Lesson Overview 12.2 The Structure of DNA anddevelopment/dna/ .

Lesson OverviewLesson Overview12.2 The Structure of DNA12.2 The Structure of DNA

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Chromosomes and their genes are made of a molecule called DNA.

DNA molecules carry the code that controls what cells are made of and what they do.

DNA stands for deoxyribonucleic

acid.

Each chromosome is a very long molecule

of tightly coiled DNA.

Which part of a DNA molecule holds this information?

What is DNA?

Lesson OverviewLesson Overview The Structure of DNAThe Structure of DNA

Nucleic Acids and Nucleotides

DNA is made up NUCLEOTIDES

3 basic components:

1. 5-carbon sugar called deoxyribose

2. a phosphate group

3. a nitrogenous base


Nitrogenous Bases and Covalent Bonds

Covalent bonds between their sugar and phosphate groups

Hydrogen bonds between base pairs (weaker)

How do you think the four bases are paired?

DNA and base pairs

There are four types of bases, and they are usually identified by their initials.

A

T

C

G

adenine

cytosine

guanine

thymine

The double helix ‘ladder’ of a DNA molecule is held together by ‘rungs’ made from pairs of chemicals called bases.

Base pairs hold the two strands of the DNA helix together.The rules for base pairing are…

There are millions of base pairs in a DNA molecule, and they always follow these rules.

‘A’ always pairs with ‘T’ ‘C’ always pairs with ‘G’

It is the sequence of these bases along a DNA molecule that forms the genetic code – it’s that simple!

How do bases pair together?

A T C G


Nitrogenous Bases and Covalent Bonds

DNA has four kinds of nitrogenous bases:

1. adenine (A) – 2 rings, PAIRS with T

2. guanine (G) – 1 ring, PAIRS with C

3. cytosine (C) – 1 ring, PAIRS with G

4. thymine (T) – 2 rings, PAIRS with A


Chargaff’s Rules

Chargaff discovered that the % of adenine [A] and thymine [T] bases are almost equal in any sample of DNA

The same thing is true for the other two nucleotides, guanine [G] and cytosine [C]

The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules, ” or base-pairing rules


The Work of Watson and Crick (and Franklin)

In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule.

Watson and Crick’s breakthrough model of DNA was a double helix, in which two strands were wound around each other


The Work of Watson and Crick

Watson and Crick’s breakthrough model of DNA was a double helix, in which two strands were wound around each other


Base Pairing

Lesson OverviewLesson Overview12.3 DNA Replication12.3 DNA Replication


The Replication Process

During replication, the DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing.

Each strand of the double helix of DNA serves as a template, or model, for the new strand.



As each new strand forms, new bases are added

If the base on the old strand is A, then T is added to the new strand

Also, G is always paired to C


Each DNA molecule resulting from replication has one original strand and one new strand



The Role of Enzymes

HELICASE – an enzyme that “unzips” a molecule of DNA by breaking the hydrogen bonds between base pairs and unwinding the two strands


The Role of Enzymes

DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA.

DNA polymerase also “proofreads” each new DNA strand, ensuring that each molecule is a perfect copy of the original


Replication in Living Cells

How does DNA replication differ in prokaryotic cells and eukaryotic cells?

• Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied. (DNA is circular)

• In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied

Lesson Overview 12.2 The Structure of DNA anddevelopment/dna/ .

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