Bio 1 Ch 12&13

Bio 1 Ch 12&13

DNA, Genetics, and Biotechnology

DNA Structure

• Nucleotides - Subunits of nucleic acids and consist of an 5 carbon sugar, a phosphate group, and a nitrogenous base

• 2 nucleic acids in living cells are DNA and RNA

DNA and RNA

• 4 nitrogenous bases in DNA

• -Adenine, Guanine, Cytosine, Thymine.

• 4 nitrogenous bases in RNA

• -Adenine, guanine, Cytosine, Uracil

• Adenine and Guanine are known as purines

• Cytosine, Thymine, Uracil are pyrimidines

DNA Scientists

• Erwin Chargaff discovered that amount of guanine on a DNA strand equals the amount of cytosine, and adenine equals thymine.

• DNA is a Double helix - twisted ladder shape,

• -Formed by two strands of nucleotides twisted around each other

Watson and Crick

• Discovered that cytosine and guanine bases pair to each other by 3 hydrogen bonds

• Also discovered that thymine and adenine bases pair to each other by two hydrogen bonds

DNA structure

• DNA is compared to a twisted ladder

• Pairs of bases (cytosine-guanine or thymine adenine) form the steps of the ladder

• Purine always binds to a pyrimidine

• Number of purines always equals the number of pyrimidines in a DNA strand.

Purines and Pyrimidines

• Cytosine and thymine are pyrimidine, adenine and guanine are purines

• C=G and A=T

• C + T = G + A

• Complementary base pairing - describes precise pairing of purine and pyrimidine bases between strands of nucleic acids

5' and 3'

• Rails of the ladder are full of carbon molecules, which can be numbered.

• They are numbered as 5’ (5 prime) and 3’ (3 Prime)

• Opposite rail is numbered 3’ to 5’

Chromosome structure

• Length of a human chromosome ranges from 51 million to 245 million base pairs.

• An 140 million nucleotide long DNA strand would be about 5 cm long if laid out on a table

• In order to fit into a cell, the strands coils around bead like structures called histones.

Replication of DNA

Semiconservative replication

• Watson and Crick introduced semiconservative replication when they presented their model of DNA to the science community

• Semiconservative replication - parental strands of DNA separate, serve as templates, and produce DNA molecules that have one strand of parental DNA and one strand of new DNA

Semi Conservative Replication

• DNA replication occurs during interphase of mitosis and meiosis

• Semiconservative replication occurs in 3 stages, unwinding, base pairing, joining

Unwinding

• DNA helicase unwinds the double helix

• Hydrogen bonds are broken, leaving single strands of DNA

• As the helix unwinds, RNA primer is added onto each DNA strand

Base pairing

• DNA polymerase - catalyzes the addition of appropriate nucleotides to the new DNA strand

• -Nucleotides are added to the 3’ end  of the new strand

• -A binds to T, C binds to G

Base Pairing

• Templates allow identical copies of original double stranded DNA to be produced

• The two strands that are being replicated have two different names

• -Leading strand is being replicated toward the helicase

• -Lagging strand is being replicated away from the helicase

Joining

• There are often many points in eukaryotic DNA that replication occurs

DNA, RNA, and Protein

The "Central Dogma of Biology"

• It was unresolved from Watson and Crick’s case how DNA serves as genetic code for synthesis of proteins.

• Geneticists have since accepted and agreed on the central dogma of biology:

• DNA codes for RNA, which guides the synthesis of proteins

RNA

• RNA - contains sugar ribose, base uracil replaces thymine, usually single stranded.

• 3 types of RNA

• Messenger RNA (mRNA)

• Ribosomal RNA (rRNA)

• Transfer RNA (tRNA)

Messenger RNA (mRNA)

• Travel from nucleus to ribosome to direct synthesis of a specific protein.

Ribosomal RNA

• Associates with proteins to form ribosomes in the cytoplasm

Transfer RNA

• Smaller segments of RNA molecules that transport amino acids to the ribosome

Transcription

• First step of central dogma involves synthesis of mRNA from DNA in transcription

• -Transcription - DNA code is transferred to mRNA in the nucleus

• -mRNA then takes the code into the cytoplasm for protein synthesis

Transcription

• RNA contains Uracil, which is matched with Adenine.

• T matches with A

• G matches with C

• C matches with A

• A matches with U

RNA transcription

• DNA is unzipped in the nucleus and RNA polymerase binds to a specific section where mRNA will be synthesized

• -RNA polymerase - regulates RNA synthesis

RNA Transcription

• RNA polymerase moves along DNA strands in the 3’ to 5’ direction, and synthesizes mRNA as a complement to the DNA nucleotides

• -Strand of DNA that is used to synthesize mRNA is the template strand

• -Strand of DNA not used is nontemplate strands

The Code

• Instructions for protein synthesis are encoded in the DNA

• Only way DNA varies among organisms was in the sequence of the bases

• 20 amino acids used to make proteins

• Discovered that codes of amino acids are in groups of three. Three base cod in DNA or mRNA is a Codon

RNA Translation

• Once mRNA is synthesized and processed, it moves to the Ribosome.

• -mRNA leaves the nucleus and enters the cytoplasm

• -Once in cytoplasm, 5’ end of mRNA connects to the ribosome. Code is read and translated to make a protein through translation

RNA Translation

• tRNA acts as interpreters of mRNA codon sequence.

• -tRNA folds into a cloverleaf shape and is activated by an enzyme that attaches a specific amino acid to the 3’ end.

• -At middle of folded strand, there is a three base coding sequence called the anticodon.

• Every anticodon is complementary to a codon on the mRNA

RNA Translation

• tRNA adds their amino acids to the polypeptide chain until a “Stop codon” is reached.

• Once stop codon is reached, translation stops and a polypeptide is released.