Introduction to Central Dogma:

DNA Replication, Transcription and Translation

What is Central Dogma?

• It is the flow of genetic information from DNA to Proteins

Transcription Translation

DNA mRNA Proteins

Replication

DNA

What is DNA?

• It is the fascinating molecule that contains the Codeof Life.

• It is the Genetic Code, which is the set of “messages” that are “translated” by ribosomes into proteins that define YOU

What Is the Code of Life?

• Recall that DNA and RNA belong to the class of biomolecules called Nucleic Acids, which are made up of nucleotide monomers

Structure of Nucleic Acids

The nucleotide subunits consist of:

1. A Nitrogen Base

2. A Sugar (either Deoxyribose or Ribose)

3. A Phosphate Group

Structure of Nucleotides

Nitrogen Bases Can Be:

Adenine

Guanine

Cytosine

Thymine (only DNA)

Uracil (only RNA)

Nitrogen Bases

A

G

C

T

U

Erwin Chargaff analyzed DNA from different organisms and found that

A = T & G = C

Chargaff’s Rule of Base Pairing

According to Chargaff, in DNA:

A always bonds with T G always bonds with C

Image by: Riedell

The Sugar Group Can Be Either:

Deoxyribose (in DNA)or-

Ribose (in RNA)

The Sugar Group

Purines consist of Adenine and Guanine bases; they are double-ring structures

Pyrimidines consist of Thymine and Guanine bases; they are single-ring structures

A Purine ALWAYS bonds with a Pyrimidine….A with T and G with C

Purines & Pyrimidines

DNA has no URACIL RNA has no THYMINE

Nitrogen Base Difference DNA & RNA

• The nitrogen bases are held together by weak hydrogen bonds

• The sugar and phosphate groups are held together by strong covalent phosphodiester bonds

Bonding in DNA

CovalentPhosphodiester

Bonds

Semi-Conservative:•Strand separation, followed by copying of each strand. •Each separated strand acts as a template for the synthesis of a new complementary strand.

DNA Replication (occurs in the nucleus)

Step 1:•Unwinding of the double helix

Helicase enzyme unwinds the double helix & creates a replication fork

DNA Replication (occurs in the nucleus)

Step 2:•One DNA strand is used as a template to create the new copy DNA DNA A – T G – C C – G A – T A – T etc

DNA Replication (occurs in the nucleus)

• Transcription produces genetic messages in the form of mRNA

DNA Transcription (occurs in the nucleus)

DNA Template

mRNA

• STEP 1: As in replication, the DNA double helix unzips

• STEP 2: RNA nucleotides line up along one strand of DNA, following the base-pairing rules

DNA Transcription (occurs in the nucleus)

DNA Template

mRNA

• STEP 3: DNA is transcribed into RNA

DNA mRNA G C T A A U G G G C T A

DNA Transcription (occurs in the nucleus)

DNA Template

mRNA

• Single-stranded mRNA peels away from DNA and prepares to move into the cytoplasm

DNA Transcription (occurs in the nucleus)

DNA

mRNA

Nucleus

Cytoplasm

• Eukaryotic RNA is processed before leaving the nucleus

The non-coding segments, called introns, are spliced out

A cap & tail are added to the ends

DNA Transcription (occurs in the nucleus)

Exon Exon ExonIntron Intron

Introns removed

Exons spliced together

Coding sequence

mRNA

DNA

RNAtranscriptwith capand tail

TranscriptionAddition of cap and tail

NUCLEUS

CYTOPLASM

Cap

Tail

• mRNA leaves the nucleus and moves into the cytoplasm where it will be “translated” into a polypeptide (a fancy word for a protein)

DNA Translation (occurs in the cytoplasm)

TRANSCRIPTION

TRANSLATION

PROTEIN

CYTOPLASM

NUCLEUS

• The words that will be translated are triplets of mRNA bases called codons

• The codons in a gene determine the amino acids in the polypeptide sequence

DNA Translation (occurs in the cytoplasm)

DNA molecule

DNA strand

TRANSCRIPTION

RNA

TRANSLATION

PolypeptideAmino acid

Gene 1

Gene 2

Gene 3

Codon Codon Codon Codon

• In the cytoplasm, a ribosome attaches to the mRNA • A tRNA pairs with each codon, adding an amino acid

to the growing polypeptide

DNA Translation (occurs in the cytoplasm)

U C A G

U

C

A

G

GACU

GACU

GACU

GACU

UUUUUCUUAUUG

CUUCUCCUACUG

AUUAUCAUAAUG

GUUGUCGUAGUG

phe

leu

leu

ile

met (start)

val

UCUUCCUCAUCG

CCUCCCCCACCG

ACUACCACAACG

GCUGCCGCAGCG

ser

pro

thr

ala

UAUUACUAAUAG

CAUCACCAACAG

AAUAAC

AAGAAA

GAUGACGAAGAG

tyr

stopstop

his

gln

asn

lys

asp

glu

UGUUGCUGAUGG

CGUCGCCGACGG

AGUAGCAGAAGG

GGUGGCGGAGGG

cys

stoptrp

arg

ser

arg

gly

First Base T

hird Base

Second Base

mRNA Genetic Code Chart

Table 14.2Types of RNA

Type of RNA Functions in Function

Messenger RNA(mRNA)

Nucleus, migratesto ribosomesin cytoplasm

Carries DNA sequenceinformation to ribosomes

Transfer RNA(tRNA)

Cytoplasm Provides linkage between mRNAand amino acids;transfers aminoacids to ribosomes

Ribosomal RNA(rRNA)

Cytoplasm Structural component of ribosomes

• Mutations are changes in the DNA base sequence Are caused by errors in DNA replication Are caused by environmental factors, such as

exposure to radiation and chemicals or temperature changes

• Mutations most likely cause altered proteins to be produced

• 2 Kinds of Mutations: Gene Mutations & Chromosomal Mutations

Mutations

• Point mutations – changes of a single DNA nucleotide – can cause sickle-cell anemia and many other disorders

Gene Mutations

Normal hemoglobin DNA Mutant hemoglobin DNA

mRNA mRNA

Normal hemoglobin Sickle-cell hemoglobin

Glu Val

• Types of Point mutations

Substitutions

Insertions

Deletions

Gene Mutations

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mRNA

NORMAL GENE

BASE SUBSTITUTION

BASE DELETION

Protein Met Lys Phe Gly Ala

Met Lys Phe Ser Ala

Met Lys Leu Ala His

Missing

Gene Mutations

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Deletion

Duplication

Inversion

Homologouschromosomes

Reciprocaltranslocatio

nNon-homologouschromosomes

Chromosomal Mutations

• Human karyotype showing addition of 1 chromosome on autosome 21 – Down Syndrome

Chromosomal Mutations

Chromosome painting