The Central Dogma of The Central Dogma of Molecular Biology Molecular Biology

DNA → RNA → ProteinsDNA → RNA → Proteins

Biology IID. Mitchell

The Central Dogma of Molecular The Central Dogma of Molecular BiologyBiology

ReverseTranscription

DNADNA

DNA-Deoxyribonucleic Acid

The discovery of DNA’s structure clarified how DNA could serve as the

genetic material.

Contributions of ScientistsContributions of ScientistsHershey and Chase

◦Confirmed that DNA carries the genetic information

Erwin Chargaff◦The same four bases found in the DNA of all

organisms◦the amount of adenine = the amount of

thymine; the amount of guanine = the amount of cytosine.

Wilkins and Franklin◦Developed X-ray diffraction

photographs of DNA ◦X-rays revealed that DNA resembled

a tightly coiled helix composed of two or three chains of nucleotides.

Watson & Crick◦Used the information from Chargaff,

Franklin, and Wilkins◦Built a model of DNA with the

configuration of a double helix, a “spiral staircase” of strands of nucleotides twisted around a central axis.

Structure of DNAStructure of DNAIs a molecule that is a double helix -two

strands twisted around each other.

Each strand is composed of nucleotides.◦ Nucleotides are made of three parts: a phosphate

group, a sugar, and a nitrogen base.

five-carbon sugar : deoxyribosenitrogen bases : adenine, thymine,

guanine, and cytosine

Connecting the DNA moleculeConnecting the DNA molecule

Rails of the DNA ladder are alternating sugar & phosphates

Rungs are composed of pairs of bases

Strands of nucleotides are held together by hydrogen bonds between bases.

Base Pairing RulesBase Pairing Rules

Rule:Rule: a purine on one strand of DNA always pairs with a pyrimidine on the

opposite strand.

Adenine and Guanine are purines. Thymine and Cytosine are pyrimidines.

Adenine always pairs with ThymineGuanine always pairs with Cytosine

Purines are 2 ringed: Adenine or Guanine

Pyrimidines are 1 ringed: Thymine or Cytosine

The strictness of the base pairing results in two complementary strands.

The sequence of bases on one strand determines the sequence of bases on the other strand.

Example: TCGAACT AGCTTGA

DNA Complementary Strand DNA Complementary Strand PracticePractice

Replication of DNA Replication of DNA Replication - the making of an exact

copy of the DNA molecule Replication occurs whenever a cell

divides The copy must be 100% accurate

(errors = death possibly)

Stages in replication Stages in replication (basic)(basic)

1. DNA molecule is split in two at the end by the work of the enzyme DNA polymerase.

1. DNA unzips slightly

1. New nucleotides attach to the free ends

1. Process continues until completed

Results of ReplicationResults of Replicationresults in two (2) double strands of DNAeach strand is 50% new and 50% old DNA

Replication PracticeReplication Practice

RNARNARibonucleic AcidRibonucleic Acid

RNARNAFunction: protein synthesisLocation: found in ribosomes and the nucleolusStructure: Single helix composed of nucleotidesNitrogenous Bases: Adenine, Uracil, Guanine, and Cytosine

5-Carbon Sugar: Ribose

Three types of RNA:Three types of RNA:rRNA = ribosomal RNA - carries

protein building instructions. Goes from the DNA in the nucleus to the ribosome

◦mRNA = messenger RNA - is the message from DNA for the construction of the new protein molecule

◦tRNA = transfer RNA - delivers amino acids to the ribosomes. Has an anticodon specific to the amino acid.

DNADNA→RNA→Protein→RNA→ProteinDNA is TRANSCRIBED to

messenger RNA (mRNA)

mRNA carries the message to transfer RNA (tRNA)

tRNA is TRANSLATED to an amino acid chain, which makes up proteins

TranscTranscrription iption The copying of one side of the DNA

molecule that results in a single strand of RNA

During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand.

Transcription of a gene produces a messenger RNA (mRNA) molecule.◦ mRNA carries the message from the nucleus to the

ribosomes

Transcription PracticeTranscription Practice

TranslationTranslationThe reading of the RNA code to make

proteins or polypeptides. (protein synthesis)

The nucleotide sequence of mRNA determines the amino acid sequence that codes for a SPECIFIC protein.◦Translation occurs at ribosomes.

mRNA Codon ChartsmRNA Codon ChartsmRNA codon Charts are used for translation.

The genetic code consists of 64 triplets or codons of nucleotides. ◦groups of three bases on a mRNA strand

With three exceptions, each codon encodes for one of the 20 amino acids used in protein synthesis.

Processes In Eukaryotes And Processes In Eukaryotes And ProkaryotesProkaryotes

The basic mechanics of transcription and translation are SIMILAR.

Because bacteria lack nuclei, transcription and translation are coupled.

In a eukaryotic cell, almost all transcription occurs in the nucleus and translation occurs mainly at ribosomes in the cytoplasm.