The Structure of RNA. The Central Dogma

Transcription

1 RNA and Protein Synthesis The Structure of RNA The Central Dogma Phenotype A gene is a SEQUENCE of DNA that codes for a protein (or functional RNA). Phenotype is the individual s observable trait resulting from its genotype (genes). Gene expression is the overall process that begins with the transcription of a DNA SEQUENCE (gene) into an RNA SEQUENCE which is translated into the amino acid SEQUENCE of a functional polypeptide/protein, resulting in a particular phenotype. This is considered the central dogma of biology because it is fundamental to how every living thing has come to be. The role of RNA in this process is as a middleman or worker that physically creates proteins according to the instructions (blueprint) in DNA. Like DNA, RNA is a nucleic acid and is similar to DNA in its variable SEQUENCE of nucleotides. 21

2 The Structure of RNA Codon Unlike DNA, RNA: 1) is a single polynucleotide 2) has ribose instead of deoxyribose 3) has uracil instead of thymine The three main types of RNA are: 1) Messenger RNA () 2) Transfer RNA (trna) 3) Ribosomal RNA (rrna) Each group of three nucleotides in is known as a codon. Messenger RNA carries the message (SEQUENCE) of a gene that is to be translated into a polypeptide/protein. 22

3 Ribosome Types of RNA Amino acid Anticodon Ribosomal RNA Ribosomes are the smallest, non membrane-bound organelles where translation (protein synthesis) occurs. Ribosomes are made of ribosomal RNA and about 80 different proteins. The function of rrna is to recognize and bind & trna, like an enzyme active site, to catalyze translation (protein synthesis). Transfer RNA trna is T or hairpin shaped. One end of trna is bound to a specific amino acid. The other end of trna has a triplet of nucleotides called the anticodon. trna interacts with both rrna and when it is time for the amino acid is carries to be added to a growing polypeptide chain. 23

4 RNA Transcription RNA RNA polymerase DNA Template Strand Coding Strand The process that produces all three types of RNA is transcription, which takes place in the nucleus. There are genes, trna genes and rrna genes. RNA polymerase, along with numerous transcription factors, binds to DNA at a promoter sequence. Once transcription is initiated, RNA polymerase unwinds and unzips the DNA. Using one strand as a template, RNA nucleotides from the nucleoplasm are matched up with DNA nucleotides according to the base pairing rule, resulting in an RNA plynucleotide complementary to the DNA template polynucleotide. In this way, the RNA nucleotide SEQUENCE is determined by the DNA nucleotide SEQUENCE. 24

5 RNA Processing Once transcribed, RNA processing takes Exon Intron place in the nucleoplasm. DNA Pre- is after it has been transcribed, but before being processed. Introns are non-coding sequences of DNA and Transcription pre- to be cut out during RNA processing. Exons are coding sequences of DNA and Pre- pre- that will be spliced together during processing. Following transcription, introns are cut out of Processing pre- and exons are spliced together, then cap and tail sequences are added at either end. This is all performed by the activity of ENZYMES! The cap and tail sequences help the Cap leave the nucleus through nuclear Tail pores and protect it from degradation by hydrolytic enzymes. RNA splicing allows for a variety of final polynucleotides to be produced from one gene and pre- by joining exons in a variety of ways. This may explain how in humans, 100,000 proteins are produced by about 25,000 genes. It also allows for the evolution of new genes through exon shuffling. 25

6 The Genetic Code The genetic code describes the way codons in DNA and are translated to amino acids during protein synthesis. Codons are nucleotide triplets in DNA and that code for one specific amino acid. Amino acids are the monomers of polypeptides/proteins. The relationship between codons and amino acids is not one-to-one. Some amino acids have more than one codon. AUG codes for the amino acid methionine, but it is also the Start codon. There are three Stop codons that do not code for amino acids. The stop codons signify the termination of translation and the end of the polypeptide being synthesized. The nucleotide SEQUENCE is read as a codon SEQUENCE that is translated into the amino acid SEQUENCE of a polypeptide/protein. 26

7 Cap Start Codon Translation Cytoplasm Nuclear Envelope Nucleus Nuclear Pores Review: - is made in the nucleoplasm by transcription and processing. - goes out into the cytoplasm and binds to a ribosome. - is able to leave the nucleus through nuclear pores due to it being a single polynucleotide (1nm wide instead of 2nm for DNA) and its cap/tail. - participates in the process of translation. Translation is the final step in gene expression that produces the resulting phenotype. Translation is the synthesis of polypeptides/proteins... 27

8 Met Translation Ala trna Lys Ribosome (subunits) Cap Anticodon Start Codon carries the nucleotide sequence of the gene to be translated. trna transfers an amino acid specific to its anticodon which is complementary to the codon. rrna provides binding sites for both and trna, allowing them to interact and perform their functions during translation. Translation begins when the capped end of binds between the ribosomal subunits. The ribosome reads along the SEQUENCE until it finds the start codon AUG. Next, transfer RNAs with anticodons complementary to the codons bind to the ribosome, delivering their specific amino acids (methionine first). Once a peptide bond forms between amino acids, the ribosome moves the distance of one codon downstream. 28

9 Polypeptide Lys trna trna Ribosome Translation Ribosome direction Next, the start trna leaves as another trna carrying the amino acid specific to the next codon/anticodon binds the ribosome. As the ribosome moves along, the cycle of trna amino acid delivery and peptide bond formation continues producing a growing polypeptide chain. The cycle stops when the ribosome reaches one of three stop codons (UGA). No trna binds and all components dissociate. The product is a polypeptide/protein composed of a VERY SPECIFIC SEQUENCE of amino acids. Different proteins have a different AA SEQUENCE! DNA SEQUENCE SEQUENCE AA SEQUENCE 29

10 DNA, RNA, Genes and Proteins This diagram represents the overall process of gene expression. Gene expression begins when the SEQUENCE of nucleotides in DNA is transcribed into the SEQUENCE of nucleotides in. Then the SEQUENCE of nucleotides in is translated into the SEQUENCE of amino acids in a polypeptide/protein. The amino acid SEQUENCE of a polypeptide/protein produces its conformation, and its conformation allows it to carry out a specific function. The highly variable nature of proteins enables them to perform a wide variety of functions crucial to living systems. DNA codes for proteins and proteins run the show! The Central Dogma Single strand of DNA Transcription & Processing Codon Codon Codon Translation Alanine Arginine Leucine Amino acids within a polypeptide 30