DNA AND PROTEIN SYNTHESIS

DNA (DEOXYRIBONUCLEIC ACID)

Nucleic acid that composes chromosomes and carries genetic information.

CHROMOSOME ORGANIZATION1. A chromosome is an enormous strand of

super coiled DNA.

2. Sections of DNA on the chromosome that code for proteins are called genes.

3. Noncoding sections of DNA are called “junk DNA” (regulatory or unknown function)

BUILDING BLOCKS OF DNA

Composed of nucleotides

Nucleotides contain three parts:1. 5-Carbon Sugar (deoxyribose)2. Phosphate Group3. Nitrogen Base (four types, adenine,

guanine, thymine and cytosine)

Adenine and Guanine are purines (composed of two rings of nitrogen atoms)

Thymine and Cytosine are pyrimidines (composed of one ring of nitrogen atoms)

STRUCTURE OF DNA

Consists of two strands of nucleotides that form a twisted ladder (double helix)

Sugar and phosphate alternate along the sides of the ladder (linked by strong covalent bonds)

Pairs of nitrogen bases form the rungs of the ladder (linked by weak hydrogen bonds).

Specific base pairing arrangement (Chargaff’s Rule)

A-T : 2 hydrogen bondsC-G : 3 hydrogen bonds

Nitrogen bases attach to the sugar portion of the side (NOT the phosphate)

Strands run in opposite directions

FUNCTION OF DNA

DNA codes for proteins (structural proteins, enzymes, and hormones)

information for building proteins is carried in the sequence of nitrogen bases

proteins determine physical and metabolic traits and regulate growth and development.

The Relationship between Genes and Proteins

The Flow of Genetic Information

RNA (RIBONUCLEIC ACID)- Nucleic acid involved in the synthesis of

proteins

Structure - Composed of nucleotides, but differs from DNA in three ways.

1. Single strand of nucleotides instead of double stranded

2. Has uracil instead of thymine3. Contains the sugar ribose instead of

deoxyribose

RNA FUNCTIONThree forms of RNA involved in protein

synthesis

1. mRNA (messenger): copies instructions in DNA and carries these to the ribosome.

2. tRNA (transfer): carries amino acids to the ribosome.

3. rRNA (ribosomal): composes the ribosome.

PROTEIN SYNTHESISCells build proteins following instructions

coded in genes (DNA).

Consists of two parts:1. Transcription – transcribing genetic information2. Translation – translating a molecular ‘language’; nucleotides (mRNA) to amino acids (polypeptide chain)

TRANSCRIPTION- DNA is copied into a complementary strand

of mRNA.

WHY? DNA cannot leave the nucleus. Proteins are

made in the cytoplasm. mRNA serves as a “messenger” and carries the protein building instructions to the ribosomes in the cytoplasm.

HOW DOES TRANSCRIPTION OCCUR?1. RNA polymerase untwists and unzips a

section of DNA (usually a single gene) from a chromosome. This is called Initiation.

2. RNA polymerase pairs free RNA nucleotides to the exposed bases of one of the DNA strands following base pair rules. This is called Elongation.

◦Uracil replaces thymine◦Only 1 strand of DNA serves as a

template, the other “hangs out”

3. Newly synthesized mRNA separates from template DNA and DNA zips back up. This is called Termination.

• mG (methyl-guanine) cap is added to starting end.

String of adenines added to the other end (a poly-A tail)

These protect the mRNA from being broken down. Cap helps mRNA attach to ribosome.

Splicing occurs to remove non-coding segments called introns.

Remaining, coding segments are exons.

RNA Processing

RESULT OF TRANSCRIPTION- mRNA strand with instructions for building a

protein that leaves the nucleus and goes to the cytoplasm.

TRANSCRIPTION EXAMPLE Transcribe the following DNA Sequence in

mRNADNA - TAC CGG ATC CTA GGA TCA mRNA - AUG GCC UAG GAU CCU AGU

Intermediary in Protein Synthesis Why would the cell want to have an intermediate

between DNA and the proteins it encodes?

1. The DNA can then stay pristine and protected, away from the caustic chemistry of the cytoplasm.

2. Gene information can be amplified by having many copies of an RNA made from one copy of DNA.

3. Regulation of gene expression can be effected by having specific controls at each element of the pathway between DNA and proteins. The more elements there are in the pathway, the more opportunities there are to control it in different circumstances.

GENETIC CODEThe “language” that translates the sequence

of nitrogen bases in DNA (mRNA) into the amino acids of a protein.

Codon = three nucleotides on DNA or mRNA

One codon specifies one amino acid

Some codons are redundant (code for the same amino acid)

The genetic code is universal to all organisms

DNA: TAC CTT GTG CAT GGG ATCmRNA AUG GAA CAC GUA CCC UAGA.A MET G.A HIS VAL PRO STOP

IMPORTANT CODONS AUG = start translation (Met) UAA, UAG, UGA= stop translation

TRANSLATION- Instructions in mRNA are used to build a

protein- Location of Translation - ribosome (in the

cytoplasm)

PROCESS OF TRANSLATION1. mRNA binds to the ribosome.

2. Ribosome searches for start codon (AUG)

3. tRNA brings correct amino acid (methionine) to the ribosome.

- Each tRNA carries one type of amino acid. - The anti-codon (three nitrogen bases on tRNA) must complement codon for amino acid to be added to protein chain

4. Ribosome reads next codon

5. tRNA’s continue lining up amino acids according to codons

6. Peptide bonds link amino acids together

7. Ribosome reaches STOP codon Amino acid chain is released

RESULT OF TRANSLATIONA Protein

PROTEIN STRUCTURE

• The amino acid sequence causes protein to fold/coil up into unique shapes. The molecule is held in shape by intermolecular bonding. These shapes give a protein its unique function.

• Primary, Secondary, Tertiary, and Quaternary structures.

- Some proteins must be modified before functional: Endoplasmic Reticulum- Then they may be sent to the membrane or out of the cell: Golgi Apparatus

In prokaryotes:◦ One circular gene.◦ 90% translated, few introns. ◦ Some additional genes may be found on plasmids,

smaller DNA circles. ◦ Plasmids can be transferred between bacteria for

a type of “sexual reproduction”.

Differences between Prokaryotes and Eukaryotes

Tiny particles that have no cells, yet they replicate, and evolve.

Must use host cells gene expression machinery. Viral genetic info can be DNA or RNA.

Retroviruses use Reverse Transcription. (ex. HIV): Genetic code in RNA. In host, viral enzyme called reverse transcriptase makes a DNA copy. DNA copy joins host DNA and more viruses are produced.

Viruses

Frame shift mutations – initiation starts 1 or 2 bases off b/c insertion or deletion. Changes the reading frame. Results in different amino acid sequence. Ex: Tay-Sachs disease

Errors in Protein Synthesis

Point mutationa. Change of 1 base pair. Usually results in no change in the organism. Ex: sickle cell anemia and achondroplasia, a form of dwarfism

Somatic cell vs. Sex cell mutationsa. Somatic cell mutation affects only one cell. Not inherited.b. Mutation of tumor suppressor genes in somatic cells may lead to cancer.c. Mutations in sex cells are inherited and will be found in all cells of the offspring.

Gene amplification

- Creates extra copies of genes within a cell. Normal part of development. Specialized cells may need several copies of a gene. • Ex: human pancreatic cells have many copies of

gene that codes for insulin.

- Ex: Oncogenes (like ras) are often amplified in cancer cells.