How DNA is used in HeredityReading the Book of Life, or

Gene Expression

DNA is precious – must be protected

• A specific gene specifies a polypeptide

– DNA is transcribed into RNA, which is translated into the polypeptide

– We are polypeptdides

Figure 10.6A

DNA

DNA

Protein

TRANSCRIPTION

TRANSLATION

RNA – the key to using DNA

• Ribo Nucleic Acid

• Single strand, 3 configurations

• sugar is Ribose

• No Thymine - Uracil

• Used to make the proteins from DNA coding

Types of RNA• Messenger

• Transfer

• Ribosomal

2 steps to Protein

Synthesis

Transcription – use DNA gene to make “working copy” - mRNA

• In nucleus

• Enzyme –> RNA polymerase

– Initiate at the “Promotor”

– Elongate

– Transcript made

– End at the “Terminator”

• An exercise in translating the genetic code

Figure 10.8B

Startcodon

RNA

Transcribed strand

StopcodonTranslation

Transcription

DNA

Polypeptide

Translation – protein synthesis• DNA gene makes proteins (structural,

enzymes, hormones)• every 3 letters of transcribed mRNA is

called a CODON• Each CODON specifies a single AA

– 64 codons available to specify the 20 AA in nature

• Uses tRNA & rRNA

• An exercise in translating the genetic code

Figure 10.8B

Startcodon

RNA

Transcribed strand

StopcodonTranslation

Transcription

DNA

Polypeptide

Editing the gene

• Eukaryotic cells have too much information coded (junk DNA)

–message has to be “edited”

–remove introns and keep exons

• “Junk” DNA – 98% of our DNA code

Editing the DNA

Figure 10.10

DNA

RNAtranscriptwith capand tail

mRNA

Exon Intron IntronExon Exon

TranscriptionAddition of cap and tail

Introns removed

Exons spliced together

Coding sequence

NUCLEUS

CYTOPLASM

Tail

Cap

Mistakes in Protein Synthesis• Changes to DNA code

– Point mutations• Deletion

• substitution

• frame shift mutation

– Kinking of DNA• Pyramidine dimer

Figure 10.16A

Normal hemoglobin DNA

mRNA

Normal hemoglobin

Glu

Mutant hemoglobin DNA

mRNA

Sickle-cell hemoglobin

Val

• Types of mutations

Figure 10.16B

mRNA

NORMAL GENE

BASE SUBSTITUTION

BASE DELETION

Protein Met Lys Phe Gly Ala

Met Lys Phe Ser Ala

Met Lys Leu Ala His

Missing

Are ALL mutations harmful?