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12-3 RNA and Protein Synthesis

The Structure of RNA12-3

A gene is a SEQUENCE of DNA that codes for a protein (or functional RNA).

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.

The Central Dogma

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.

Phenotype

Phenotype is the individual’s observable trait resulting from its genotype (genes).

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RNA & Protein Synthesis The Structure of RNA

Unlike DNA, RNA:1) is a single polynucleotide2) has ribose instead of deoxyribose3) has uracil instead of thymineThe three main types of RNA are:1) Messenger RNA (mRNA)2) Transfer RNA (tRNA)3) Ribosomal RNA (rRNA)Each group of three nucleotides in mRNA is known as a codon.Messenger RNA carries the message (SEQUENCE) of a gene that is to be translated into a polypeptide/protein.

Codon

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Amino acid

Transfer RNA

Types of RNA

Ribosome

Ribosomal RNA

RNA & Protein Synthesis

Ribosomes are made of ribosomal RNA and about 80 different proteins.

Ribosomes are the smallest, non membrane-bound organelles where translation (protein synthesis) occurs.

The function of rRNA is to recognize and bind mRNA & tRNA, like an enzyme active site, to catalyze translation (protein synthesis).

The other end of tRNA has a triplet of nucleotides called the anticodon.

One end of tRNA is bound to a specific amino acid.

tRNA is “T” or “hairpin” shaped.

tRNA interacts with both rRNA and mRNA when it is time for the amino acid is carries to be added to a growing polypeptide chain.

Anticodon

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RNA

RNA polymerase

DNA

RNA & Protein Synthesis RNA Transcription

The process that produces all three types of RNA is transcription, which takes place in the nucleus. There are mRNA 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.

Template Strand

Coding Strand

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RNA Processing

Exon IntronDNA

Pre-mRNA

mRNA

Cap Tail

RNA & Protein Synthesis

Pre-mRNA is mRNA after it has been transcribed, but before being processed.

Once transcribed, RNA processing takes place in the nucleoplasm.

Introns are non-coding sequences of DNA and pre-mRNA to be cut out during RNA processing.Exons are coding sequences of DNA and pre-mRNA that will be spliced together during processing.Following transcription, introns are cut out of pre-mRNA and exons are spliced together, then cap and tail sequences are added at either end.

The cap and tail sequences help the mRNA leave the nucleus through nuclear pores and protect it from degradation by hydrolytic enzymes.

This is all performed by the activity of ENZYMES!

RNA splicing allows for a variety of final mRNA polynucleotides to be produced from one gene and pre-mRNA 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”.

Transcription

Processing

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The Genetic Code

The genetic code describes the way codons in DNA and mRNA are translated to amino acids during protein synthesis.Codons are nucleotide triplets in DNA and mRNA that code for one specific amino acid.Amino acids are the monomers of polypeptides/proteins.

The mRNA nucleotide SEQUENCE is read as a codon SEQUENCE that is translated into the amino acid SEQUENCE of a polypeptide/protein.

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.

mRNA

RNA & Protein Synthesis

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Nucleus

mRNA

TranslationRNA & Protein Synthesis

Translation is the final step in gene expression that produces the resulting phenotype.

Translation is the synthesis of polypeptides/proteins...

Review:- mRNA is made in the nucleoplasm by transcription and processing.- mRNA goes out into the cytoplasm and binds to a ribosome.- mRNA 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.- mRNA participates in the process of translation.

Nuclear Envelope

Nuclear Pores

Start Codon

Cap

Cytoplasm

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TranslationRNA & Protein Synthesis

Translation begins when the capped end of mRNA binds between the ribosomal subunits. The ribosome reads along the mRNA SEQUENCE until it finds the start codon AUG.

mRNA carries the nucleotide sequence of the gene to be translated. tRNA transfers an amino acid specific to its anticodon which is complementary to the mRNA codon. rRNA provides binding sites for both mRNA and tRNA, allowing them to interact and perform their functions during translation.

Once a peptide bond forms between

Next, transfer RNAs with anticodons

mRNAStart Codon

Ribosome(subunits)

MetAla Lys

AnticodonCap

tRNA

complementary to the mRNA codons bind to the ribosome, delivering their specific amino acids (methionine first).amino acids, the ribosome moves the distance of one codon downstream.

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tRNA

Ribosome mRNA

Lys

Translation direction

Polypeptide

Ribosome

tRNA

mRNA

RNA & Protein Synthesis

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 mRNA, 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 mRNA SEQUENCE AA SEQUENCE

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DNA, RNA, Genes and Proteins

Codon Codon Codon

mRNA

Alanine Arginine Leucine

Amino acids within a polypeptide

Single strand of DNA

RNA & Protein Synthesis

This diagram represents the overall process of gene expression.

The Central Dogma

Gene expression begins when the SEQUENCE of nucleotides in DNA is transcribed into the SEQUENCE of nucleotides in mRNA. Then the SEQUENCE of nucleotides in mRNA 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!

Transcription& Processing

Translation