Biology ch7 DNA Structure and gene function

DNA

a molecule of nucleic acid made of nucleotides; stores the info the cell needs to produce more cells 

Erwin Chargaff

discovered base pairing A and T G and C

Rosalind Franklin

X Ray diffraction images show DNA is a repeating double helix

number of base pairs in each human chromosome

140 million

1 DNA molecules is made of 2 strands of 

nucleotides

each nucleotide consists of

1 phosphate group, 1 molecule of the 5 carbon sugar deoxyribose, 1 nitrogenous base adenine, guanine, cytosine, or thymine

adenine to 

thymine

guanine

cytosine

nucleotide sequence

order of the nitrogeneous bases in a strand

DNA strands held togetehr by 

base pairing

DNA strands are 

antiparallel

5 prime

where phosphate group is attatched to deoxyribose

3 prime

where OH group is attatched to deoxyribose

protein production starts with

DNA

gene

small region of a chromosome

the sequence of DNA in each gene encodes a 

specific protein

Central Dogma

DNA to RNA to Protein

stages of protein synthesis

transcription and translation

transcription

RNA synthesis; uses DNA as a template to produce RNA; RNA sequence is complementary to the DNA

Translation

protein synthesis; takes place at ribosomes; 3 types of RNA interact to carry out (mRNA, rRNA, and tRNA) 

messenger RNA

brings the info from DNA in the nucleus to ribosomes in the cytoplasm 

ribosomal RNA

makes up the ribosomes which read the message

transfer RNA

brings the amino acids to the ribosomes for protein synthesis

3 steps of of RNA nucleotides pairing with DNA nucleotides (Transcription) 

Initiation, Elongation, Termination 

Transcription Initiation

RNA polymerase binds to the promoter. RNA polymerase unwinds the 2 strands of DNA. The DNA template strand encodes the RNA molecule. The other DNA strand doesn’t participate in transcription

promoter

beginning of the gene

Transcription Elongation

RNA polymerase moves along the template strand, with the template strand making an RNA copy. RNA nucleotides base pair with the template strand. RNA polymerase joins them together into a strand of RNA. The 3’ end of RNA matches the 5’ end of DNA. 

Transcription Termination

RNA polymerase reaches the terminator, the RNA, DNA, and RNA polymerase separate from each other. DNA becomes a double helix again. 

terminator

region of DNA found at the end of the gene

RNA is processed in the 

nucleus 

RNA processing protects 

RNA

RNA processing 

a cap structure is added to the 5-prime end of mRNA and a poly-A tail structure is added to the 3-prime end of mRNA 

The cap and tail protect mRNA from

degradation

RNA processing removes

extra sequences aka introns 

introns

sequences in genes that are not used for producing a protein 

Exons

the sequences that specify amino acids

processed RNA is ready for

translation because it is now a functional molecule

Translation builds

the protein

all cells have the same

genetic code

Codon

3 nucleotide sequence that encodes 1 amino acid

genetic code shows mRNA

which mRNA codons correspond to which amino acids

unit of the genetic code

consists of codons

each of the 20 amino acids found in proteins is uniquely specified by 

1 or more codons 

mRNA bases are 

the symbols used by the genetic code (letters of the genetic alphabet) 

genetic alphabet

U, A, C, G

codons in the genetic code are how many bases long 

3

64 possible arrangements of

codons; the words

genetic language has how many words

64

genetic code is universal

all organisms encode the same 20 amino acids with the same 64 triplets

codons are exclusive

none code for 2 or more amino acids

tRNA translates the 

genetic code 

Stop codons 

UAA, UAG

Start codon (met)

AUG

tRNA molecules bring the amino acids to the 

ribosome

anticodon

binds to codon on mRNA during translation

tRNA binds to an mRNA codon at the anticodon and binds to the 

amino acids (translation) 

each step in translation happens at 

ribosomes 

large subunit binds to 

tRNA

small subunit binds to

mRNA

Translation initiation 

small ribosomal subunit binds to mRNA, large ribosomal subunit binds to tRNA. tRNA anticodon matches up to the start codon in mRNA

Translation elongation

the second tRNA enters the ribsome next to the initiator tRNA, its anticodon matches the 2nd mRNA codon. The amino acids are joined together when enzymes in the ribosome form a peptide bond. The 1st tRNA leaves the ribosome but its amino acid stays behind, the ribsome moves to the right and a 3rd tRNA comes in. Enzymes form another peptide bond to join the amino acids. 

Translation termination

The ribosome reaches the stop codon which is at the end of the gene. A protein called release factor binds to the stop codon, now no tRNA can bind there so no more amino acids will be added. The polypeptide chain detaches from mRNA and folds into a functional protein

translation is efficient 

multiple ribosomes attach to an mRNA molecule simultaneously

prokaryotes regulate 

several genes at once 

genes in prokaryotes are organized into

operons

operons 

groups of genes that are always transcribed together 

the lac operon includes 

3 genes that encode lactose digesting enzymes 

the promoter 

region of DNA where RNA polymerase binds to initiate transcription 

the operator 

region of DNA where regulatory proteins bind, which can affect the activity of RNA polymerase

repressor proteins block

transcription

when lactose is absent

repressor protein binds to operator blocking transcription

when lactose is present

it binds to the repressor and changes its shape, protein synthesis of lactose digesting enzymes occurs

Eukaryotes have multiple levels of 

gene regulation 

gene regulation starts in the

nucleus 

transcription factors bind to

enhancers

certain proteins can hold mRNAs inside the

nucleus

mutation

change in a cell’s DNA sequence

point mutation 

changes one or a few base pairs in a gene

nucleotide substitution cause small changes in protein structure

only 1 codon altered so only 1 amino acid will be affected

sickle cell disease caused by 

a single base substitution in a hemoglobin gene 

frame shift mutations

cause large changes in protein structure, changes every codon after the insertion