1.6.2: DNA Replication & 1.6.3: Mutations

DNA Replication

Before mitosis a cell must copy DNA so each daughter cell gets an identical copy

DNA Replication process

1. Enzyme helicase breaks bonds b/w nucleotides on the 2 DNA strands, unwinding the double helix

2. Enzyme DNA polymerase reads the sequence of nucelotides on one DNA strand (template strand)

3. Polymerase builds a new strand by matching nucleotides to those on template strand

End product of DNA Replication

2 identical double helixes (product of polymerase)

____ adds complementary nucleotides to the growing DNA strand during replication.

DNA polymerase

Important components of the nucleotide

a nitrogenous base, deoxyribose (5-carbon sugar), and a phosphate group

Nucleotide named based on ___

the nitrogenous base

purine

nucleotide with nitrogenous base of adenine(A) or guanine (G)

Pyrimidine

Nucleotide with nitrogenous base of Cytosine(C) or Thymine (T)

DNA IS ______ to form structures called nucleosomes

wrapped around proteins known as histones

Histones

evolutionarily conserved proteins that are rich in basic amino acids and form an octamer

Dideoxy chain termination method

DNA sequencing technique invented by Fred Sanger

Electrophoresis

Technique used to separate DNA fragments of different sizes; the smallest fragments will be farthest from the well (where the DNA was loaded), and the heavier molecular weight fragments will be closest to the well.

Supercoiling

DNA twisted in order to fit within small cells (done within prokaryotic cells)

“beads on a string” DNA structure

The DNA (which is negatively charged because of the phosphate groups) is wrapped tightly around the histone core. This nucleosome is linked to the next one with the help of a linker DNA.

heterochromatin

tightly packaged region of eukaryotic chromosomes; usually contains genes that are not expressed; found in the regions of the centromere and telomeres

Euchromatin

less dense region of eukaryotic chromosomes; usually contained genes that are transcribed with DNA packaged around nucleosomes but not further compacted

What are the 3 models of DNA Replication?

Conservative, semi-conservative, & dispersive

Conservative model of DNA replication

Parental DNA remains together & the newly formed daughter strands are together

Semi-conservative model of DNA replication

suggests that each of the two parental DNA strands act as a template for new DNA to be synthesized; after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand

Dispersive model of DNA Replication

both copies of DNA have double-stranded segments of parental DNA and newly synthesized DNA interspersed

Basics of DNA replication

each of the two strands that make up the double helix serves as a template from which new strands are copied. The new strand will be complementary to the parental or “old” strand. When two daughter DNA copies are formed, they have the same sequence and are divided equally into the two daughter cells

DNA replication in Prokaryotes

Quick replication of nucleotides; The addition of nucleotides requires energy; this energy is obtained from the nucleotides that have three phosphates attached to them, similar to ATP which has three phosphate groups attached

DNA Replication in Eukaryotes

Slower than prokaryotic DNA replication; The essential steps of replication are the same as in prokaryotes.

Mutations

happen as a result of mistakes during DNA replication or due to damage to the DNA; can also be the result of the addition of a base, known as an insertion, or the removal of a base, also known as deletion

Proofreading

repair mechanic for DNA replication error; DNA pol reads the newly added base before adding the next one, if it has not paired correctly the enzyme makes a cut at the phosphodiester bond and releases the wrong nucleotide

Mismatch repair

errors corrected after replication is completed

Nucleotide excision repair

enzymes replace incorrect bases by making a cut on both the 3' and 5' ends of the incorrect base. The segment of DNA is removed and replaced with the correctly paired nucleotides by the action of DNA pol.

Induced mutations

result from exposure to chemicals, UV rays, X-rays, or other environmental agent

Spontaneous mutations

occur without any exposure to any environmental agent; they are a result of natural reactions taking place within the body

Silent mutations

non expressed mutations

Point mutation

 affect a single nucleotide, potentially altering the function of a gene

Transition substitution

a purine or pyrimidine being replaced by a base of the same kind; for example, a purine such as adenine may be replaced by the purine guanine.

Transversion

substitution refers to a purine being replaced by a pyrimidine, or vice versa; for example, cytosine, a pyrimidine, is replaced by adenine, a purine

translocation.

a piece of DNA from one chromosome may get translocated to another chromosome or to another region of the same chromosome

Substituion mutation

wrong nucleotide used

frameshifts mutation

extra nucleotides added or essential one deleted