explanation of alternative splicing: | Alternative splicing lets one primary transcript be spliced in different ways to make more than one kind of mRNA and protein. |
| What is translation? | the process of making a protein from an mRNA molecule. The ribosome reads the mRNA and helps link amino acids together to form a polypeptide |
| What is a ribosome? | A ribosome is the cellular machine that carries out translation. It brings mRNA and tRNAs together and helps form the peptide bonds that build a protein. |
| What is a ribosome made of? | two subunits, and each subunit contains: rRNA (ribosomal RNA) Proteins |
| What is the small subunit? | The small subunit is the part of the ribosome that binds to mRNA first. It is also made of rRNA + ribosomal proteins. |
| How is eukaryotic transcription different from prokaryotic transcription? | Eukaryotes: Have introns → require RNA splicing. Add a 5′ methylated cap and 3′ poly-A tail. Promoter recognition affected by enhancers and chromatin structure. |
| Does Transcription and Translation happen at the same time in Euks? | No |
| Why do transcription and translation NOT occur at the same time in eukaryotes? | Eukaryotes have a nuclear membrane, so transcription occurs in the nucleus, while translation happens in the cytoplasm. Because these processes occur in separate compartments, they cannot be coupled like they are in prokaryotes. |
| How do enhancer proteins affect transcription in eukaryotes? | Enhancer sequences—sometimes thousands of base pairs away—affect the stability of RNA polymerase’s interaction with the promoter and are required for efficient transcription of eukaryotic genes. |
| Organization of DNA in eukaryotes | Eukaryotic chromosomes are tightly wound around histone proteins forming chromatin & nucleosomes. Before transcription can occur, the promoter region must be unwound so RNA polymerase can access it. |
| What must happen before a piece of DNA can get transcribed? | the nucleosomes that are tightly wrapped around proteins must get unwound. |
| How is translation initiation started in eukaryotes? | by recognition of the 5′ methylated cap: The small ribosomal subunit (40S) recognizes initiation factors bound to the 5’ cap of the mature mRNA. 40S ribosomal subunit scans the mRNA from 5’→3’ until it reaches the first AUG, which becomes the start site. |
| How does the ribosome bind mRNA? | The structural features mRNA must have in order to help ribosome bind are :A 5’ methylated cap & a long poly-A tail |
| What is the initiation codon? | AUG |
| What does the initiation codon code for? | In eukaryotes, AUG codes for unmodified methionine (Met) carried by the special initiator tRNA. |
| What post-translational modifications can be made to a protein? | Cleavage like the removal of amino acids such as the N-terminal methionine or Chemical additions like phosphorylation, glycosylation, or lipidation |
| What is a zymogen? | A zymogen is a protein synthesized in an inactive form that becomes active only after enzymatic cleavage that removes an N-terminal pro-segment. (Inactive portion that was removed is found in the blood stream) |
| How does the presence of a nuclear membrane in eukaryotes affect transcription and translation? | Euks have a nuclear membrane, so transcript occurs in the nucleus & translation occurs in the cytoplasm. Bc these take place in diff places, they can't occur at the same time. Whereas, proks lack a nuclear membrane so those can happen @ same time |
| What are enhancers and where are they found? | Only found in EuKs! They are DNA sequences required for efficient transcription of eukaryotic genes. They can be located thousands of base pairs away from the promoter and help stabilize RNA polymerase’s interaction with the promoter |
| What are nucleosomes and where are they found? | pieces of DNA tightly wound around histone proteins, forming the basic unit of chromatin and is found in eukaryotic chromosomes |
| General idea of mutations in the genetic code: | This is any change in DNA that may alter the amino acid sequence of a protein. When amino acids change, protein function may become normal, impaired, enhanced, lost, or altered. |
| What is a silent mutation? | This mutation is a base pair change, it changes a codon to another codon (A to a T) that specifies for the same amino acid, usually by altering the third nucleotide (wobble position). NO change to the protein. |
| What is a missense mutation? | This mutation changes a codon so that it encodes a different amino acid. (Glycine --> Arginine) |
| What is the result of a missense mutation? | If they’re different sizes or has a different charge then it provides more opportunity for the protein not to perform it’s job (conservative vs nonconservative substitutions) |
| What are conservative substitutions in missense mutations? | The new amino acid has similar chemical properties, so function may be preserved. |
| What are nonconservative substitutions in missense mutations? | The new amino acid has very different properties (size, charge, polarity), which increases the chance the protein will not function properly. |
| What is a nonsense mutation? | a mutation that generates a premature stop codon (UAA, UAG, UGA) is a nonfunctional protein (truncated). |
| What is a frameshift mutation? | a mutation that shifts the reading frame; The addition or removal of a base or bases. Insert or delete a base |
| How can mutations outside the coding sequence affect gene expression? | these mutations can influence the production of a protein, promoters (RNA polymerase will no longer recognize/bind it), enhancers (transcription drops), 5'cap & polyA tail (reduce translation), & introns (proper splicing disrupted) |
| What mutations are categorized as loss of function mutations? and what does that mean? | missense, nonsense, & frameshift; severe mutation that reduces or eliminates the activity of a protein |
| Which mutations are categorized as null mutations? | nonsense, frameshift, or large deletions; A specific type of loss-of-function mutation in which the gene produces no functional protein at all (complete loss of activity) |
| *Transcription occurs in the ________ and translation occurs in the __________ in eukaryotes | nucleus, cytoplasm |
| *During maturation of a eukaryotic mRNA, regions that are spliced out are: ? | introns |
| *A complex of proteins and small nuclear RNAs that form discrete particles to remove introns is a | splicesome |
| *Site of protein synthesis is: ? | ribosome |
| What are the steps of transcription? | Initiation, Elongation, Termination, RNA processing (Specific to EuKs!) --> This is where 5' methylated cap is added and 3' polyA tail is added and introns are spliced out. |
| What are the steps of translation? | Initiation, Elongation, Termination; and if needed Post-translational modification follows |
| What comes first transcription or translation? | Transcription; this is bc translation can only occur after the mature mRNA reaches the cytoplasm. |
| How does a mutation in the promoter affect gene expression? | reduces or prevents RNA polymerase from binding; transcription is diminished or may not occur at all, which decreases or eliminates protein production |
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