DNA The Genetic Material
QUESTION Is the structure and function of DNA the same in almost all organisms?
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
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 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 fork (Y) • At the replication fork, the enzyme DNA polymerase 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.
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 exons (code for amino acids) • Noncoding sequences areintrons (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.