CHAPTER 10 “HOW PROTEINS ARE MADE”

Learning Targets  I will compare the structure of RNA with that of DNA.  I will summarize the process of transcription.

Review  4 Macromolecules  Lipids  Carbohydrates  Proteins  Nucleic Acids (DNA & RNA)

Why Proteins?  Traits are determined by proteins that are built according to instructions coded in DNA  Proteins are vital to the functioning of cells  Important structural component  Act as enzymes  Cellular communication  Cell membrane channels

How Are Proteins Built?  To build proteins, the cell uses instructions contained in DNA molecules  DNA is the blueprint that describes how to build a protein  Proteins are not built directly from DNA another molecule is involved, RNA  (ribonucleic acid)

What is RNA?  RNA is a nucleic acid just like DNA but there are a few differences… DNARNA Sugar deoxyriboseribose Nitrogen BasesA,T,C,GA,U,C,G Strand DoubleSingle Location NucleusAnywhere in the cell

RNA Strand

RNA  There are three types of RNA (more on this later) 1. mRNA: messenger Copies the information in the DNA molecule in the nucleus 2. tRNA: transfer 3. rRNA: ribosomal

Back to Proteins  In eukaryotes, DNA is located inside the nucleus but proteins are assembled in the cytoplasm outside the nucleus

Back To Proteins  If DNA makes proteins how does the information leave the nucleus?  Copied  Transcribed (RNA is the transcription molecule)  Translated

How To Build A Protein aka Gene Expression

What is Transcription and Translation?  Transcription: the process in which genetic information from DNA is copied to mRNA  (3 main steps)  Translation: the process in which the base sequence in a strand of mRNA is converted, or “translated”, into the amino acid sequence of a protein

Transcription Overview  Step 1: RNA polymerase binds to the gene’s promoter  Step 2: Two DNA strands unwind and separate  Step 3: Complementary RNA nucleotides are added

Transcription: Making RNA  Step 1: RNA polymerase binds to the gene’s promoter  Promoter: A specific sequence of DNA that acts as a “start” signal for transcription

Transcription: Making RNA  Step 2: Two DNA strands unwind and separate  Proteins separate the DNA strands  Bonds between the nucleotide bases break  The phosphate backbone remains intact

Transcription: Making RNA  Step 3: Complementary RNA nucleotides are added by RNA polymerase  Only one of the strands will be transcribed into mRNA. This is called the transcribed strand  RNA polymerase matches individual mRNA nucleotides with complementary DNA nucleotides on the transcribed strand  Linked together by covalent bonds

Transcription Review  Step 1: RNA polymerase binds to the gene’s promoter  Step 2: Two DNA strands unwind and separate  Step 3: Complementary RNA nucleotides are added.

Review Question Describe two roles for proteins in the transcription process  Before an mRNA strand can be constructed, the two strands of a DNA molecule must be separated. Proteins separate the DNA strands. Once the strands are separated, RNA polymerases (which are also proteins) match the mRNA nucleotides to the DNA strand and build the mRNA strand.

Learning Targets  I will relate the role of codons to the sequence of amino acids that results after translation.  I will outline the major steps of translation.  I will discuss the evolutionary significance of the genetic code.

The Genetic Code: Three-Nucleotide “Words”  Different types of RNA are made during transcription  mRNA is made when a cell needs a particular protein  mRNA carries instructions for making a protein from a gene and delivers it to the site of translation

mRNA  RNA instructions are written as a series of three- nucleotide sequences on the mRNA called codons  Each codon signifies a start or stop signal for translation  1 codon= 1 amino acid

Genetic Code  The amino acids and “start” and “stop” signals that are coded for by each of the possible 64 mRNA codons

Translation: From RNA to Protein  Occurs in the cytoplasm of the cell  Translation is carried out by the ribosomes  What is the function of the ribosomes?  To build proteins  They are small structures made of rRNA and proteins that are found in all cell types

Translation: From RNA to Protein  In the cytoplasm, tRNA molecules and ribosomes help to synthesize proteins  tRNA: single strand of RNA that temporarily carries a specific amino acid at each end  Each tRNA has an anticodon  Anticodon: a three-nucleotide sequence on a tRNA that is complementary to a mRNA codon

Translation: From RNA to Protein  rRNA: are RNA molecules that are part of the structure of ribosomes  A cell’s cytoplasm contains thousands of ribosomes  Each ribosomes temporarily holds one mRNA and two tRNA molecules.

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Translation

 Three steps of RNA translation: 1. Initiation 2. Elongation 3. Termination

Initiation  Ribosome small subunit binds to start of mRNA strand  tRNA carrying amino acid methionine binds to start codon  Start sequence is codon AUG  Specifies amino acid methionine  Then large subunit of ribosome binds  Initiates protein synthesis

Initiation

Elongation  Addition of amino acids by forming peptide bonds  mRNA shifts through ribosome  Next codon is read and amino acid is added  Continues until all codons are read

Termination  Ribosome continues until the stop codon  UAA, UAG, UGA  No tRNA molecules can recognize this codon  Releases new protein and mRNA  Polypeptide chain forms into its native shape  Acts as a functional protein in the cell

Termination

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