Chapter 7: DNA Structure and Gene Function

Hershey and Chase Experiment (1952)

The experiment demonstrated that DNA, not protein, was the genetic material encoding characteristics of a cell.

Describe the structure of DNA

- Double-stranded

-Helical shape

-Has a sugar (deoxyribose)-phosphate backbone

-Contains the nitrogen bases Adenine, Guanine, Cytosine, Thymine

-Strands are held together by hydrogen bonds

Nucleotide

A unit of DNA or RNA consisting of a nitrogen base, deoxyribose or ribose sugar, and a phosphate.

Describe the structure of RNA

-Single-stranded

-Contains the nitrogen bases URACIL, adenine, guanine, and cytosine.

-has a ribose sugar-phosphate backbone

What are the 3 types of RNA?

Messenger RNA (mRNA)

Ribosomal RNA (rRNA)

Transfer RNA (tRNA)

mRNA

Messenger RNA; carries the genetic information from DNA to the ribosomes

rRNA

Ribosomal RNA; along with proteins, makes up the ribosomes.

Ribosome

A complex made of RNA and proteins that serves as platform or physical site where proteins are made.

tRNA

Transfer RNA; transfers amino acids to ribosomes for protein synthesis.

Steps of Transcription

1. Initiation

2. Elongation

3. Termination

RNA Polymerase

Enzyme in transcription that separates the hydrogen bonds in DNA and builds a complementary strand of RNA from the exposed DNA.

What nitrogen base substitutes for thymine in RNA

Uracil

Introns

Sequences in DNA that do not code for anything.

Exons

Sequences in DNA that code for proteins

What happens during mRNA processing?

-A 7-methylguanosine "cap" is added to the 5' end

-poly-A tail is added to the 3' end of the strand.

-Introns are cut out of the strand and exons are spliced together

Initiation step of transcription

RNA polymerase binds to the promoter region of DNA. The hydrogen bonds break in DNA and allow the strands to open up.

Elongation step of transcription

RNA polymerase reads the exposed DNA strands and builds a mRNA strand.

Termination step of transcription

RNA polymerase reaches a "stop" sequence in the DNA molecule and finishes building the new RNA strand.

Where does translation occur in the cell?

on a ribosome in the cytoplasm of the cell.

Where does transcription take place in the cell?

in the nucleus

Codon

A sequence of 3 nitrogen bases in DNA that code for an amino acid.

Steps of translation

-Initiation

-Elongation

-Termination

Initiation step of Translation

mRNA binds to a ribosome, which reads the mRNA 3 bases at a time

Elongation step of Translation

tRNA carries an amino acid to the ribosome, which matches the codons in the DNA to the complementary anticodon of the tRNA

Termination step of Translation

Once the ribosome reaches a stop codon, the protein is completed and released from the ribosome.

Operon

A cluster of structural and regulatory genes that act as a unit.

Gene

A specific sequence of DNA that controls the expression of one or more traits, such as blood type.

Gene expression

The process by which DNA is used to make proteins that determine specific traits.

Promoter

A portion of DNA that acts as a "starting line" for transcription.

Operator

A portion of the DNA that acts as a site for a repressor protein to bind. Acts as a switch that turns on/off transcription.

Repressor Protein

When attached to the operator portion, can prevent transcription by acting as a "roadblock" to RNA polymerase.

Gene expression in Prokaryotes

Prokaryotes have operons, clusters of genes, that can be "turned off" when they are not needed.

Lac Operon

-Present in E. coli

-Controls bacterial cells' ability to use lactose as energy

-When lactose is present, it binds to a repressor protein. Repressor detaches from the operator region of the gene and allows to use lactose.

Gene Expression in Eukaryotes

More complex than prokaryotes due to specialized cell genes.

-Five levels of gene control:

DNA availability/transcription factors

mRNA processing mRNA exit from nucleus

RNA degradation

Protein processing and degradation

How does DNA availability affect gene control?

Genes that are not used are packaged tightly until needed, preventing transcription.

How does mRNA processing affect gene control?

After transcription, mRNA can be rearranged during processing, producing different proteins from the same DNA information.

How does RNA degradation affect gene control?

The cell can control how many times mRNA is reused before it is broken down based on need for proteins.

How does protein processing and degradation affect gene control?

-Proteins that are needed only occasionally may be "turned off" and saved for later use.

-Proteins needed for single use may be broken down shortly after use.

Gene mutation

A change in the sequence of bases within a gene.

-Not always harmful.

Causes of mutations

-Errors in replication; happens rarely

-Mutagens

-Viruses

Why are errors in replication rare?

DNA polymerase proofreads strands and corrects errors in DNA

Mutagen

Environmental influences such as UV radiation and chemicals

Types of Gene Mutations

-Point mutations

-Frameshift mutations

Point mutation

Type of mutation in which one nucleotide is substituted for another.

Examples:

silent mutations, nonsense mutations missense mutations

Silent Mutations

Point mutation that does not result in the production of a different protein; harmless

Nonsense Mutations

Result in a "STOP" codon

Missense Mutations

Result in a change in the amino acid made, which may change the protein made.

Frameshift Mutation

-Mutation in which one or more nucleotides are inserted or deleted

-Results in a different sequence of RNA, which will code for the wrong amino acids

Cancer

Uncontrolled cell division caused by changes in DNA

Characteristics of Cancer

-Many different variations

-Improper cell cycle regulation

-Loss of normal cell shape

-Loss of contact inhibition

-Shortened telomeres due to inability to produce telomerase.

Telomere

Nucleotide sequences as the ends of chromosomes that function like a protective cap for DNA.

Steps to forming recombinant DNA

1. Cut the desired DNA by restriction sites

2. Amplify the genes copies by PCR

3. Insert the genes into the vectors.

4. Transfer the vectors into a host organism

5. Obtaining the products of recombinant genes

Polymerase Chain Reaction (PCR)

Process that can make billions of copies of a specific DNA region by using Taq polymerase, specific primers, and as exposure to varying temperatures.

How is PCR useful in forensics cases?

PCR can be used to amplify samples of genetic material found at crime scenes, which can be analyzed for genetic markers. The markers can be compared to the DNA samples of suspects.

Human Genome Project

(1990-2003) An international collaborative effort to sequence and map all the genes in the human genome. As a result, scientists are able to identify genetic markers for diseases and develop gene therapies.

In DNA, Adenine always binds with ....

Thymine

In DNA, Guanine always binds with.....

Cytosine

What is the complementary DNA sequence to the following:

ATTGCGA

TAACGCT

What is the complementary RNA sequence to the following:

TAGATTC

AUCUAAG