III. What do genes do? Gene expression
A. Genes
1. specific sequences of nucleotides within a molecule of DNA
2. occupy a specific locus on a chromosome
3. can mutate (change nucleotide sequence)
4. specify genetic information – regulation and protein synthesis
a. DNA used to make RNA (transcription)
i.
RNA structure – like DNA except ribose replaces deoxyribose
ii. types of RNA

mRNA = messenger RNA
specifies info needed to make proteins

rRNA = ribosomal RNA
combines with proteins to make ribosomes (organelle that synthesizes
proteins)

tRNA = transfer RNA
transport amino acids to ribosome during protein synthesis
http://www.youtube.com/watch?v=41_Ne5mS2ls transcription and translation
b. RNA used to make proteins (translation)
why you need proteins
i.
enzymes are proteins – catalyze cellular reactions
ii. structural proteins – ex: collagen
iii. storage proteins – store nutrients
ex: ovalbumin in egg white
zein in corn
iv. transport proteins
hemoglobin transports oxygen
proteins in cell membrane transport substances into and out of cell
v. regulatory proteins
some hormones are proteins
some proteins turn genes on and off
vi. motile proteins
actin filaments (made of protein) in spindle fibers
actin and myosin in muscles
vii. protective proteins
antibodies
B. Transcription = mRNA synthesis
using DNA as a template to make RNA
1. the basic rules – for all types of cells
a. only genes are transcribed (not all DNA is used)
genes are identified by:
i.
a promoter sequence (in front of the gene)
ii. a terminator sequence at the end
b. only one strand of DNA is transcribed
(though genes are not all on the same DNA strand)
c. sequence of RNA is complementary to DNA
i.
if DNA is C then RNA complementary base is G
ii. if DNA is G then RNA complementary base is C
iii. if DNA is T then RNA complementary base is A
iv. if DNA is A then RNA complementary base is U
d. new nucleotides are added to the 3’ end of the RNA
e. synthesis is antiparallel
f.
requires RNA polymerase (an enzyme)
i.
unwinds double-stranded DNA
ii. separates double-stranded DNA into 2 strands of single-stranded DNA
iii. reads DNA template
iv. adds RNS nucleotides
v. recognizes promoters and terminators
2. Additional rules for eukaryotes
a. RNA polymerase needs transcription factors to locate and attach to promoter
transcription factors are proteins that attach to the DNA near the promoter and
form a transcription complex
b. introns must be removed from RNA (primary transcript) = RNA splicing
produces mRNA
introns = non-coding regions (not used to make protein)
exons = coding regions (used to make protein)
accomplished by spliceosomes
c. add 5’ cap - a modified guanine attaches to 5’ end of RNA
protects RNA from degradation
d. create poly-A tail - the addition of many (hundreds) of adenine to the 3’ end of the
RNA.
required to transport RNA out of the nucleus
C. Translation = polypeptide synthesis
using mRNA as a template to make polypeptides/proteins
translation from nucleotide sequence to amino acid sequence
1. the genetic code
a. 4 different nucleotides need to code for 20 different amino acids
b. codon = series of 3 RNA nucleotides that specify an amino acid
c. 3 codons do not specify any amino acid = STOP codons
2. the basic rules
a. use a ribosome to capture and hold mRNA
b. translate 3 RNA nucleotides (1 codon) at a time into 1 amino acid
this requires a tRNA to bind to codon and bring the appropriate amino acid
c. read RNA from 5’ end to 3’ end
d. always start at AUG
chose the 5’AUG3’ closest to the 5’ end of the mRNA
e. continue until reach a STOP codon
a codon that can’t be recognized by a tRNA
STOP codons: UAG, UAA, UGA