RNA Gene Expression Gene A segment of DNA

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RNA & Gene Expression • Gene: A segment of DNA that specifies the amino

RNA & Gene Expression • Gene: A segment of DNA that specifies the amino acid sequence of a polypeptide • DNA does not directly control protein synthesis, instead its information is transcribed into RNA • The “Central Dogma”: 1

RNA properties • RNA (ribonucleic acid) 2

RNA properties • RNA (ribonucleic acid) 2

Types of RNA • Three Classes of RNA – Messenger RNA (m. RNA) •

Types of RNA • Three Classes of RNA – Messenger RNA (m. RNA) • Takes a message from DNA to the ribosomes • strand – Ribosomal RNA (r. RNA) • Makes up ribosomes (along with proteins) • globular – Transfer RNA (t. RNA) • Transfers amino acids to ribosomes • Hairpin shape 3

Gene Expression • Gene Expression Requires Two Steps: • Transcription – Is the synthesis

Gene Expression • Gene Expression Requires Two Steps: • Transcription – Is the synthesis of RNA under the direction of DNA – Produces messenger RNA (m. RNA) • Translation – Is the actual synthesis of a polypeptide, which occurs under the direction of m. RNA – Occurs on ribosomes http: //highered. mcgrawhill. com/sites/dl/free/0072835125 /126997/animation 1. html 4

Ribonucleic Acid • Why would the cell want to have an intermediate between DNA

Ribonucleic Acid • Why would the cell want to have an intermediate between DNA and the proteins it encodes? – The DNA can then stay pristine and protected, away from the caustic chemistry of the cytoplasm. – Gene information can be amplified by having many copies of an RNA made from one copy of DNA. – Regulation of gene expression can be effected by having specific controls at each element of the pathway between DNA and proteins. – The more elements there are in the pathway, the more opportunities there are to control it in different circumstances. 5

24. 2 Gene Expression • Transcription – During transcription, a segment of the DNA

24. 2 Gene Expression • Transcription – During transcription, a segment of the DNA serves as a template for the production of an RNA molecule – Messenger RNA (m. RNA) • RNA polymerase (enzyme) binds to a promoter (“start” sequence) • DNA helix is opened so complementary base pairing can occur • RNA polymerase joins new RNA nucleotides in a sequence complementary to that on the DNA, in a 5’ to 3’ direction 6

Transcription of DNA to form m. RNA 7

Transcription of DNA to form m. RNA 7

Messenger RNA • m. RNA - of the 64 possible 3 -base combinations: –

Messenger RNA • m. RNA - of the 64 possible 3 -base combinations: – 61 code for the twenty different amino acids – 3 code for "stop"; i. e. chain termination • Specific nucleotide sequences call for “start” of transcription (usually AUG = methionine) = PROMOTOR sequence • “stop” of m. RNA synthesis = TERMINATION sequence (UAA, UGA, UAG) • Finished m. RNA strands are ~500 -10, 000 nucleotides long 8

 • During transcription – The gene determines the sequence of bases along the

• During transcription – The gene determines the sequence of bases along the length of an m. RNA molecule Gene 2 DNA molecule Gene 1 Gene 3 DNA strand 3 5 A C C A A A C C G A G T (template) TRANSCRIPTION m. RNA 5 U G G U U U G G C U C A 3 Codon TRANSLATION Protein Figure 17. 4 Trp Amino acid Phe Gly Ser 9

24. 2 Processing of m. RNA • After Transcription • Primary “Pre-”m. RNA must

24. 2 Processing of m. RNA • After Transcription • Primary “Pre-”m. RNA must be modified into mature m. RNA – Introns are intragene segments (often, junk) – Exons are the portion of a gene that is expressed • Intron sequences are removed, and a poly-A tail is added – Ribozyme splices exon segments together – http: //highered. mcgrawhill. com/sites/dl/free/0072835125/126997/animation 20. html 10

m. RNA Processing pre-RNA must be modified before translation 11

m. RNA Processing pre-RNA must be modified before translation 11

The Functional and Evolutionary Importance of Introns • The presence of introns – Allows

The Functional and Evolutionary Importance of Introns • The presence of introns – Allows for alternative RNA splicing – Animations of RNA processing: http: //highered. mcgraw hill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 535: : /sites/dl/free/00 72437316/120077/bio 25. swf: : Processing%20 of%20 Gene%20 Infor mation%20 -%20 Prokaryotes%20 versus%20 Eukaryotes – http: //highered. mcgrawhill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 535: : /sites/dl/free/00 72437316/120077/bio 30. swf: : How%20 Spliceosomes%20 Process% 20 RNA 12

Transposons • “jumping genes” • Can move to new locations and disrupt gene sequences

Transposons • “jumping genes” • Can move to new locations and disrupt gene sequences 13

24. 2 Gene Expression • Translation – The Genetic Code • Triplet code: each

24. 2 Gene Expression • Translation – The Genetic Code • Triplet code: each 3 -nucleotide unit of a m. RNA molecule is called a codon • There are 64 different m. RNA codons – 61 code for particular amino acids » Redundant code; some amino acids have numerous code words » Provides some protection against mutations – 3 are stop codons signal polypeptide termination 14

Messenger RNA Codons 15

Messenger RNA Codons 15

Overview of Gene Expression Protein synthesis http: //highered. mcgrawhill. com/olcweb/cgi/plugi npop. cgi? it=swf: :

Overview of Gene Expression Protein synthesis http: //highered. mcgrawhill. com/olcweb/cgi/plugi npop. cgi? it=swf: : 535: : 53 5: : /sites/dl/free/0072437 316/120077/micro 06. sw f: : Protein%20 Synthesis 16

24. 2 Gene Expression • Transfer RNA – t. RNA transports amino acids to

24. 2 Gene Expression • Transfer RNA – t. RNA transports amino acids to the ribosomes (creates polypeptide chain) – Single stranded nucleic acid that correlates a specific nucleotide sequence with a specific amino acid – Amino acid binds to one end, the opposite end has an anticodon – the order of m. RNA codons determines the order in which t. RNA brings in amino acids Protein synthesis http: //highered. mcgrawhill. com/olcweb/cgi/pluginp op. cgi? it=swf: : 535: : /si tes/dl/free/0072437316/12 0077/micro 06. swf: : Protein %20 Synthesis 17

Transfer RNA: Amino Acid Carrier 18

Transfer RNA: Amino Acid Carrier 18

r. RNA – Ribosomal RNA is the most abundant type of RNA in cells

r. RNA – Ribosomal RNA is the most abundant type of RNA in cells – Ribosomes: comprised of subunits 2/3 RNA, 1/3 protein • Two populations of ribosomes are evident in cells, Free and bound 19

“Bound” vs “Free” Ribosomes • F �ree ribosomes are located in the cytoplasm of

“Bound” vs “Free” Ribosomes • F �ree ribosomes are located in the cytoplasm of the cell. They are not attached to any structure, but they may group together with other ribosomes to form polysomes (polyribosomes). In the cytoplasm, ribosomes are free floating. They can move all around the cell. • Bound ribosomes are located on the surface of the endoplasmic reticulum. The endoplasmic reticulum that contains ribosomes is described as the rough endoplasmic reticulum because of the bumpy surface. Bound ribosomes can not move to other areas of the cell. They are attached to the cytosolic side of the endoplasmic reticulum. • Free ribosomes produce proteins for the cell, while bound ribosomes produce proteins that are transported out of the cell. 20

Polyribosomes • A number of ribosomes can translate a single m. RNA molecule simultaneously

Polyribosomes • A number of ribosomes can translate a single m. RNA molecule simultaneously – Forming a polyribosome Completed polypeptide Growing polypeptides Incoming ribosomal subunits Polyribosom Start of e m. RNA (5 end) End of m. RNA (3 end) (a) An m. RNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes. Ribosomes m. RNA 0. 1 µm Figure 17. 20 a, b (b) This micrograph shows a large polyribosome in a prokaryotic cell (TEM). 21

 • The ribosome has three binding sites for t. RNA – The P

• The ribosome has three binding sites for t. RNA – The P site – The A site – The E site P site (Peptidyl-t. RNA binding site) A site (Aminoacylt. RNA binding site) E site (Exit site) E m. RNA binding site Figure 17. 16 b P A Large subunit Small subunit (b) Schematic model showing binding sites. A ribosome has an m. RNA binding site and three t. RNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams. 22

 • Concept 17. 4: Translation is the RNAdirected synthesis of a polypeptide: a

• Concept 17. 4: Translation is the RNAdirected synthesis of a polypeptide: a closer look Quicktime movie: http: //carbon. cudenver. edu/~bstith/transla. MOV Narrated animation: http: //highered. mcgrawhill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 535: : /sites/dl/free/0072437316/ 120077/micro 06. swf: : Protein%20 Synthesis Interactive practice: http: //learn. genetics. utah. edu/content/begin/dna/transcribe/ 23

24. 2 Gene Expression • Ribosome and Ribosomal RNA – Ribosome has a binding

24. 2 Gene Expression • Ribosome and Ribosomal RNA – Ribosome has a binding site for m. RNA and for 2 t. RNAs – Facilitates complementary base pairing – Ribosome moves along m. RNA and new t. RNAs come in and line up in order – This brings amino acids in line in a specific order to form a polypeptide – Several ribosomes may move along the same m. RNA • Multiple copies of a polypeptide may be made • The entire complex is called a polyribosome 24

Translation (Building a polypeptide) requires Three Steps: – Initiation (requires energy) – Elongation (requires

Translation (Building a polypeptide) requires Three Steps: – Initiation (requires energy) – Elongation (requires energy) – Termination Amino end Growing polypeptide Next amino acid to be added to polypeptide chain t. RNA 3 m. RNA 5 Codons (c) Schematic model with m. RNA and t. RNA. A t. RNA fits into a binding site when its anticodon base-pairs with an m. RNA codon. The P site holds the t. RNA attached to the growing polypeptide. The A site holds the t. RNA carrying the next amino acid to be added to the polypeptide chain. Discharged t. RNA leaves via the E site. 25

Summary of Gene Expression 26

Summary of Gene Expression 26

24. 2 Gene Expression • Genes and Gene Mutations – A gene mutation is

24. 2 Gene Expression • Genes and Gene Mutations – A gene mutation is a change in the sequence of bases within a gene. – Gene mutations can lead to malfunctioning proteins in cells. 27

24. 2 Gene Expression • Genes and Gene Mutations – Causes of Mutations •

24. 2 Gene Expression • Genes and Gene Mutations – Causes of Mutations • Errors in replication – Rare – DNA polymerase “proofreads” new strands and errors are cleaved out • Mutagens – Environmental influences – Radiation, UV light, chemicals – Rate is still fairly low because DNA repair enzymes monitor and repair DNA 28

Types of Gene Mutations Point Mutations – The substitution of one nucleotide for another

Types of Gene Mutations Point Mutations – The substitution of one nucleotide for another • Missense mutations – a point mutation in which a single nucleotide is changed, resulting in a codon that codes for a different amino acid – Missense mutations are responsible for about 75% of the mutations in the p 53 gene. Mutations of this gene are responsible for about 30 -50% of cancers in humans • Silent mutations • Nonsense mutations – mutations that change an amino acid to a stop codon 29

Types of Gene Mutations • Frameshift Mutations – One or more nucleotides are inserted

Types of Gene Mutations • Frameshift Mutations – One or more nucleotides are inserted or deleted – Results in a polypeptide that codes for the wrong sequence of amino acids – Codons must be read in the correct reading frame for the specified polypeptide to be produced 30