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Biology Final Chap 17 - How Genes are Expressed
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43
Biology
Undergraduate 2
12/13/2011

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Term
What is the central dogma of biology?
Definition

DNA--Translation--> RNA--Transcription--> Protein


^ an easy way of summarizing how the genes in DNA code for proteins

Term
What are the three types of RNA?
Definition

messenger RNA - mRNA

transfer RNA - tRNA

ribosomal RNA - rRNA

Term
What is the structure and function of messenger RNA (mRNA)?
Definition

structure: single stranded

has a poly A tail (many adenines) 

GTP cap (guanosine triphosphate) which stabilizes the molecule 

function: carries the code for a gene from the nucleus to the cytoplasm 

Term
What is the structure and function of transfer RNA (tRNA)?
Definition

structure: clover leaf shaped with 3 loops

2 ends - amino acid attatchment site and anticodon end with a specific sequence of 3 nitrogen bases

function: translates the code (sequence of N bases) on mRNA into a specific sequence of amino acids in a polypeptide. each tRNA carries one type of amino acid

Term
What is the structure and function of ribosomal RNA (rRNA)?
Definition

structure: has one small subunit and one large subunit

has 3 tRNA binding sites (E,P,A) and 1 mRNA binding site

has a tunnel where the polypeptide exits

function: binds to ribosomal proteins to make ribosomes - the site of protein synthesis 

Term
How does the structure of RNA differ from the structure of DNA?
Definition

RNA is single stranded, RNA is never helical, RNA and DNA have different nitrogen bases

RNA - A to U and C to G

DNA - A to T and C to G

In RNA U replaces T

Term
What type of binding sites are on ribosomes?
Definition

3 tRNA binding sites and 1 mRNA binding sites

3 tRNA binding sites - A (enter), P (translocation), E (exit) 

Term
How many subunits do ribosomes have?
Definition
2 - one small subunit and one large subunit
Term
What are the 2 binding sites on tRNA and what do they bind to?
Definition
?
Term
How many nitrogen bases make up a codon of the genetic code?
Definition
3 nitrogen bases make a codon
Term
What do codons code for?
Definition
amino acids
Term
How does the genetic code work?
Definition
  1. for every gene the template, or, sense strand serves as a pattern
  2. RNA base pairing binding to the sense strand produces mRNA
  3. mRNA is translated by tRNA into a specific sequence of amino acids 
Term
What are some codons that exist that do not actually code for amino acids?
Definition
3 stop codons and 1 start codon
Term
What are the characteristics of the genetic code (definite, redundent, universal)?
Definition

it is a triplet code - codons consist of 3 nitrogen bases ex. ACA

it is definite - ACA codes for theonine and only theonine (exclusive)

it is redundent - most amino acids are coded for by more than one codon ex. ACA, ACG, ACU all code for theonine - also known as the "wobble effect"

it is universal - all living things use the same code to translate their DNA into proteins 

Term
What is the "wobble effect" and what is its benefit? 
Definition

amino acids are coded for by more than 1 codon...

for example: ACA, ACG, ACC and ACU are all theonine 

the wobble effect makes it less likely for there to be errors or mutations 

Term
How does the universality of the genetic code make genetic engineering possible?
Definition
if all living things follow the same genetic code for their amino acids, they can be given the same code and it will have the same effect
Term
Understand the steps of transcription including the enzymes involved and what happens in each step.
Definition
  1. Initiation - an RNA polymerase molecule binds to the promotor region of a transcription unit and the DNA "unzips" in that area - RNA polymerase begins to form an RNA transcript  
  2. Elongation - RNA polymerase  moves down stream unzipping the DNA and elongating the RNA by adding free RNA nucleotides with completementary base pairing 
  3. Termination - at the end of the transcription unit the RNA transcript is released from the DNA and polymerase enzyme detatches 
Term
What happens at a promoter region?
Definition

RNA polymerase binds to the promotor region to initiate the RNA sythesis

promotor regions have a "TATA" box or, an "on switch"

transcription factors must bind to the DNA "TATA" box for RNA polymerase to bind 

this is how transcription is regulated 

Term
What happens when RNA is processed?
Definition

GTP cap and poly A tail are added 

cap and tail protect mRNA from degradation and allow it to bind to a ribosome for translation

poly A tail makes it more stable

 

during RNA splicing introns are cut out with spliceosome complex and exons are sealed together yielding a shorter mature RNA

(like a sentence without spaces)

Term
What is the difference between exons and introns?
Definition

exons are expressed portions of the transcription unit 

introns are not expressed  and are inbetween the exons

Term
What do exons code for?
Definition
specific protein domains - 3 dimensional regions of a polypeptide with a particular shape and function 
Term
What is the benefit of a cap and tail?
Definition
they protect the mRNA from degradation and allow it to bind to a ribosome for translation. the longer the tail the more stable it is
Term
What are domains of a polypeptide?
Definition
3 dimensional regions of a polypeptide with a particular shape and function
Term
What are anticodons?
Definition
?
Term
Understand the steps of translation and what happens in each step.
Definition
  1. Initation - mRNA binds to the small ribosomal subunit. intiatior tRNA with anticodon (ex. UAC) binds to the start codon with complementary base pair (AUG). then the large ribosomal subunit binds to the small ribsomal subunit forming an initiation complex 
  2. Elongation of polypeptide chain - a tRNA with complementary anticodon binds to mRNA at the empty A (enter) site of the ribosome. a peptide bond forms between amino acid at site A and site P. (the growing polypeptide is now at site) then the mRNA shifts over. now the tRNA that was at A moves to P and P moves to E and is released. A is now empty and holds the next codon to be translated by the correct tRNA
  3. Termination - when a stop codon is reached a release factor binds to A site instead of tRNA. the release factor breaks bond between the P site and the last amino acid of the polypeptide. the polypeptide and the last tRNA leave the ribosome
Term
Where in a eukaryotic cell does transcription occur?
Definition
Term
Where does translation occur?
Definition
cytoplasm
Term
What is produced at the ribosome during translation?
Definition
a polypeptide 
Term
What types of bonds hold together the amino acids from translation?
Definition
peptide bonds
Term
What is the function of a release factor?
Definition

it breaks the bond between the tRNA at the P site and the last amino acid of the polypeptide 

allows for the termination of translation and the release of the polypeptide from the ribosome 

Term
What is a polysome and why is it beneficial to a cell?
Definition

when mRNA is translated by many ribosomes simultaneously 

many copies of the same polypeptide are polysomes

this way a cell can produce many polypeptides only having to use one strand of mRNA 

less effort for more production

Term
What are some modifications to the polypeptide that can occur after translation?
Definition
  • it can be enzymatically cut into 2 parts
  • it may have sugars, lipids, or phosphate groups added
  • some amino acids at the end of the polypeptide may be removed 
 
modifications help polypeptide become a functional protein
Term
What are point mutations? 
Definition
chemical changes in one base pair of a gene
Term
What are the different types of point mutations?
Definition

insertions

deletions

substitutions

Term
In point mutations, what are substitutions?
Definition

where one N base is subsituted for another

leads to 

missense - where one amino acid is different

or

nonsense - if there is the formation of an early stop codon

Term
In point mutations, what are insertions and deletions?
Definition

both result in frame shift mutation 

insertions - where N base (s) are added

deletions - where N base (s) are deleted 

Term
Which of the point mutations lead to frameshift mutations?
Definition
insertions and deletions
Term
Which of the point mutations are potentionally more harmful: substitutions or frameshift mutations?
Definition

frameshift mutations because they can lead to: immediate nonsense if a stop forms

or

extensive nonsense where all of the following amino acids are incorrect

Term
Why is the rate of mutations in a cell low?
Definition
they are spontaneous, occur in 1 in every 1010 nucleotides 
Term
What can happen if a mutation ocurrs in a germ cell?
Definition
gives rise to a gamete that can pass on mutations to the next generation 
Term
What are mutagens?
Definition

chemical or physical agents in the environment that change DNA

ex. UV radiation, X rays, Viruses, Carcinogens 

Term
What are some ways genetic expression can be regulated?
Definition

through feedback inhibition 

when the product accumulates it binds to the first enzyme in a feedback inhibition pathway and shuts it down

ex. abundence of tryptophan 

Term
What do we now understand a gene to be?
Definition

a region of DNA whose final product is either a polypeptide or an RNA molecule

 

they can contain: non-coding and promoter regions that do not code for a polypeptide but are still functional parts of the gene

 

some genes code for mRNA, tRNA, and rRNA

 

some genes code for more than 1 polypeptide 

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