DNA and Genes

Unit 4

Chapter 11

DNA structure

DNA controls cellular activity because it regulates the production of proteins.

DNA is the blueprint for proteins that are necessary for cellular metabolism.

Why are proteins so important?

Some proteins become important structures, such as the filaments in muscle tissue.

• Other proteins, such as enzymes, control chemical reactions that perform key life functions—breaking down glucose molecules in cellular respiration, digesting food, or making spindle fibers during mitosis.

Who discovered that DNA is the blueprint for life? In 1952 Alfred Hershey and Martha Chase

performed an experiment using radioactively labeled viruses that infect bacteria.

Because viruses are protein and DNA only, they figured out that viral DNA (not viral protein) could force the bacteria to make new viruses.

This was evidence that DNA can determine cell activity.

DNA is a polymer.

Polymer: chemical structure made of repeating units

DNA is made of repeating nucleotide units. DNA nucleotides always have a phosphate

group, deoxyribose sugar, and a nitrogen base.

DNA nucleotide

Four DNA nitrogenous bases

A nitrogenous base is a carbon ring with nitrogen atoms and determines the name of the nucleotide.

In DNA, there are four possible nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

Adenine (A) Guanine (G) Thymine (T)Cytosine (C)

Nucleotides join together to form long chains, with the phosphate group of one nucleotide bonding to the deoxyribose sugar of an adjacent nucleotide.

The phosphate groups and deoxyribose molecules form the backbone of the chain, and the nitrogenous bases stick out like the teeth of a zipper.

The structure of nucleotidesThe structure of nucleotides

DNA is a double helix and looks like a twisted ladder.

The outer parts are the sugar-phosphate backbone.

Two nitrogen bases of the nucleotides face inward and form the rungs of the helix ladder.

Adenine always binds to thymine.

Cytosine always binds to guanine..

Who discovered the double helix structure? In 1953, Watson and

Crick proposed that DNA is made of two chains of nucleotides held together by nitrogenous bases and twisted together.

They used Rosalind Franklin’s X-ray crystallography work to figure this out.

The importance of comparing DNA nucleotide sequences

Each species has its own unique DNA sequence.

The more closely related two individuals are, the more likely they will share the same DNA nucleotide sequence.

Comparing DNA base pairs of two species will show their evolutionary history.

DNA replication – making copies of the DNA code Necessary before a cell

undergoes mitosis, occurs in interphase

Click on image to play video.

Copying DNA

How does DNA code for proteins?

The sequence of nucleotides in each gene contains information for assembling the string of amino acids that make up a single protein.

The ribosomes required to make proteins cannot read DNA.

Therefore, for DNA to code for proteins, an RNA molecule must be made.

Ribosomes can read RNA.

RNA is another nucleic acid, nucleotide polymer.

RNA differs from DNA structure in three ways. Single stranded

instead of double stranded

Ribose sugar instead of deoxyribose

Uracil instead of thymine nitrogen base

Ribose sugar

Three types of RNA

Messenger RNA: carries the DNA code (message) to the ribosomes

Ribosomal RNA: makes up the ribosomes that reads the mRNA to build the correct amino acid sequence

Transfer RNA: brings the amino acids to the ribosome

Transfer RNA

Click on the image to play the video.

Transcription

The process of building an RNA strand from the DNA template

Click on image to play the video.

In eukaryotes, this occurs inside the nucleus.

In prokaryotes, this occurs in the cytoplasm.

Transcription

mRNA processing in eukaryotes

Since much of the DNA code is useless or codes for multiple proteins, the unnecessary portions of DNA that were coded into mRNA must be removed.

The useless portions of RNA (introns) are removed. The coding portions (exons) are linked together to make the final mRNA.

mRNA codes for amino acids.

Three mRNA nucleotides code for one amino acid, but more than one combination codes for the same amino acid.

Translation

The process of ribosomes reading the mRNA code to properly make an amino acid chain that is folded into a usable protein

Translation

The ribosome binds to AUG, the starting code (codon). The ribosome directs the methionine tRNA to bring transfer the methionine (met) amino acid.

Translation

1. The ribosome read the next codon and directs the appropriate tRNA to transfer the amino acid.

Translation process

1. The ribosome joins the amino acids together and continues this process until the codon indicates stop.

What happens if there is a mistake (mutation) in the DNA code? Possibly proteins won’t be made or are made

improperly. If the mutations occur in the gametes, the offspring’s

DNA will be affected positively, negatively, or neutrally.

What can cause a mutation? Replication error Transcription error Cell division error Chemical agents (mutagens) Spontaneous changes

Point mutation

A point mutation is a change in a single base pair in DNA.

A change in a single nitrogenous base can change the entire structure of a protein because a change in a single amino acid can affect the shape of the protein.

Point mutations

May change the amino acid code if the mutations occurs in the right place in the code.

Normal

Point mutation

mRNA

ProteinStop

Stop

mRNA

Protein

Replace G with A

Frameshift mutations

Losing a single nucleotide base This mutation would cause nearly every

amino acid in the protein after the deletion to be changed

mRNA

Protein

Deletion of U

Changes to the chromosome

When a part of a chromosome is left out, a deletion occurs

When part of a chromatid breaks off and attaches to its sister chromatid, an insertion occurs.

Deletion

A B C D E F G H A B C E F G H

Insertion

A B C D E F G H A B C B C D E F G H

Changes to the chromosome

When part of a chromosome breaks off and reattaches backwards, an inversion occurs.

When part of one chromosome breaks off and is added to a different chromosome, a translocation occurs.

Inversion

A B C D E F G H A D C B E F G H

A B E FDCBX AWC HGGE HD F

W X Y Z Y ZTranslocation

Repairing DNA

Enzymes proofread the DNA and replace incorrect nucleotides with correct nucleotides.

The greater the exposure to a mutagen such as UV light, the more likely is the chance that a mistake will not be corrected.