Genetic Coding in Cells

Genetic Coding in Cells

DNA vs. RNA• Double stranded

• Deoxyribose sugar

• Bases: C,G A,T

• Self replicate

• Single stranded

• Ribose sugar

• Bases: C,G,A,U

• Can’t self replicate

• mRNA, tRNA, rRNA

Both contain a sugar, phosphate, and base.

DNA Replication1. DNA divides in half by splitting down the center

of the ladder starting at one end.

2. Each rung separates in the middle, between the 2 bases. C splits from G and A from T.

DNA Replication cont.

3. The cell has spare DNA Units.

4. The correct DNA unit attaches itself to the appropriate rungs on each of the 2 half-ladders as the DNA molecule splits. A spare AT, a spare TA, a spare GC, & a spare CG.

5. After the DNA finishes “unzipping” and the spare DNA units join up with the rungs on the half ladders, 2 identical molecules are formed.

Introduction to The Central Dogma of Molecular Biology

Protein SynthesisFlow of Information:DNA RNA Proteins Transcription Translation

Transcription is the process by which a molecule of DNA is copied into a complementary strand of RNA.

This is called messenger RNA (mRNA) because it acts as a messenger between DNA and the ribosomes where protein synthesis is carried out.

Transcription occurs in the Nucleus.1. RNA polymerase (an

enzyme) attaches to DNA at a special sequence that serves as a “start signal”.

2. The DNA strands are separated and one strand serves as a template.

3. The RNA bases attach to the complementary DNA template, thus synthesizing mRNA.

Transcription cont.

4. The RNA polymerase recognizes a termination site on the DNA molecule and releases the new pre - mRNA.

5. Next the pre - mRNA under goes splicing. This is when the non-coding sequences or introns are eliminated. The coding mRNA sequence can be described as an exon.

6. Now the mature mRNA leaves the nucleus and travels to the ribosome in the cytoplasm.

Transcription

TranslationTranslation is the process of decoding a mRNA molecule into a polypeptide chain or protein.

Each combination of 3 nucleotides on mRNA is called a codon or 3-letter code word.

Each codon specifies a particular amino acid that is to be placed in the polypeptide chain (protein).

Translation•A three-letter code is used because there are 20 amino acids that are used to make proteins.

SU

GA

R-P

HO

SP

HA

TE

BA

CK

BO

NE

B A

S E

S

H

PO

O

HO

O

O

CH2NH2N

NH

N

N

HOH

P

O

O

HO

O

O

CH2

NH2

N

N

N

N

H

P

O

OH

HO

O

O

CH2

NH2

N

N

N

N

O

A Codon

GuanineGuanine

AdenineAdenine

AdenineAdenine

Arginine

Translation•There is a total of 64 codons with mRNA, only 61 specify a particular amino acid.

•There are more than 1 codon for each of the 20 amino acids.

•The remaining three codons (UAA, UAG, & UGA) are stop codons, which signify the end of a polypeptide chain (protein).

•Besides selecting the amino acid methionine, the codon AUG also serves as the “initiator” codon, which starts the synthesis of a protein.

TranslationTransfer RNA (tRNA)

•Each tRNA molecule has 2 important sites of attachment.

1.anticodon, binds to the codon on the mRNA molecule.

2.The other site attaches to a particular amino acid.

•During protein synthesis, the anticodon of a tRNA molecule base pairs with the appropriate mRNA codon.

MethionineMet-tRNA

U*

9

262223Pu

16

12Py 10

25

20:1

G*

17:1

Pu

A20:2

1713

20G

A5051

656463

G

62

52

CPu

59

A*

C

Py

T49

39

4142

31

2928

Pu*

43127

U35

38

36

Py*

34

403047:1

47:15

46

Py47:16

4544

47

73CCA

707172

66676869

321

7654

A CU

Anticodon

TranslationRibosome:

•Are made up of 2 subunits, a large one and a smaller one, each subunit contains ribosomal RNA (rRNA) & proteins.

•Protein synthesis starts when the two subunits bind to mRNA.

•The initiator codon AUG binds to the first anticodon of tRNA, signaling the start of a protein.

Protein Synthesis: TranslationRibosome:

•The anticodon of another tRNA binds to the next mRNA codon, bringing the 2nd amino acid to be placed in the protein.

•As each anticodon & codon bind together a peptide bond forms between the two amino acids.

Protein Synthesis: TranslationRibosome:

•The protein chain continues to grow until a stop codon reaches the ribosome, which results in the release of the new protein and mRNA, completing the process of translation.

Protein Synthesis: Translation

AE

Large subunit

P

Small subunit

Translation - Initiation

fMet

UACGAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA5’mRNA

3’

AE

Ribosome P UCU

Arg

Aminoacyl tRNA

PheLeu

Met

SerGly

Polypeptide

CCA

Translation - Elongation

GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA5’mRNA

3’

AE

Ribosome P

PheLeu

Met

SerGly

Polypeptide

Arg

Aminoacyl tRNA

UCUCCA



3’

ANYTHING

ACIDAMINE

Protein Synthesis

C

O

OHCN

H

HH

C

HO H

C

H

O

CN

H

HH

C

H H

C

H

O

OHCN

H

HH

C

HO H

Serine

C

H

O

OHCN

H

HH

C

H H

AlanineH

C

O

OHC

R

N

H

H

Amino Acid

H2O

AE

Ribosome P

CCA

Arg

UCU

PheLeu

Met

SerGly

Polypeptide



3’

AE

Ribosome P


Aminoacyl tRNA

CGA

Ala

CCA

Arg

UCU

PheLeu

Met

SerGly

Polypeptide


3’

AE

Ribosome P


CCA

Arg

UCU

PheLeu

Met

SerGly

Polypeptide

CGA

Ala


3’

3’

5’

5’

3’

Transcription And Translation In Prokaryotes

Ribosome

Ribosome5’

mRNA

RNAPol.

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Genetic Coding in Cells