DNA Structure and Function

DNA Structure DNA is a macromolecule that stores and transfers information in living cells. It is found in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. It is also found in mitochondria and chloroplasts.

DNA Structure DNA consists of long strands of nucleotides. Each nucleotide consists of –Deoxyribose (5-carbon sugar) –Phosphate group –A nitrogen-containing base Four bases –Adenine, Guanine, Thymine, Cytosine

Nucleotide Bases phosphate group deoxyribose ADENINE (A) THYMINE (T) CYTOSINE (C) GUANINE (G)

Composition of DNA Chargaff showed: –Amount of adenine relative to guanine differs among species –Amount of adenine always equals amount of thymine and amount of guanine always equals amount of cytosine A=T and G=C

Rosalind Franklin’s Work Was an expert in x-ray crystallography Used this technique to examine DNA fibers Concluded that DNA was some sort of helix

Structure of the Hereditary Material Experiments in the 1950s showed that DNA is the hereditary material Scientists raced to determine the structure of DNA Watson and Crick proposed that DNA is a double helix nal_games/medicine/dna_do uble_helix/ nal_games/medicine/dna_do uble_helix/

Watson-Crick Model DNA consists of two nucleotide strands Strands run in opposite directions Strands are held together by hydrogen bonds between bases A binds with T and C with G Molecule is a double helix

Watson-Crick Model h.edu/units/basics/build dna/

DNA Function DNA can copy itself - REPLICATION DNA transfers information to new strands of DNA. DNA can transfer its information to RNA - TRANSCRIPTION RNA then translates information into an amino acid sequence - TRANSLATION

DNA Structure Helps Explain How it Duplicates DNA is two nucleotide strands held together by hydrogen bonds Hydrogen bonds between two strands are easily broken Each single strand then serves as template for new strand

DNA Replication new old Each parent strand remains intact Every DNA molecule is half “old” and half “new”

Base Pairing During Replication Each old strand serves as the template for complementary new strand

Enzymes in Replication DNA helicase unwind the two strands DNA polymerase attaches complementary nucleotides DNA ligase fills in gaps Other enzymes wind two strands together.

A Closer Look at Strand Assembly Energy for strand assembly is provided by removal of two phosphate groups from free nucleotides newly forming DNA strand one parent DNA strand

Continuous and Discontinuous Assembly Strands can only be assembled in the 5’ to 3’ direction. This refers to the number of the carbon of the sugar

DNA Repair Mistakes can occur during replication DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand

Same two steps produce ALL proteins: 1) DNA is transcribed to form RNA –Occurs in the nucleus –RNA moves into cytoplasm 2) RNA is translated to form polypeptide chains, which fold to form proteins Steps from DNA to Proteins

Three Classes of RNAs Messenger RNA –Carries protein-building instruction Ribosomal RNA –Major component of ribosomes Transfer RNA –Delivers amino acids to ribosomes

A Nucleotide Subunit of RNA phosphate group sugar (ribose) uracil (base)

Base Pairing During Transcription A new RNA strand can be put together on a DNA region according to base- pairing rules As in DNA, C pairs with G Uracil (U) pairs with adenine (A)

Transcription & DNA Replication Like DNA replication –Nucleotides added in 5’ to 3’ direction Unlike DNA replication –Only small stretch is template –RNA polymerase catalyzes nucleotide addition –Product is a single strand of RNA

Promoter A base sequence in the DNA that signals the start of a gene For transcription to occur, RNA polymerase must first bind to a promoter transcribed DNA winds up again DNA to be transcribed unwinds mRNA transcript RNA polymerase

Adding Nucleotides growing RNA transcript 5’ 3’ 5’ 3’ direction of transcription

Transcript Modification unit of transcription in a DNA strand exonintron mature mRNA transcript poly-A tail 5’ 3’ snipped out exon intron cap transcription into pre-mRNA 3’5’

Genetic Code Set of 64 base triplets – ex: AGA CCC Codons - Nucleotide bases read in blocks of three 61 of the base triplets specify amino acids 3 stop codons – UGA, UAA, UAG e/gene-code/how.htmlhttp://nobelprize.org/educational_games/medicin e/gene-code/how.html

Genetic Code

Code Is Redundant Twenty kinds of amino acids are specified by 61 codons Most amino acids can be specified by more than one codon Six codons specify leucine –UUA, UUG, CUU, CUC, CUA, CUG

tRNA Structure codon in mRNA anticodon in tRNA amino acid OH tRNA molecule’s attachment site for amino acid

Ribosomes tunnel small ribosomal subunitlarge ribosomal subunitintact ribosome