2

a physical molecule and genetic information

A polymer = a chain of identical or nearly identical subunitsDNA has 4 kinds of base subunitsDNA has 2 strands whose bases have complementary shapes and charges

The order of the bases along the chain specifies protein sequences and other genetic functions.The 2 strands have complementary information. This permits:

1. error correction (‘informational redundancy’)2. copying of information3. readout of information into RNA.

DNA

Complementarity and hydrogen bonding:1. hold the strands together2. direct DNA replication3. direct synthesis of RNA

2

3

Sequence complementarity is a physical property of the bases; the shape of each base determines which other base it can pair with.

A G

CT

A G

GA

CT

CT AGA

T

G

C T C

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More about the physical properties that matter:

The strands must physically separate (‘unzip’) for DNA replication (copying) and RNA transcription (readout).

The sides of the paired bases are exposed and can interact with the regulatory proteins that sense DNA sequence.

The bases, and the bonds connecting them, are asymmetric; in the 2 strands they run in opposite directions (‘polarity’).

The DNA in each chromosome is 2 very very long single DNA strands wound around each other (the ‘double helix’).

Image from Wikimedia Commons4

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RNA: • Like DNA, a nucleic acid• Slightly different backbone than

DNA (ribose, not deoxyribose)• T bases replaced by U bases• Usually a single strand,but can fold

up by internal base pairing

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Protein: • Not a nucleic acid• A polymer of amino acids• More in Module 3

Images from Wikimedia Commons

• Chromosome structure :• One very long molecule of DNA. (e.g. human X = 1.5 x 108 bp)

• Bound to and wrapped around proteins

chromosomes• 23 different ones (2 versions of

each in each person)

• Named 1 to 22, from largest to smallest, and X and Y (the two versions of the ‘sex chromosomes’0

• ~50 – 250 million base pairs long

• ~400 – 4000 genes on each

• Each chromosome has different genes

For a molecule of DNA to function as a chromosome, it must carry specific information:1. Sequences recognized by DNA replication proteins (‘origins’ where DNA

replication starts and ‘telomeres’ at the chromosome ends).

2. A ‘centromere’ sequence that creates an attachment point for the fibers that pull replicated chromosomes apart in cell division.

3. (Genes)

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Griffith’s demonstration of bacterial transformation

Avery, MacLeod, McCarty’s Experiment

BEADLE AND TATUM’S EXPERIMENT

Lederberg and Tatum’s demonstration of genetic recombination between bacterial cells.

BERNARD AND DAVIS’s EXPERIMENTS

Bacteria can transfer genes from one strain to another by three different mechanisms:

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There are two sex chromosomes (the X and Y), which determine the gender of the person carrying them. The other 22 chromosomes are called autosomes. Somatic cells in diploid eukaryotes contain pairs of each autosome plus two sex chromosomes

Human genome

One person

Another person

~99.9% of sequence positions are identical.

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Within-species diversity in most plants and anmials

One individual

of a species

Another individual

of the same

species

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Humans are a young species; we are more similar to each other than individuals of most other species.

~99% of sequence positions are identical.

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Introns and splicing

Most protein-coding genes contain segments called introns that are ‘spliced’ out of the RNA before it is translated into protein. The other segments are called exons.

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noncoding

coding region

IntronsNot transcribed Exons

Transcribed into RNA

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Introns and splicing

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promoter terminator

transcribed segment (specifies RNA)

mature (spliced) mRNA

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The cellular machinery that does the splicing (a protein-RNA complex called the spliceosome) recognizes signals in the RNA specifying the parts to be cut out.

Vectors for Gene Cloning: Plasmids and Bacteriophages

Transgenic Organisms

• One example of this application of transgenics is the production for commercial reasons of plants which are resistant to herbicide.

• Plants are being developed which are more resistant to various stresses such as high temperature, freezing and high salinity.

• enhance their value as food, for example by changing the balance of fatty acids present in the oil that they produce to a more healthy one.