THE STRUCTURE

OF DNA

Chapter 12… section 12.1 & 12.2

What do we know about DNA?

What is the importance of DNA?

Chromosomes contain many genes made of DNA.

A gene is a segment of DNA that codes for a particular trait.

GENES (and DNA)DO 3 CRITICAL THINGS ….

1. carry information from one generation to the next

2. determine the inheritable characteristics of an organism

3. can be easily copied

Deoxyribonucleic Acid (DNA)

DNA is a long thin molecule in a human cell with over 6 billion nucleotides.

DNA- is a long molecule made up of units called nucleotides.- Determines the production of proteins.

A nucleotide has 3 parts:1. Deoxyribose Sugar2. Phosphate Group3. Nitrogenous Base

There are 4 kinds ofnitrogenous bases in DNA…..

1. Adenine (A) 2. Guanine (G)3. Cytosine (C) 4. Thymine (T)

Adenine and Guanine are PURINESand have a “double” ring structure

Thymine and Cytosine are PYRIMIDINES and have a “single” ring structure

DNA looks like a twisted ladder….the sides (BACKBONE)

are composed of alternating…

deoxyribose sugars and

phosphate groups(Covalently bonded together)

the steps (RUNGS) are composed of ….

complementary pairs of nitrogenous bases

“untwisted” “twisted”

The nucleotides can be joined in any order….

this means that, ANY SEQUENCE of N bases is possible…

thereby allowing for great diversity in living things – even among members of the same species

Watson and Crick – determined the structure of DNA to be a DOUBLE HELIX in which two strands were wound around each other

DNA is like a spiral

staircase

HYDROGEN BONDS…bond between a positive H atom of one molecule and a negative atom of another molecule; intermolecular force

weak bond no electrons involved

Form between the N bases in DNA

provide enough force to hold the 2 strands of DNA together

Hydrogen Bonds form between certain base pairs

ADENINE (purine) and THYMINE (pyrimidine) – 2 H bonds

GUANINE (purine) and CYTOSINE (pyrimidine) – 3 H bonds

because the N bases can only pair in a certain way this is called…..

COMPLEMENTARY BASE PAIRING

a purine will always bond with a pyrimidine

HYDROGEN BONDS

Chargaff’s Rule (because of complementary base pairing) for every adenine there is exactly 1 thymine

for every cytosine there is exactly 1 guanine

Covalent Bonds – occur when 2 atoms share electrons; intramolecular force

strong bonds that are not easily broken found between…

• the sugars and phosphates • the sugars and nitrogen bases

maintain the….• backbone (sides) of the DNA molecule• the integrity of the DNA code (sequence of N bases)

DNA Structure- 10 base pairs make up one full twist of DNA

What is the complement?

If the sequence of N bases on 1 strand of DNA is….

A T C G G C T T A A T A T C GWhat is the sequence on the other strand?

REMEMBER…DNA is a double stranded molecule

T A G C C G A A T T A T A G C

During most of the cell cycle chromosomes are not visible, instead the DNA is seen in a form called chromatin.

During cell division, the chromatin condenses and coils around proteins (called histones) to form chromosomes.

Semiconservative DNA REPLICATION…. (hyperlink)making an exact copy of DNAoccurs before cell division (mitosis + meiosis) - interphase highly accurateMUTATIONS may occur ….

changes in the (sequence of nucleotides) DNA

Watson and Crick realized that each strand of a DNA molecule has all the

information needed to RECONSTRUCT THE OTHER STRAND

DNA Replication a simplistic view….

DNA Replication – 3 Steps

1. UNWINDING … DNA Helicase (enzyme) separates the DNA

molecule at Hydrogen bonds2. BASE PAIRING… DNA Polymerase (enzyme) adds new

nucleotides forming new strands of DNA3. JOINING… DNA ligase (enzyme) connects new

“daughter” strands together

The sites where separation and replication occur are called replication forks

DNA replication produces ….

2 new complementary strands(of DNA) Following (Chargaff’s) rules of base

pairing

Each “parental” (old) strand of DNA serves as a template (pattern) for the making of (2) “daughter” (new) strands

When DNA replication is complete…. 2 molecules of DNA are made

Each molecule is composed of… 1 old strand and 1 new strand

Prokaryotic Cells…. have no nucleus have no membrane bound organelles… mitochondria, golgi bodies, vacuoles,… DNA is in the cytoplasm have a single, circular chromosome DNA replication begins at a single point

and often proceeds in 2 directions until the entire DNA is replicated

Eukaryotic Cells… have a nucleus have membrane bound organelles… mitochondria, golgi bodies, vacuoles,… DNA is in the nucleus have more DNA…. DNA replication occurs at hundreds of places

and produces segments called OKAZAKI FRAGMENTS

replication proceeds in both directions until the DNA is completely copied

DNA is usually in a form called CHROMATIN

But during cell division, the chromatin Condenses and coils around proteins (called histones)to form CHROMOSOMES (of DNA and protein)

chromosome

chromatin