The Flow of Genetic Information

Transcription

1 Chapter 17

2 The Flow of Genetic Information The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins and of RNA molecules involved in protein synthesis. Proteins are the link between genotype and phenotype GENE EXPRESSION is the process by which DNA directs the synthesis of proteins. This occurs in two stages 1. Transcription 2. Translation

3 Basic Principles of Transcription and Translation Genes provide the instructions for making proteins but do not build a protein directly. The bridge between DNA and protein synthesis is RNA Structure of RNA Ribose instead of Deoxyribose Uracil instead of Thymine Single stranded

4 Transcription The synthesis of RNA under the direction of DNA The information is transcribed (copied) from one molecule to another DNA is a template just like during replication The type of RNA created during transcription is messenger RNA (mrna)

5 Transcription DNA Code: A T A T G G C A T RNA Code: U A U A C C G U A

6 Translation The synthesis of a polypeptide under the direction of mrna The base sequence of mrna must be converted to the amino acid sequence. This occurs on ribosomes.

7 Transcription and Translation Both processes occur in all organisms Transcription of a protein coding eukaryotic gene results in pre-mrna and further processing results in the final molecule Called a primary transcript

8 The Genetic Code There are 20 amino acids and 64 codons Triplet code (codons) 3 Nucleotides code for a single amino acid. During transcription, the gene determines the sequence of bases along the length of an mrna molecule. For each gene, only one of the two DNA strands is transcribed. This is called the TEMPLATE STRAND This means mrna is complementary rather than identical C G A U Non-template strand is sometimes called the coding strand

9 Codons AUG Start codon Also codes for methionine Signals protein synthesis and begins translating mrna at that location Notice some amino acids are coded for by more than one codon Codons must be read in the correct reading frame The correct grouping is necessary The red dog ate the bug vs. Her edd oga tet heb ug

10 Evolution of the Genetic code Nearly universal In labs, genes can be transcribed and translated after being transplanted from one species to another Bacteria can be programmed by the insertion of human genes to synthesize certain human proteins for medical use Leads to developments in biotechnology Recombinant DNA

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13 Chapter 17.2 RNA polymerase opens the DNA and joins RNA nucleotides Only functions in 5 3 direction RNA polymerase does NOT require a primer Specific sequences of nucleotides along DNA mark where transcription of a gene begins and ends PROMOTER sequence where transcription begins TERMINATOR sequence where transcription ends The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit

14 Synthesis of an RNA Transcript Three Stages of Transcription 1. Initiation 2. Elongation 3. Termination

15 Transcription Initiation Complex Initiation Promoter includes starting point Also determines which strand will serve as a template Transcription factors Mediate the binding of RNA polymerase and the initiation of transcription After certain transcription factors are attached RNA polymerase II binds to it The whole complex of transcription factors and RNA polymerase II TATA box Important promoter DNA sequence Protein-protein interactions are important in controlling eukaryotic transcription Seen with the interaction between RNA polymerase II and transcription factors

16 Elongation of the RNA strand RNA polymerase moves along DNA opening the helix Enzyme adds nucleotides at the 3 end of the growing RNA molecule RNA template peels away from the DNA template and it reforms A single gene can be transcribed simultaneously by several molecules of RNA polymerase following each other Why is this important???

17 Termination of Transcription In bacteria, transcription moves through the terminator which serves as a termination signal Polymerase detaches from the DNA and releases the transcript In eukaryotes, RNA polymerase II transcribes a POLYADENYLATION SIGNAL SEQUENCE AAUAAA in pre-mrna Protein later down the growing transcript cut it free from polymerase and release it Polymerase continues transcribing DNA for hundreds of nucleotides past the site where mrna was released.

18 17.3 Eukaryotic cells modify RNA after Transcription Enzymes in the eukaryotic nucleus modify pre-mrna in specific ways before genetic messages are sent to the cytoplasm This is RNA processing Both ends of the primary transcript are altered

19 Alteration of mrna Ends The 5 end is synthesized first and received a 5 cap a modified form of a guanine nucleotide added after transcription The 3 end of the pre-mrna molecule is also modified After the polyadenylation signal an enzyme adds more adenine nucleotides This forms a poly-a tail IMPORTANCE OF MODIFICATIONS 1. These modifications facilitate the export of mature mrna from the nucleus 2. Help protect mrna from degradation 3. Help ribosomes attach to the 5 of the mrna

20 Split genes and RNA splicing RNA splicing Removal of large portions of the RNA molecule that is initially synthesized Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides, which are not translated The sequence of DNA nucleotides that codes for a eukaryotic polypeptide is not continuous Introns (used to describe both DNA and RNA) The noncoding segments of nucleic acid that lie between coding regions Exons (used to describe both DNA and RNA) Sequences that are eventually expressed and translated to amino acids There are exceptions, but exons always exit the nucleus

21 Primary Transcript Modifications RNA polymerase II transcribes both introns and exons from the DNA mrna that enters the cytoplasm has the introns removed The introns are cut out Exons are joined to form an mrna molecule with continuous coding sequence RNA splicing Signal for splicing is a short nucleotide code at each end of an introl Small nuclear ribonucleoprotiens (snrnps snurps ) recognize these splice sites snrnps are located in the nucleus and composed of protein and RNA

22 Spliceosome A larger assembly molecule composed of several snrnps Interacts with certain sites on an intron, releasing the intron and joining the exons Strong evidence that snrnps catalyze these processes and may also build splicosomes

23 1. Small snrnps and other proteins form a sliceosome on a pre-mrna molecule 2. Within the spliceosome, snrnps base-pairs with nucleotides at specific sites along the intron 3. The spliceosome cuts the pre-mrna released the intron and splicing the exons together

24 Ribozymes RNA molecules that function as enzymes The intron RNA functions as a ribozyme and catalyzes its own excision Three properties of RNA enable some RNA molecules to function as enzymes RNA is single-stranded a region of an RNA molecule may base-pair with a complementary region in the same molecule creating a 3-D structure Some of the bases in RNA contain functional groups that may participate in catalysis The ability of RNA to hydrogen-bond with other nucleic acid molecules

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