Chapter 14 - DNA Structure and Function

Mitochondrial DNA is inherited only from the

mother

One set of 23 human chromosomes represents the human

haploid genome

Transformation discovered by _______

Griffith

Transformation

a chemical substance from one cell can pass information to another cell

R strain; S strain

was not deadly to mice; deadly

Transformation Experiment

-Inject R —→mouse lives

-Inject S —→mouse dies

-Inject S destroyed by heat —→ mouse lives

-Inject a mixture of destroyed S with live R —→ mouse dies!

Transforming factor

Genetic material escaped from the dead S cells and entered the live R cells.

Avery, McCarty, and MacLeod Discovered

the CAUSE of transformation

____ is responsible for the genetic transformation.

DNA

Hershey and Chase Confirmed that DNA was responsible

How:

--They used bacteriophage with either the DNA or the protein labeled.

--They looked to see if DNA or protein was injected into bacteria (to turn them into phage-making factories.

They showed that only rough cells given smooth cell’s DNA could be ________

transformed

The Hershey and Chase Experiment

-Label DNA with ³²P (no P in proteins). If it enters the cell, then DNA is the genetic material

-Label bacteriophage proteins with 35S (no S in DNA). If it enters the cell, then protein is the genetic material.

When Phage infect bacteria, the _________ is injected into the bacterium. The phage stays outside.

genetic material

If phage DNA enters the bacterial cell _________

DNA must be the genetic material

If phage protein does NOT enter the bacterial cell ________

Protein is NOT the genetic material

Complementary Base-pairing

(Adenenine) A = T (Thymine)

(Guanine) G = C (Cytosine)

If humans have 30% A and 20 % G, what percent T and G would be expected?

50%

The Building Blocks of DNA are _________

Nucelotides

Nucleotides

have a phosphate, sugar (ribose or deoxyribose), and base (A, T, G, C or U)

Pyrimidines

nucleotide base with a single ring structure: C, T and U

Purines

nucleotide base with a double ring structure: A and G

Watson and Crick Discover _____

structure of DNA

DNA is a ________

helix

The Structure of DNA

Sugar–phosphate backbones are on the outside of the helix.

Nitrogenous bases point toward center of the helix.

Complementary base pairing with H-bonds helps hold the strands together.

A always pairs with T (two hydrogen bonds).

G always pairs with C (three hydrogen bonds).

The Structure of DNA (Part 2)

Antiparallel strands run in opposite directions.

Each strand has a 5’ end (phosphate) and a 3’ end (-OH group hydroxyl).

One strands runs 5’ to 3’ from top to bottom (left), the other strand runs 5’ to 3’ from

bottom to top (right). NOTE: DNA is made in 5’-3’ direction (by adding to the 3’ end)

DNA Sequencing

Fred Sanger discovered

Dye-labeled ddNTPs are added to stop DNA chain from being synthesized at different lengths.

The DNA sequence is determined by passing the DNA fragments through a capillary tube. A laser detects each labeled ddNTP at the end of each fragment.

Three suggested models of DNA replication:

1) Conservative

2) Semi-conservative
3) Dispersive

Meselson and Stahl Showed DNA Replication is

Semi-conservative

___________ helped them distinguish new strands from old strands.

Radioisotope-labeled DNA

Semiconservative Replication Experiment Results:

New DNA is Half Heavy (¹⁵N) Half Light (¹⁴N)

DNA Replication in Prokaryotes

The single circular chromosome is replicated bidirectionally, starting at the origin

A DNA Replication Fork

DNA replication creates a

Replication Fork

DNA is unwound by

Helicase

DNA needs help to start replication from

RNA Primers

Primase

adds the RNA primer, required for new DNA to have something to attach to.

DNA polymerase III

adds new DNA continuously in the 5’-3’ direction towards the replication fork.

RNA primers are replaced by ______

DNA Polymerase I

Leading vs Lagging

Synthesized Continuously vs Synthesized in Fragments

DNA polymerase synthesizes DNA in 5’-3’ direction away from replication fork, creating

Okakzi Fragments

DNA pol I replaces _________ with DNA

RNA Primers

DNA Ligase

Links the fragments (during lagging)

DNA Replication Summary

DNA Replication in Eukaryotes

The large, multiple, linear chromosomes are replicated bidirectionally, simultaneously from many origins.

Telomeres (Only in eukaryotes)

The sequences at the ends, known as telomeres, do not contain functional genes.

Human chromosome telomeres have

TTAGGG

Chromosomes can lose _____ base pairs with each replication

50-200

After 20–30 divisions, most cells

die

Telomerase

enzyme that adds back the telomere sequences

Telomerase is found in 90% of ______ cells

cancer

DNA Repair Corrects ________ and Damage

Mutations

If proofreading fails, ___________ can recognize incorrect base pairs

and fix them. This can only happen very soon after DNA replication.

mismatch repair

Nucleotide excision repair

can fix problems even long after DNA replication.

Mutations in ____ have been known to cause cancer.

repair genes

Proofreading

corrects errors during replication. (Done by DNA Polymerase)

Mismatch Repair

-Incorrectly added base is detected soon after replication.

-Proteins detect this base and remove it from the newly synthesized strand by nuclease action.

-The gap is now filled with the correctly paired base

Nucleotide Excision Repair

-Nucleotide excision repair can repair thymine dimers.

-When exposed to UV, thymines lying adjacent to each other can form thymine dimers. In normalcells, they are excised and replaced.

Types of Mutations in DNA

Mutations are variations in the nucleotide sequence of DNA.

Induced mutations

result from an exposure to chemicals, UV rays, x-rays, or some 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.

Difference between Prokaryotic and Eukaryotic DNA Replication