Chapter 10

DNA, Nucleic Acids and Protein Synthesis

Structure of DNA

• DNA is a polymer.

• DNA is made from monomers called nucleotides.

• A nucleotide is made of three parts: deoxyribose (sugar), phosphate group, and a nitrogen-containing base

Nitrogenous Bases

• DNA is made from 4 bases.

• Adenine, guanine, cytosine and thymine.

• They are represented A, G, C, T.

• Adenine and Guanine are classified as purines and contain two rings of carbon.

• Thymine and Cytosine are classified as pyrimidines and contain one ring of carbon.

The Double Helix

• In 1953, James Watson and Francis Crick suggested a model for the structure of DNA.

• The shape is called a Double Helix and is a spiral.

The Double Helix•Other scientists contributed to the discovery of the double helix.

•Rosalind Franklin took pictures of DNA crystals and revealed the helix shape.

•Maurice Wilkins assisted Rosalind Franklin in her research.

•The double helix was found to be linked together by hydrogen bonds.

Base Pairing

• Cytosine always pairs with guanine using 3 hydrogen bonds.

• Adenine always pairs with thymine using 2 hydrogen bonds.

• This base pairing rule is important because when DNA is replicated, all copies must be the same.

DNA Replication

• The process of copying DNA is called DNA Replication.

• Replication can begin and end at many points on the molecule simultaneously.

• Step 1: Helicase separates the double helix into two strands.

• Step 2: DNA polymerase binds to the separated strands and constructs a new strand based off the old one using base pairing rules.

• Step 3: The Double Helix winds up again.

Accuracy and Repair

• The process of DNA Replication occurs with a high degree of accuracy.

• Sometimes a mutation can cause a change in a base pair.

• The body contains enzymes to help find the mistakes and repair the errors.

• Some errors are missed and have serious consequences for new cells.

• Mutations can occur because of chance, chemicals, ultraviolet light and other agents.

RNA

• RNA is similar to DNA because it is made of nucleotides. There are some differences though.

• Difference 1: RNA contains the sugar ribose.

• Difference 2: RNA is single stranded all the time.

• Difference 3: RNA does not contain thymine but has uracil instead.

Types of RNA

• RNA has three types.• Messenger RNA (mRNA) is a single uncoiled

chain that carries information from DNA in the nucleus to the cytoplasm to make proteins.

• Transfer RNA (tRNA) is a coiled single chain shaped like a T that binds to specific amino acids.

• Ribosomal RNA (rRNA) are in a glob shape and make up ribsomes where proteins are made.

Transcription

• One function of RNA is to carry genetic information from DNA in the nucleus to the cytoplasm when it can be used to make proteins.

• The process by which genetic information is coped from DNA to RNA is called transcription.

Steps of Transcription

• Step 1: DNA unwinds.• Step 2: RNA polymerase binds to the DNA and

begins to bring in RNA nucleotides.• Step 3: RNA is paired with the DNA according to

base pair rules until an entire RNA strand is made.

• Step 4: The RNA is released and the DNA winds up again.

• All three types of RNA can be made this way.

Splicing

• Once the mRNA is made from the DNA, it must be edited to remove information that is not required for protein synthesis.

• Raw mRNA is made of introns and exons.

• During splicing, introns are removed and the exons merge together.

Protein Synthesis

• Proteins are polymers or polypeptides.• The proteins are made of monomers called amino acids.• There are 20 different amino acids.• During protein synthesis, mRNA is translated into

specific amino acids.• The genetic information necessary to make amino acids

is in the form of triplets called codons.• Codons are three nucleotides together as a set.• All living beings contain codons and make proteins the

same way. This evidence supports that all living creatures are related.

Translation•The process of assembling proteins from information coded in mRNA is called translation.•mRNA leaves the nucleus and then migrates to the cytoplasm to dock with ribosomes.•Amino acids float freely in the cytoplasm waiting to be picked up by tRNA molecules.

Steps to Translation

• Step 1: mRNA moves from the nucleus to the cytoplasm.

• Step 2: mRNA docks with one or many ribsomes.• Step 3: tRNA each with specific anticodons dock with

the codons on the mRNA starting with the start codon AUG.

• Step 4: As each tRNA docks, it brings with it an amino acid.

• Step 5: The amino acids bind together and become long chains called proteins.

• Step 6: Once the stop codon is reached, the protein breaks off and leaves the cell to be used.

Problems During Synthesis

• As with all mechanisms and processes, these can have problems if a mutation changes a base.

• Once the base of a DNA molecule is changed, that changes the mRNA which in turn changes the protein made.

• This can cause serious illness and disease or it may cause nothing.