DNA / RNA

Chapter 08

DNA

• Deoxyribonucleic acid (DNA) is a nucleic acid that contains the blueprint for making the proteins the cell needs.

• DNA contains genes.• Genes are specific messages instructing the

cell on how to construct a protein.

DNA

• DNA is the chemical used to pass genetic information on to the next generation of organisms.

• DNA controls the synthesis of proteins, which helps determine the characteristics of the organism and regulate the cell’s metabolism.

DNA

• DNA contains the genetic instructions used in the development of all known living organisms and some viruses.

• DNA molecules are used for long term storage of information.

• DNA carries the instructions necessary to create RNA and proteins; therefore, it is often compared to a blueprint.

DNA Structure

• DNA is a nucleic acid.• Nucleic acids are large polymers of

nucleotides.

DNA Structure

• DNA consists of two long polymers of simple units known as nucleotides.

• These two strands run in opposite directions to each other and are therefore known as anti-parallel.

• The strands have backbones made of sugars with phosphate groups attached.

DNA Structure

• Attached to each sugar is one of four types of molecules called bases.

• Information is encoded in the sequence of these four bases along the backbone.

• The information is read using the genetic code.

DNA Structure

• The genetic code specifies the sequence of amino acids within proteins.

• The code is read by copying stretches of DNA into RNA (A process known as transcription).

DNA Structure

• A nucleotide consists of a sugar molecule, a phosphate group, and a nitrogenous base.

• There are four different nitrogenous bases in DNA:

DNA Structure

– Adenine (A), guanine (G), cytosine (C), and thymine (T).

• The DNA nucleotides can combine into a long linear DNA molecule that can pair with another linear DNA molecule.

DNA Structure

• The two paired strands of DNA form a double helix with sugars and phosphates on the outside and the nitrogenous bases on the inside.

• The nucleotides form hydrogen bonds with one another, which helps to stabilize the helical structure.

DNA Structure

• Adenine pairs with Thymine (A-T).• Guanine pairs with Cytosine (G-C).

Nitrogenous Bases

• The nucleotide bases are nitrogenous bases that are involved in pairing in DNA and RNA. This is known as base pairing.

• In genetics they are simply called bases.• Adenine, Guanine, Cytosine, and Thymine are

DNA bases.• Adenine, Guanine, Cytosine, and Uracil are

RNA bases.

Adenine

Guanine

Thymine

Cytosine

Uracil

Chromosomes

• Within cells, DNA is organized into structures called chromosomes.

Chromosomes

• The chromosomes are duplicated before the cell divides, a process known as DNA replication.

• Within the chromosomes, chromatin proteins such as histones compact and organize DNA. The chromatins help determine which parts of the DNA are transcribed.

Eukaryotes Vs. Prokaryotes

• Eukaryotic organisms (animals, plants, fungi, and protists) store their DNA inside the cell nucleus.

• Prokaryotic organisms (bacteria and archae) have no nucleus; therefore, the DNA is found in the cytoplasm.

DNA Replication

• When a cell grows and divides, two new cells result.

• DNA replication is the process by which a cell makes another copy of its DNA.

• Base pairing rules and many enzymes make replication possible.

DNA Replication

• DNA replication is the process of copying a double-stranded DNA molecule to form two double-stranded molecules.

DNA Replication

• Each DNA strand holds the same genetic information; therefore, both strands can serve as a template for the reproduction of the complementary strand.

• The template strand is conserved in its entirety and the new strand is assembled from nucleotides. This is known as semiconservative replication.

DNA Replication

• The resulting double-stranded DNA molecules are identical.

• DNA replication must happen before cell division can occur.

DNA Replication

• Helicases are enzymes that bind to the DNA and separate the two strands of DNA.

• DNA polymerase incorporates DNA nucleotides into the new DNA strand. The nucleotides enter according to the base pairing rules.

DNA Replication

• In prokaryotic cells, this replication process starts at only one place along the DNA molecule (origin of replication).

• In eukaryotic cells, the replication starts at the same time along several different places of the DNA molecule.

DNA Replication

• Two new identical, double-stranded DNA molecules are formed.

• The new strands of DNA form on each side of the old DNA strands.

DNA Replication

• The exposed nitrogenous bases of the original DNA serve as the pattern on which the new DNA is formed.

• Two double helices are formed with identical nucleotide sequences.

• A portion of the DNA polymerase molecule edits the newly created DNA molecule and makes corrections if needed.

DNA Replication

Repair of Genetic Information

• If an error or damage occurs to the DNA helix on one strand, the pairing arrangement of nitrogenous bases on the other undamaged strand can be read.

• This information is used to repair the damaged strand.

DNA Code

• DNA stores information.• The order of the nitrogenous bases is the

genetic information that codes for proteins.• The nucleotides are read in sets of three.• Each sequence of three nucleotides is a

codeword for a single amino acid.• The information to code one protein can be

thousands of nucleotides long.

RNA Structure And Function

• Ribonucleic Acid (RNA) is important in protein production.

• RNA’s nucleotides contain a ribose sugar whereas DNA’s nucleotides contain a deoxyribose sugar.– Ribose has an –OH group and deoxyribose has an

–H group on the second carbon atom.

RNA Structure And Function

• RNA contains the nitrogenous bases Uracil (U), guanine (G), cytosine (C), and adenine (A).

• DNA is found in the cell’s nucleus, while RNA is made in the nucleus and then moves out into the cytoplasm of the cell.

RNA Structure And Function

• DNA directs protein synthesis by using RNA.• RNA is made by enzymes that read the protein

coding information in DNA.• RNA nucleotides pair with DNA nucleotides.– RNA contains Uracil instead of Thymine so

adenine in DNA pairs with Uracil in RNA.

Nucleic Acid Base Pairing RulesDNA pairs with DNA DNA pairs with RNA RNA pairs with RNA

A pairs with T A pairs with U A pairs with U

T pairs with A T pairs with A U pairs with A

G pairs with C G pairs with C G pairs with C

C pairs with G C pairs with G C pairs with G

Transcription

• Transcription is the process of using DNA as a template to synthesize RNA.

• The RNA polymerase enzyme reads the sequence of DNA nucleotides and follows the base pairing rules between DNA and RNA to build the new RNA molecule.

Transcription

• The two strands of the double stranded DNA molecule are separated to expose the nitrogenous bases.

• The DNA’s nitrogenous bases are read and paired with the RNA nucleotides.

• Only one strand of the DNA molecule is read (the coding strand). The other strand is referred to as the non-coding strand.

Transcription

• Promoter sequences are specific sequences of DNA nucleotides that RNA polymerase uses to find a protein-coding region of DNA and to find out which strand of DNA is the coding strand.

Transcription

• Termination sequences are DNA nucleotide sequences that indicate when RNA polymerase should finish making an RNA molecule.

3 Types of RNA

• Messenger RNA (mRNA) – carries the blueprint for making the necessary protein.

• Transfer RNA (tRNA) – reads mRNA and brings in the necessary amino acids.

• Ribosomal RNA (rRNA) – reads the mRNA and brings in the necessary amino acids.

Translation

• Translation is the process of using information in RNA to direct protein synthesis.

• mRNA is read in sets of three nucleotides called codons.

Translation

• A codon is a set of three nucleotides that codes for a specific amino acid.

• The ribosome is made up of proteins and ribosomal RNA (rRNA).

• The ribsome holds the mRNA in place and reads it’s codons.

3 Phases of Translation

• Initiation• Elongation• Termination

Initiation

• The small ribosomal subunit binds to the mRNA and moves along until it reaches an AUG codon to signal the beginning of translation.

• Transfer RNA (tRNA) carries amino acids to the mRNA complex.

Initiation

• The anticodon portion of the tRNA interacts with the mRNA to match the correct amino acid to the codon in the mRNA nucleotide sequence.

• The tRNA that binds to the AUG codon that signals the beginning of translation carries the amino acid methionine; therefore, every protein begins with this amino acid.

Elongation

• The ribosome functions as an assembly line.• New amino acids are carried by tRNA to the

corresponding mRNA segment.• The anticodon on tRNA matches with the

codon on mRNA.• The amino acid is then attached to the end of

the chain and the protein becomes elongated.

Termination

• The ribosome will continue to add new amino acids until a stop signal is reached on the mRNA molecule.

• The stop codon can be either UAA, UAG, or UGA.

Termination

• When these codons are encountered, a release factor enters the ribosome. The ribosomal subunits release mRNA.

• The mRNA can then either be reused or broken down to stop protein production.

Translation

Nearly Universal Genetic Code

• The code for making protein from DNA is the same for nearly all cells.

• Bacteria, protists, plants, fungi, and animals all use DNA to store their genetic information.

• They all transcribe information in DNA to RNA.• They all translate the RNA to synthesize

protein using a ribosome.

Nearly Universal Genetic Code

• Almost all use the same three nucleotide codons to code for the same amino acid.

• In eukaryotic cells, transcription always occurs in the nucleu, and translation always occurs in the cytoplasm.

Nearly Universal Genetic Code

• These similarities make it possible to use bacteria to synthesize human proteins (i.e. insulin).

• Some viruses use RNA to store their genetic information (retroviruses). HIV is an example of this. Retroviruses use RNA to make DNA, which is then used to make proteins.

Gene Expression

• Gene expression occurs when a cell transcribes and translates a gene.

• Cells control which genes are used to make proteins.

• The different cell types in the human body are due to which proteins the cell is producing.

Controlling Protein Quantity

• An enzymes activity can be regulated by controlling how much of that enzyme is made.

• The cell controls how much mRNA is available for translation, which in turn determines the quantity of the protein produced.

Controlling Protein Quantity

• Enhancer and silencer sequences affect the ability of RNA polymerase to transcribe a specific protein.

• Enhancer sequences increase protein synthesis by increasing transcription.

• Silencer sequences decrease protein production by decreasing transcription.

RNA Degradation

• Cells regulate gene expression by limiting the length of time that mRNA is available for translation.

• Enzymes in the cell break down mRNA.

Mutations

• A mutation is any change in the DNA sequence of an organism.

• Errors during DNA replication can cause mutation.

• External factors can cause mutation:

Mutations

– Radiation, carcinogens, drugs, viruses.

• Not all mutations cause a change in the organism.

• If the mutation occurs away from the protein-coding sequence of the DNA, it is unlikely to be harmful to the organism.

Silent Mutation

• A silent mutation is a change that does not change the amino acids used to build a protein.

Nonsense Mutation

• A nonsense mutation causes a ribosome to stop protein synthesis by introducing a stop codon too early.

• This prevents the formation of functional proteins.

Missense Mutation

• A missense mutation causes the wrong amino acid to be used in making a protein.

• This will change the shape of the protein and affect its active sites.

• This can cause an abnormally functioning protein.

Insertions And Deletions

• Some mutations involve larger spans of DNA than a change in a single nucleotide.

• An insertion mutation adds one or more nucleotides to the normals DNA sequence.

Insertions And Deletions

– This can add amino acids to the protein and change its function.

• A deletion mutation removes one or more nucleotides.– This can delete amino acids from the protein and

change its function.