Chapter 11- DNA and Genes

DNA

        deoxyribonucleic acids

language of genetics

codes for proteins (everything is made of or by

proteins)

polymer of nucleotides

Four bases, therefore 4 possible nucleotides

All DNA of all organisms are composed of the same nt.  Diff btw organisms is based on difference in nt sequence

DNA sequence similarity depends on degree of relatedness btw

organisms

Watson and Crick (1953)

DNA - 2 chains of nucleotides (nt)

    joined by h-bonding between bases

    2 sugar phosphate backbone with bases pointed

inward like teeth of a zipper

            Zipper is twisted: double helix

Complementary base pairing:

Adenine always H-bonds Thymine (2 h-bonds)

Guanine always H-bonds Cytosine (3 h-bonds)

Therefore, if you have the sequence of one strand, you know the sequence of the other strand.

Replication

Cells divide - daughters must each have a copy of chromosomes

Basically, DNA is "unzipped" and each strand is copied

1.  DNA is bound by enzymes called helicases- break H-bonds

and unzip double helix into single strands

This starts the replication fork.

2.  DNA polymerase binds to and "reads" sequence of single

strands of template DNA. 

Grabs free nt floating around in nucleus and strings them

together according to the sequence of template DNA

From DNA to protein:

1 gene = 1 protein

~80 000 genes in a cell

DNA-----transcription-----> RNA ------translation------->protein

RNA

Ribose, not deoxyribose

Single stranded

Uracil, instead of thymine

3 types of RNA

1.  Messenger RNA (mRNA) - copy of DNA that is used to make protein

2.  Ribosomal RNA (rRNA) - structural, along with protein makes up ribosomes

3.  Transfer RNA (tRNA) - RNA bound to amino acids, used in decoding mRNA into proteins

Transcription

DNA is copied into RNA

Similar, but not identical, to  replication

RNA polymerase, helicases

The Genetic Code

if there are only  4 nt, how are 20 amino acids coded for?

Codons- DNA and RNA nt are grouped into threes

                64 possible combinations of 3

                Each codon corresponds to one amino acid (more

than 1 codon per amino acid)

AUG - start

UAG, UAA, UGA - stop

Table 11.2 - p 298

All organisms use the same code- universal - evidence that all life evolved from same origin

Translation

RNA ------> protein

on ribosomes in cytoplasm

tRNA is the key- bound to amino acids, have anitcodons to find proper mRNA sequence

1.  Ribosome binds to mRNA in cytoplasm

2.  Ribosomes bind tRNA for 1st codon (start) in "A-site"

3.  a. Ribosome moves, tRNA₁ moves to "P-site," 

     b. tRNA₂ binds to A-site

     c.  peptide bond formed between tRNA₁ and tRNA₂

4. a. Ribosome moves, tRNA₂ moves to "P-site," 

     b. tRNA₃ binds to A-site

     c.  peptide bond formed between tRNA₂ and tRNA₃

Etc. until ribosome reaches stop codon.

Protein Structure

Primary- 1° - amino acid sequence

Secondary - 2° - h-bonding between stretches of amino acids

        alpha-helix

        beta sheets

Tertiary- 3° - 3D folding of protein

Quaternary- 4° - proteins associate with other proteins

Genetic Changes

Mutation- any change in DNA

Mutations in germ line (repro cells)- will carry to offspring

        can have no effect, deleterious (birth defect), or

beneficial (adaptations)

Mutations in somatic cells (body cells) - not passed to offspring,

passed to daughter cells (mitosis)

Cause of aging

cancer - mutation of gene that encodes protein

involved in cell cycle

Types of Mutations:

DNA polymerase makes ~ 1 mistake every 10 000 bases

Proofreading- mispairing causes distortion of DNA shape

                        enzymes detect mispair and correct

1.  Point Mutation- 1 nt. changed

        silent (does not change amino acid) or not (does

change)

THE DOG BIT THE CAT

        THE DOG BIT THE CAR

2.  Frameshift Mutation

        caused by insertion or deletion of nt.

        changes all downstream codons

        THE DOG BIT THE CAT

        THE DOB ITT HEC AT                   (DELETION)

        THE DOG GBI TTH ECA T             (INSERTION)

3.  Chromosomal Mutations

        chromosomes will break and rejoin improperly

        very common in plants

        results in nondisjunction- failure of chro. to                                 separate

        extra or not enough chromosomes

        zygote usually dies, or is sterile

                Down's Syndrome (trisomy-21)

                Klinefelters Syndrome (XXY)

                Turner Syndrome (XO)

Causes of Mutations

1. Mistake in replication

2. Mutagen- any agent that can cause DNA change

        Radiation - X-ray, UV, gamma - break DNA apart, repair    

                        can result in deletions or substitutions

        Chemical- dioxins, asbestos, benzene, cyanide,

formaldehyde