Recall : the 4 types of organic molecules

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids ****

Monomer: Nucleotide

Polymer: DNA/RNA

What is DNA?• DNA : Deoxyribose Nucleic Acid

FUNCTION : It’s primary function is to store and transmit information

• Specifically: How and when to make a protein

STRUCTURE :• Nucleotide (monomer) has 3 parts:

- Sugar- Phosphate- Nitrogen Base

Discovery of DNA• During the mid-1900’s, scientists all over

the world were competing to see who could discover the structure of this genetic material

• In 1953, James Watson and Francis Crick published a paper suggesting a model for the structure of deoxyribonucleic acid

• They relied heavily (and somewhat unethically) on scientific evidence reported by other scientists

Example: Rosalind Franklin’s X-ray crystallographic images

• “We’ve found the secret of life!” ~ Francis Crick (1916-2004)

Watson and Crick determined DNA to have the shape of a Double Helix (twisted ladder)

Watson (left) and Crick (right)

More On Nucleotides…There are 4 different types of N-bases in DNATherefore, there are 4 different DNA nucleotides

Adenine (A) Guanine (G)

↑ PURINES

These bases contain 2 carbon rings

Cytosine (C) Thymine (T)

↑PYRIMIDINES

These bases contain 1 carbon ring

The 4 different nucleotides differ only in their nitrogen-bases

The sugar and phosphate groups are identical in each nucleotide

DNA vs. RNADNA

-Deoxyribose Sugar-Double-stranded- Bases A, T, G, C

RNA

- Ribose Sugar- Single-stranded- Bases A, U, G, C

Molecular Modeling: NUCLEOTIDES

Chargaff’s Data• Originally it was thought that the bases

A, T, G, and C occurred in all living things in the same repeated pattern, such as:

… ATGC ATGC ATGC ATGC ATGC …

• If this were true, then DNA could not be the hereditary molecule because it would not be able to provide the variety needed for a molecule containing the genetic code.

• Following WWII, the research of biochemist Erwin Chargaff revealed the percentage of each base (A, T, G, and C) found in the DNA of various organisms.

The following table contains some of his actual data…

Nitrogen Base Make-Up of Different Organismal DNA (by %)

ORGANISM A T G CMycobacterium tuberculosis 15.1 14.6 34.9 35.4

Yeast 31.3 32.9 18.7 17.1

Wheat 27.3 27.1 22.7 22.8

Sea Urchin 32.8 32.1 17.7 17.3

Marine Crab 47.3 47.3 2.7 2.7

Turtle 29.7 27.9 22.0 21.3

Rat 28.6 28.4 21.4 21.5

Human 30.9 29.4 19.9 19.8

Question: Do you observe any patterns in this data?

Complementary Base-Pairing• DNA is said to have a “sugar-phosphate” backbone• The rungs of the “ladder” are made of the N-bases• The makeup of the rungs follows some specific rules:

– Each rung contains 1 purine and 1 pyrimidine– A always pairs with T– G always pairs with C

• The pairing is the result of hydrogen bonding between the N-bases

TRY IT!• Carefully copy the single-stranded sequence of

DNA into your notebook. Then, directly below it, write the sequence of the complimentary strand that would bind to it.

A C T G G C C T A T T →

T G A C C G G A T A A

G G T A C A A T G T A→

C C A T G T T A C A T

DNA Replication• Cells reproduce by dividing (in half) through the process of MITOSIS• Before it divides, a cell must copy all of its DNA … WHY?• How can this be done efficiently? (so that there are NO mistakes!!!)

- The enzyme HELICASE unwinds the DNA molecule (hydrogen bonds break apart, creating 2 single DNA strands)

- The enzyme DNA POLYMERASE binds to the single stranded DNA

- DNA polymerase adds available nucleotides, one at a time, to the free strands (following Chargaff’s rules)

- This results in two DNA molecules with identical base sequences

- This does not happen at just one site, but at many sites simultaneously on the DNA molecule

- Occasionally, the polymerase may make a mistake, changing the original base sequence (MUTATION)

CHROMOSOMESDuring mitosis (cell division), the DNA of eukaryotic cells is packaged into compact structures called CHROMOSOMES

Chromosomes are:• Rod-like structures • Easily visible by microscopy• DNA coiled around proteins • Histones : Proteins that help

maintain chromosome shape and tight packaging of DNA

- Right before division, a chromosome consists of two identical halves- Each half is called a (sister) chromatid- The sister chromatids are joined by a structure called a centromere- When the cell divides, each daughter cell will receive one chromatid

Chromosome Numbers• Each species has a characteristic number of chromosomes in each of its cells

- Humans have 46 chromosomes- The fruit fly has 8 chromosomes- Horses have 64 chromosomes

• Diploid (2n) cells have 2 copies of each chromosome (1 from each parent)• Haploid (1n) cells have only 1 copy

• In sexually reproducing organisms, a chromosome is either:

Sex Chromosome AutosomeIn humans, there are 2:

XX = female

XY = male

In humans, there are 44

Actually, they are 22 pairs of

homologous chromosomes

Chromosome Analysis• KARYOTYPE• A photomicrograph of

all the chromosomes in a (dividing) cell

• Homologous pairs (the 2 copies of each chromosome a cell has) are arranged by size

• Used to analyze the genetic condition of an individual (that is, their sex and the possible presence of a disorder)