Module 9 - DNA and its role in Heredity

Circumstantial evidence for DNA

1. Present in the nucleus of a cell and in chromosomes

2. Doubles during S phase of the cell cycle

3. Diploid cells have twice as much as haploid cells

Transformation experiment

found that DNA is required for bacteria to transform (change their genetic makeup)

Hershey-Chase experiment

found that bacteriophages (a type of virus) inject DNA (not protein) into cells in order to reproduce

Transformation

Bacteria can incorporate environmental DNA into their own DNA

Structure of DNA

1. Each strand has a sugar-phosphate backbone

2. complementary base pairing between strands

3. bases are perpendicular to the antiparallel strands

Semiconservative

each parental strand is a template for a new strand

Conservative

both strands act as a single template and produce one double-stranded daughter molecule

Dispersive

parent molecule is dispersed among two daughter molecules

Meselson-Stahl experiment

found that DNA replication was semiconservative

DNA replication occurs in 3 steps

1. Initiation

2. Elongation

3. Termination

Initiation

unwinding the DNA double helix and synthesizing RNA primers

Elongation

synthesizing new strands of DNA using each of the parental strands as templates

Termination

DNA synthesis ends

Initiation steps

1. pre-replication complex binds to site(s) or origin (ori)

2. Replication bubble forms at ori with replication forks at each end

3. DNA helicase move away from the ori, separating the strands

Elongation steps

1. DNA polymerase attaches to primers and adds nucleotides 3’ end to build a new strand

   1. a primer is required

2. During DNA synthesis

Leading strand

built continuous, completes whole strand

Lagging strand

built discontinuous, primer on end is removed, but can’t be replaced

Termination steps

1. two replication forks moving toward each other meet (proteins bind to stop replication)

2. or a replication fork reaches the end of the chromosome

End-replication problem

single-stranded regions of DNA at each end of chromosome are cut off, resulting in a slightly shortened chromosome after each replication

Telomeres

repetitive non-coding sequences at the end of eukaryotic chromosomes to protect coding regions

Telomerase

a form of DNA polymerase that can increase telomere length

Causes of mutations

1. errors during DNA replication

2. DNA damage by chemicals or other agents

3. errors during cell division

Somatic mutations

occur in somatic (body) cells

a. may impact an individual, but not passed offspring

Germline mutations

occur in germ line cells (produce gametes)

a. passed to offspring and future generations

Incorporation error rate

probability that an incorrect base will be inserted in about 1 in 100,000

1. result is a mismatch between complementary strands

2. most replication mistakes are repaired as they happen or shortly afterward

Proofreading

DNA polymerase recognizes a mismatch, backs up, removes mismatched nucleotide, then recommences synthesis

1. 99% of mismatches are recognized and removed

Mismatch repair

after replication, protein complex scans DNA for mismatched bases by searching for abnormal hydrogen bonding

Base-pair substitution

a type of point mutation, which is any mutation where a single nucleotide is changed, inserted, and deleted

Tautomeric shift

a base temporarily forms its rare tautomer (same chemical formula, different arrangement) which can pair with a different base

1. result: a mismatch between strands

Deamination

loss of an amino (NH2) group in cytosine, forming uracil (a base in RNA)

Spontaneous mutations

caused by polymerase errors or spontaneous chemical changes in bases

Induced mutations

caused by mutagens (certain chemicals or radiation) that damage DNA

Excision repair

removes damaged nucleotides and replaces them with normal ones

Direct repair

for some types of DNA damage, mutations can be repaired directly

Silent mutations

do not affect protein function

Loss of function mutations

prevent genet transcription or produce nonfunctional proteins

Gain of function mutations

lead to a protein with altered function

Conditional mutations

produce a protein that only functions under certain environmental conditions

deleterious

cause harm or damage

Deletions

a portion of a chromosome is lost

occurs when: chromosome breaks in two locations and rejoins without the middle segment

result: missing genes can have severe or fatal consequences

Duplications

a portion of a chromosome is repeated

occurs when: homologous chromosomes break at different positions when crossing over

result: one chromosomes will lack a segment (deletion) and other will have two copies (a duplication)

Translocations

a portion of a chromosome in an incorrect location

occurs when: two non-homologous chromosomes break and exchange segments (not always reciprocal)

result: chromosomes with sequences belonging to another

Chromosomal rearrangements

1. deletions

2. duplications

3. translocations

4. inversions

Inversions

a portion of a chromosome is flipped

occurs when: chromosome breaks twice and rejoins, but the segment is inverted

result: can cause loss-of-function mutations