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Ch16: Molecular Basis of Inheritance (Replication (Mutations can change…
Ch16: Molecular Basis of Inheritance
DNA is Genetic Material
Transformation: assimilation of external DNA by a cell.
phage DNA enters the host cells but the phage protein does not
Phage DNA is small and simple enough to be transformed into bacterial dna
DNA is a polymer of nucleotides made up of three components: a phosphate group, a sugar(deoxyribose), and a nitrogenous base
There are 4 nitrogenous bases: adenine, thymine, cytosine, and guanine
Chargraffs rule: dna varies between species; A/T and C/G are roughly equal
Double helix: two stranded helix shape of DNA
Antiparallel: subunits run in opposite directions
purines: nitrogenous bases with two organic rings
pyrimidines: nitrogenous pairs with a single ring.
Each base has chemical side groups that can form hydrogen bonds with its appropriate partner
Replication
When a cell copies a DNA molecule, each strand serves as a template for ordering nucleotides into a new, complementary strand
Semiconservative model: when a double helix replicates, each of the daughter molecules will have one parental strand, and one newly made strand
Conservative model: the two parental strands somehow come back together after the process
E. coli DNA has one chromosome, while human dna has 46
Origin of replication: specific sequence of nucleotides where the replication of a DNA molecule begins
Bacterial chromosomes have one origin of replication: eukaryotic chromosomes may have hundreds
The two strands are therein separated into a “bubble”, then proceeds in both directions until the entire molecule is copied
Replication fork: edge of a bubble where DNA is being separated
Helicases unzip DNA, while single-strand binding proteins keeping both sides from reforming
Topoisomerase is an enzyme that prepares for splitting by breaking, swiveling, and rejoining DNA strands
The initial nucleotide chain that is produced during DNA synthesis is actually a short stretch of RNA(a primer), not DNA
DNA polymerases catalyze the synthesis of new DNA by adding nucleotides to the 3’ end
two new strands formed during DNA replication must also be antiparallel to their template strands
DNA ligase: the sugar-phosphate backbones of all the Okazaki fragments into a continuous DNA strand
The lagging strand is so named because its synthesis is delayed slightly relative to synthesis of the leading strand
Initial pairing errors between incoming nucleotides and those in the template strand occur at a rate of one in 10^5 nucleotides
mismatch repair: other enzymes remove and replace incorrectly paired nucleotides that have resulted from replication errors
Mutations can change the phenotype of an organism
DNA molecules are constantly subjected to potentially harmful chemical and physical agents, such as X-rays
mutations in the skin cells caused by ultraviolet light are left uncorrected often result in skin cancer
mutations can be passed on from generation to generation
mutations are the original source of the variation on which Evolution works
point mutations: changes in a single nucleotide pair of a gene, occurs in a gamete or in a cell that gives rise to gametes
silent mutation: has no observable effect on the phenotype.
Misspene mutations: Substitutions that change one amino acid to another one are called:
Nonsense mutations: a point mutation that can also change a codon for an amino acid into a stop codon
Insert/deleting mutations: additions or losses of nucleotide pairs in a gene :
frameshift mutation: when the number of nucleotides inserted or deleted is not a multiple of three
Mutagen: physical and chemical agents that cause mutations
Telomere: at the end of eukaryotic chromosomes; don’t have genes, DNA typically consists of multiple repetitions of one short nucleotide sequence.
prevent the staggered ends of the daughter molecule from activating the cell’s systems for monitoring DNA damage
telomeric DNA acts as a kind of buffer zone that provides some protection against the organism’s genes shortening
Chromosomes
Bacteria has 100x viral DNA but 1/1000 eukaryotic DNA
DNA of an E. coli cell would measure about a millimeter in length, which is 500 times longer than the cell
eukaryotic DNA is Combined in a complex called chromatin
Heterochromatin: tightly packed chromatin during interphase
Euchromatin: dispersed chromatin
the looser packing of euchromatin makes its DNA accessible to this machinery, so the genes present in euchromatin can be transcribed
