DNA

The Genetic Material

QUESTION

Is the structure and function of DNA the same in almost all organisms?

Fredrick Griffith and Transformation

Hershey and Chase Show That Genes Are Made of DNA

Structure of DNANucleotides Are the Building Blocks.

• There are three components to a nucleotide– Sugar (5 carbon)– Nitrogenous base

(adenine, quanine, thymine,

cytosine)– Phosphate group

Semi-conservative Model of DNA Replication

• Prokayotes– One circular

chromosome– 5 million base

pairs– Replicates in less

than one hour

• Eukaryotes– Humans have 46

chromosomes– 6 billion base pairs– Replicated in a

few hours– DNA is separated

from most of the replication machinery by the nuclear envelop.

Both carry out replication very quickly and with Both carry out replication very quickly and with tremendous accuracy – 1 error /billion nucleotides.tremendous accuracy – 1 error /billion nucleotides.

The Bases Are Either Purines or Pyrimidines.

A always pairs with T and C always pairs with G. This is because of the number of bonds formed between the bases. Two hydrogen bonds form between A and T and three between C and G.

Write the compliment for GGCTATTGGCA.

DNA Replication

• Steps in replication– DNA helix is unwound by helicase.– The point where the DNA strands separate is

called the replication forkreplication fork (Y)– At the replication fork, the enzyme DNA DNA

polymerasepolymerase adds bases according to the base-pairing rules.

– Two new DNA helixes are formed.

Replication Begins at the Origin of Replication

• Prokaryotes have only one (1) origin of replication.

• Eukaryotes have 100’s or 1000’s of origins of replications.

• Replication proceeds in both directions, forming a replication bubble.

• At the ends of the replication bubble is the replication fork – a” Y shaped” region.

• DNA polymerase has the ability to proofread and make corrections if an error has been made. After proofreading the rate of errors is one per billion nucleotides.

• There are multiple DNA replication forks in a strand of linear DNA creating replicating “bubbles.” (Without multiple replication forks, it would take 16 days to copy one DNA molecule.)

What Is the Energy Source for Replication?

• The energy source is a nucleoside triphosphate.

• This molecule is similar to ATP except the sugar is deoxyribose and the sugar in ATP is ribose.

• The energy comes from the hydrolysis of the phosphate tail.

DNA Is Anti-parallel

Structure of A Gene

• Genes are DNA –encoded information that specifies particular proteins; each gene is made of a specific sequence of nucleotides.

• Genes are composed of coding and noncoding sequences.– Coding sequences are are exonsexons (code for amino

acids)– Noncoding sequences are intronsintrons (intervening)

Telomeres

• Telomeres are special nucleotide sequences located at the end of chromosomes.

• They do not contain genes but mutliple repetitions of short nucleotide sequences (100-1000)

• Telomerase

When the introns are removed, the resulting fragments are stitched together to make a protein.