DNA REPLICATION REVIEW

1. When does DNA divide?

2. Why does it happen at this time?

• What is the first step in DNA replication?

• Next, what is the job of the enzyme Helicase?

What is the replication fork?

1. What happens next?

2. What enzyme is responsible for this action?

3. What is the significance of the three phosphates on the free nucleotides?

LEADING STRAND VS. LAGGING STRAND

Why does the DNA always have to build from the original DNA’s 3’?

Why does the DNA always have to build from the original DNA’s 3’?

The Flow of Genetic Information.How you get from a gene to a protein.

transcriptiontranslation

replication

proteinRNADNADNA gets

all the glory, but proteins do

all the work!

We’ve talked about replication, now we will focus on TRANSCRIPTION…

TranscriptionThe process of copying

a segment of DNAinto RNA.

This is the first step in protein synthesis.

DNA holds the instructions to make proteins, but can NOT leave the nucleus.

Proteins are made outside of the nucleus on the tiny organelles called ribosomes.

The DNA needs a messenger to take a copy of the instructions to the

ribosomes so the proteins can be made.

What’s the problem?

• Other than DNA, what is another type of nucleic acid?

• There are 3 types of RNA, but today we will only mostly focus on one- mRNA.

Messenger RNA (mRNA) delivers a copy of DNA’s instructions from the

nucleus to the ribosomes.

SIDE-NOTE!

• RNA’s structure differs from DNA’s structure in three ways.

# 1

RNA is single stranded—it looks like one-half of a zipper—whereas DNA is double stranded.

• The sugar in RNA is ribose;

• REMEMBER:DNA’s sugar is deoxyribose.

# 2

• Finally, both DNA and RNA can contain four nitrogenous bases, BUT RNA does not have

Thymine.

• Thymine is replaced by a similar base called uracil (U).

Uracil forms a base pair with adenine in

RNA.

# 3

What do you notice

instead?

Steps of Transcription

1. DNA is uncoiled by enzymes and unzips using Helicase, just as it does in DNA

replication.

Steps of Transcription

2. An enzyme called RNA polymerase recognizes and binds to a DNA at a site

called a “promoter”.

A promoter is a base sequence in DNA that signals the start of a gene.

Steps of Transcription

3. The RNA Polymerase then moves along the DNA strand’s 3’ 5’prime direction, joining one free RNA nucleotide after another until

it reaches a termination sequence.

Steps of Transcription

4. Once the termination sequence is reached, the entire ‘coding gene’ has been

transcribed into an mRNA strand, and the mRNA is cut away from the DNA.

Transcription differs from DNA replication in three key respects…

1. Instead of copy the whole DNA strand, only a selected gene within the DNA is copied.

2. Instead of DNA polymerase attaching free nucleotides, it is RNA polymerase.

3. At the end of transcription there is a single, free strand of RNA nucleotides, not a double

helix.

Transcription is complete at this point. The mRNA at this point is considered “pre-mRNA”.

HOWEVER, the mRNA is not yet ready to leave the nucleus and deliver DNA’s message.

It first needs some “finishing touches”…… called Post-Transcriptional mRNA processing.

A A AA

A3' poly-A tail

mRNA

5'5' cap

3'

G PPP

50-250 A’s

Post-transcriptional processing• First a cap and tail are added to the mRNA strand.

– Why? • The cap helps the mRNA bind to the ribosome

5 GTP cap (modified Guanine nucleotide)

• The tail keeps enzymes in the cytoplasm from “attacking” the mRNA strand.

poly-A tail (string of Adenine nucleotides)

Remember - Eukaryotic genes have “junk”!

• Eukaryotic genes are not continuous– exons = the real gene

• expressed / coding DNA

– introns = the junk, stuff you don’t need in order to build a protein.• inbetween sequence

eukaryotic DNA

exon = protein coding (expressed) sequence

intron = non-coding (in-between) sequence

mRNA splicing – Taking out the “Trash”

eukaryotic DNA

exon = coding (expressed) sequence

intron = noncoding (inbetween) sequence

primary mRNAtranscript

mature mRNAtranscript

pre-mRNA

spliced mRNA

• Edit out the introns

• A “mature” mRNA strand has now been formed– It is much shorter than the original– Exons are Exported, Introns stay IN the nucleus

~10,000 bases

~1,000 bases

Splicing must be accurate• No room for mistakes!

– a single base added or lost affects the entire protein

AUG|CGG|UCC|GAU|AAG|GGC|CAU

AUGCGGCTATGGGUCCGAUAAGGGCCAUAUGCGGUCCGAUAAGGGCCAU

AUG|CGG|GUC|CGA|UAA|GGG|CCA|U

AUGCGGCTATGGGUCCGAUAAGGGCCAUAUGCGGGUCCGAUAAGGGCCAU

Met|Arg|Ser|Asp|Lys|Gly|His

Met|Arg|Val|Arg|STOP|

RNA splicing enzymes

snRNPs

exonexon intron

snRNA

5' 3'

spliceosome

exonexcisedintron

5'

5'

3'

3'

3'

lariat

exonmature mRNA

5'

• Spliceosome– Made of small nuclear

ribosomes

– recognize splice site

sequence• cut & paste gene

mRNA

From gene to protein

DNAtranscription

nucleus cytoplasm

aa

aa

aa

aa

aa

aa

aa

aa

aa

aa

aa

ribosome

proteintranslation