A 4-nucleotide- long segment of DNA. (arbitrary choice of bases) Only variation comes from nitrogenous base side chains.

A 4-nucleotide-long segment of DNA. (arbitrary

choice of bases)

Only variation comes from nitrogenous

base side chains

The general structure of a nucleic acid in terms of nucleotide subunits.

DNA varies from RNA in using deoxyribose sugar not ribose,

and having thymine instead of uracil.Otherwise it’s the same composition.

DNA GEOMETRY

In 3-D, two DNA strands twist into a double helix (spiral staircase).

Rosalind Franklin in early 1950s used X-ray crystallography to study DNA structure.

Watson and Crick used her info to develop a DNA double helix shape.

DNA double helix like a spiral staircase

antiparallel strands: each strand oriented 5‘3'

in opposite directions.Strands are

complementary, not identical.

Central bases pair up to form “steps”

Base pairs composed of one purine paired with one pyrimidine.

Width of the helix stays roughly equivalent with this arrangement

Phosphate–Sugar ester bonds are covalent and very strong.

Base pairs are held together with weak Hydrogen bonds.

This becomes important when we see how DNA works.

Thymine to Adenine

Cytosine to Guanine

(a) Thymine to adenine base pairing involves 2 H bonds.

(b) Cytosine to guanine base pairing involves 3 H bonds.

DNA double helix emphasizing the

hydrogen bonding between bases on the

two chains.

Keep in mind the weak H bonds in the interior, they are key!

For Heredity to Work: Biological info must be accurately copied and

transmitted from each cell to all of its progeny.

Most cells divide frequently, so DNA has to be easily copied (replicated) and separated

before a cell splits in two.

So how is DNA replicated?

DNA replication: 2 strands of DNA double helix unwind (helicase

enzymes) Separated strands = templates for new DNA

strands. Free floating nucleotides pair with

complementary bases on separated strands. Result is replication of DNA molecule.

The enzyme DNA polymerase catalyzes base pairing and acts only in 5’-to-3’ direction.

One strand (top) grows continuously in the direction of the unwinding, the other strand grows

in segments in the opposite direction.

The segments (Okazaki fragments) in this chain are

connected by a different enzyme, DNA ligase.

DNA replication usually occurs at multiple sites within a molecule,

and replication is bidirectional from these sites.

Summary of DNA

Replication

Two identical daughter strands

form.

Then cell

divides.

Replication animation

Review: can you…•Compare and contrast the structure and function of different types of nucleic acids•Draw the basic structure of nucleosides and nucleotides•Explain the primary structure of nucleic acids and compare it to protein structure•Describe the structural properties of the DNA double helix•Draw the steps involved in DNA replication •Compare & contrast RNA to DNA

Chromosomes are DNA

strands wrapped around proteins.

DNA is the template for protein production.

RNA is similar to DNA but with these differences:•The sugar in RNA is ribose not deoxyribose •The nitrogen base uracil replaces thymine. •RNA molecules are single, not double stranded. •RNA molecules are much smaller than DNA molecules.•RNA enters & leaves the nucleus

Protein Synthesis• Proteins are synthesized in the cell

through a series of steps involving– Transcription: DNA --> RNA– Translation: RNA --> Protein

Genes are a segment on a DNA strand that code for a hnRNA/mRNA molecule.

The process of copying a

complementary strand of mRNA off of a DNA

gene is called transcription

The transcription of DNA to form RNA involves an unwinding of a portion of the DNA double helix. Only one strand of the DNA is copied

during transcription.

RNA transcription

Initially heterogenous nuclear RNA (hnRNA) is formedNon-coding pieces (introns) are spliced out using small nuclear RNA and protein particles called “snurps” (snRNP).