Structure and modes of DNA repliction

The central dogma

describes the flow of genetic info within a cell

Replication —> Transcription —> RNA processing —> Translation —> Protein Folding

DNA Replication

DNA is copied to ensure genetic material is passed to new cells or offspring.

Transcription

DNA is transcribed into messanger mRNA by RNA polymerase

Translation

mRNA is translated into a protein at the ribosome, with the help of tRNA and rRNA

DNA strcuture

1. A deoxyribose sugar

2. A phosphate group

3. 1 of 4 nitrogenous bases (adenine, thymine, cytosine, or guanine)

A purine always pairs with a _______ to maintain _____ of DNA double helix

pyrimidine; width

A is a _____ and pairs with ______ which is a _____ they share __ hydrogen bonds

purine; T; pyrimidine; 2

C is a ____ and pairs with ____ which is a ______ they share ____ hydrogen bonds

pyrimidine; G; purine; 3

Which base pair is stronger, therefore harder to break

G → C because there are 3 hydrogen bonds

Double Helix structure

• DNA strands are antiparallel, meaning one strand runs from 5’ to 3’ while the other runs from 3’ to 5’.

• The structure is a right-handed double helix (B-DNA), the most common form in living cells.

Structure of the DNA turns of the helix

• 10 bases per helical turn, with 0.34nm in between each base pair

• Each full helical turn measures 3.4nm

Who proposed the semi-conservative double helical strcuture of DNA?

Watson and Crick

What were the options of models of DNA replication methods

1. Conservative: the parental model strands conserved and a completely new double stranded DNA molecule is synthesized. Result: one DNA is entirely parental the other is entirely new → incorrect

2. Dispersive: random sequences are mixed up → incorrect

3. Semi- the two strands of the parental DNA molecule separate, and each serves as a template for synthesizing a new complementary strand, After one round of DNA replication each DNA contains 1 parental and 1 newly synthesized strand. 

Modes of DNA replication

1. Theta

2. Rolling circle

3. Linear chromosome

Theta replication

• occurs in circular DNA such as bacterial chromosomes

• 1 origin of replication

• Bidirectional

Theta replication steps

1. DS DNA unwinds at replication origin, producing two SS templates for new DNA

2. fork proceeds around circle

3. Eventually two circular DNA molecules are produced

Rolling circle replication

• found in F factor of E. coli and some viruses

• 1 origin of replication

• Unidirectional replication

Rolling circle replication steps

1. Break in one strand of circular DNA

2. synthesis begins at 3’ end of broken strand and the unbroken strand is used as a template

3. 5’ end of broken strand flys away after being cleaved

4. the linear DNA may circularize

5. the products are multiple circular molecules

Linear chromosome replication (eukaryotes)

• multiple origins of replication

• bidirectional

Linear chromosome replication steps

1. At each origin, the DNA unwinds, producing a replication bubble

2. DNA synthesis takes place on both strands at each end of the bubble as the replication fork proceeds outward

3. Eventually the forks of adjacent bubbles run into each other and fuse.

This fusing produces two identical linear DNA molecules

If there is no breakage of a DNA strand, like we see in rolling circle…

Then we must pull the strands apart by breaking hydrogen bonds.