RNA: The other nucleic acid Chapter 5 – Protein Synthesis

RNA: ribonucleic acid Nucleic acid formed from nucleotides linked in a long chain Pentose sugar + phosphate group + nitrogenous base 3 different types of RNA: Messenger RNA (mRNA) – in the nucleus Transfer RNA (tRNA) – in the cytoplasm Ribosomal RNA (rRNA) – in the cytoplasm

Differences between DNA & RNA Type of sugar Deoxyribose Ribose Form Double strand Single strand Nitrogenous bases A, C, G, T A, C, G, U (Uracil) Location Nucleus Nucleus & cytoplasm

Molecular difference between the 2 sugars

Role of RNA DNA carries the code to produce every protein in the cell, BUT: DNA cannot leave the nucleus So, RNA plays an important role in the synthesis of proteins RNA can leave the nucleus & bring the information from DNA to the ribosomes where proteins are produced Done by mRNA

tRNA is responsible for transferring the correct amino acids to the ribosome rRNA brings the 2 units of the ribosome together to synthesize proteins

Proteins – essential for life!

Proteins The human body produces at least 50 000 different proteins (& maybe more!). Essential for life. Each protein has a specific form & function Generally, proteins are only viable for 2 days before they are broken down into their chemical components & recycled or eliminated as waste

Production of a protein takes less than a minute A cell can produce hundreds each second because it contains thousands of assembly points - ribosomes Each cell produces billions of proteins

All proteins are composed of 20 possible amino acids linked together to form polypeptides AAs can be attached together in different orientations to produce different proteins Finished proteins are chains that are folded & rolled up

Protein functions Enzymes: facilitate chemical reactions in organisms Hormones: chemical messengers in the body Antibodies: defend the body against foreign agents Storage: make essential substances readily available Transport: carry substances through body fluids Structure: form the structural elements of cells, maintain support & shape of the cell, also regulate the passage of substances through the cell membrane

Renin regulates blood pressure BTB protein helps facilitate interaction between other proteins Renin regulates blood pressure Hemoglobin helps red blood cells transport oxygen BRCA1 protects against breast & ovarian cancer Keratin forms fingernails & hair Myosin is responsible for muscle contractions Amylase breaks down complex sugars in saliva & the intestines

Protein Alphabet 20 amino acids Each amino acid is represented on the strand of DNA as a sequence of 3 nucleotides (codon) More than one codon can code for single amino acid

So... segments of DNA carry the codes to create proteins These segments are called genes A gene is a sequence of nucleotides that has the potential to be expressed in the form of a protein

Protein Synthesis 2 stages Transcription Translation Transcribe: make a copy of the information, preserving the original language Translate: convert the information to a new language

Key Terms: Codon: sequence of 3 nitrogenous bases in DNA or mRNA which codes for a specific amino acid Anticodon: group of 3 tRNA bases that combine with corresponding mRNA bases Start codon: codon that tells the mRNA where to start transcription Stop codon: signal for mRNA to end transcription

Transcription 1st step in protein synthesis Goal of transcription is to make an exact copy of a small piece of the genome on a strand of RNA Consists of the construction of RNA along a strand of DNA Happens in the nucleus of the cell

RNA polymerase attaches to a molecule of DNA that will be copied at the “promoter” Promoter – particular sequence of DNA nucleotides that serves as a binding site for RNA polymerase Rich in adenine & thymine

Nucleotides are added to the exposed strand of DNA (sense strand) RNA polymerase opens a section of the double helix and begins to synthesize a complementary strand of mRNA Transcribed strand or sense strand – has the instructions for synthesizing the protein Non-transcribed strand or anti-sense strand – identical to the synthesized mRNA Nucleotides are added to the exposed strand of DNA (sense strand)

RNA polymerase continues until it reaches a “stop” Stop – a sequence that signals the end of the section of DNA being copied After transcription, the mRNA separates from the RNA polymerase, then the RNA polymerase separates from the DNA molecule The enzyme can continue to another promoter & start a new process of transcription The 2 strands of DNA reattach to each other

Transcription Summary Initiation: RNA polymerase attaches to DNA at the promoter Elongation: production of a strand of mRNA Termination: RNA polymerase reaches the Stop, then releases the mRNA

Translation: RNA to proteins After transcription, mRNA leaves the nucleus In the cytoplasm, mRNA attaches to a ribosome to form a template mRNA carries the DNA nucleotide code in groups of 3 (codons), each one codes for an amino acid Different combinations of codons produce different proteins

tRNA reads the mRNA code & attaches the correct amino acid AAs are floating free in the cytoplasm tRNA is composed of anticodons which are complementary to the mRNA

tRNA lines its anticodon up with the mRNA codon Amino acids form a chain which will fold up to become a protein rRNA helps with the formation of bonds between the amino acids Ribosome releases the new protein The mRNA breaks down into its individual nucleotides & returns to the nucleus