Download Chapter 9 DNA: The Genetic Material Read 192

DNA, RNA &
Protein Synthesis
What is DNA?
• The hereditary material in the nucleus of
cells that tells the cell when to make
proteins and what proteins it should make.
• Regions of DNA that code for proteins are
called genes.
• These proteins give organisms their traits,
such as eye color.
• DNA stands for deoxyribonucleic acid.
DNA Structure
• Discovered in 1953 by James Watson and
Francis Crick.
• DNA is a double-helix = 2 strands twisted
around each other like a spiral staircase.
• DNA is made of repeating subunits called
nucleotides.
• Nucleotides have 3 parts:
– Deoxyribose sugar
– Nitrogen base
– Phosphate group
DNA Nucleotide
DNA Structure continued…
• The sides of the DNA ladder are repeating
phosphates and sugars.
• The steps of the ladder are the nitrogen
bases.
• There are 4 different nitrogen bases in
DNA:
– Adenine
– Guanine
– Cytosine
– Thymine
Purines = Have a double ring structure
Pyrimidines = Have a single ring structure
Nitrogen Bases in DNA
Complementary Base Pairing
• Watson and Crick discovered that the
following base pairing rule exists in DNA
structure:
– Adenine is complementary (fits together) with
Thymine
• Held together by 2 hydrogen bonds
– Cytosine is complementary with Guanine
• Held together by 3 hydrogen bonds
• Each pairing includes one purine and one
pyrimidine.
Label the small portion of DNA
shown below:
Label the following:
• Sugars
• Phosphates
• Nitrogen bases
• Hydrogen bonds
Build a DNA Model
•
•
•
•
•
•
•
Black= sugar
White hollow= phosphate
White solid= hydrogen bond
Blue= cytosine
Green= thymine
Orange= adenine
Yellow= guanine
½ of the DNA has the sequence
CTGGACT
• If the sequence of half of a DNA strand is
TAGGCC, draw the entire DNA strand.
Label a phosphate group, deoxyribose sugar, all nitrogen
bases, and a hydrogen bond.
Strawberry DNA Extraction
10 ml
Strawberry DNA Extraction cont..
Replication
• During interphase, chromosomes create new
chromatids. This process of making new DNA
strands is called replication.
• This process happens in the nucleus of the cell.
• Each new DNA produced has ½ from the original
strand and ½ that is newly built.
• DNA helicase unwinds the DNA and breaks the
hydrogen bonds that hold the 2 strands of DNA
together.
• DNA polymerase forms the new halves of DNA by
putting the correct nucleotides into position. It also
proofreads the new DNA built for any errors.
• This process occurs once in a cell’s lifetime- right
before it divides.
Below is a strand of DNA. If the strand of
DNA goes through 2 rounds of replication,
draw the 4 resulting DNA molecules. Newly
built DNA halves are represented by a
dotted line.
Transcription
• Transcription is the making of RNA by using the DNA
code as a template.
• RNA is single stranded so only ½ of the DNA is used as
a template. This half is called the sense strand. The
other is the non-sense strand.
• RNA polymerase starts at a promoter on the DNA and
builds the new strand of RNA until it reaches a
termination signal.
• RNA produced leaves the nucleus through nuclear
pores (openings in the membrane) to enter the
cytoplasm. Why can it leave?
• 3 types of RNA are produced: messenger RNA
(mRNA), ribosomal RNA (rRNA) and transfer RNA
(tRNA)
Contrasting DNA and RNA
RNA
DNA
•
•
•
•
2 strands
Deoxyribose sugar
Remains in the nucleus
Bases
– ATCG
A pairs with T
G pairs with C
•
•
•
•
1 strand
Ribose sugar
Exits nucleus
Bases
– AUGC
A pairs with U
G pairs with C
Review Question
• Draw a DNA model with the sequence
TGCCACTG and label all parts.
• The side of the DNA listed above is the
sense strand.
• If the promoter is after the 1st “T” and the
termination signal is before the last “T”, what
is the sequence of the RNA strand
produced?
3 Types of RNA
1. mRNA (messenger)- carries the
DNA’s instructions for making proteins
out of the nucleus to the ribosomes in the
cytosol.
• Every 3 nucleotides on mRNA is called a
codon. Each codon codes for an
amino acid.
– See codon table
2. tRNA (transfer)- carries a specific
amino acid on one end and an anticodon
on the other end. The anticodon
complements the codon on mRNA.
Draw the tRNA structure:
3. rRNA (ribosomal)- Along with protein,
this makes up the structure of a ribosome.
Translation (Protein Synthesis)
1. mRNA leaves the nucleus and enters the cytosol.
2. mRNA attaches to a ribosome.
3. At the start codon (AUG), a tRNA brings its amino
acid into position.
4. Adjacent amino acids bond together with a peptide
bond and the tRNA leaves.
5. The ribosome then moves onto the next codon.
6. This process continues until the STOP codon is
reached.
7. The new protein is released.
Draw the translation process
The sense strand of DNA is TACGCCCTGAAAATT
(promoter and termination signals are at the ends)
Cell Differentiation
• All human somatic cells (non-sex) have the
same genetic material.
– Then how do we have 220 cells types that look and
function differently?
• Cell differentiation (specialization) occurs when
genes (segments of DNA that code for proteins)
do not experience transcription (turned into
RNA) or translation (RNA turned into protein)
because the cell turns off the gene.
• This means that differentiated cells have the
ability to behave like any other cell if given the
right gene regulations.
Gene Mutations
• Mutation- A change in the DNA sequence of a gene.
– Rare
– Are mutations passed on to offspring?
• Only if the mutation is in a gamete (sex cell)!
– Point mutations are changes in one or a few
nucleotides in a gene.
• Substitution= When 1 nucleotide is substituted
for another (A in place of C).
• Addition= When 1 (or a few) nucleotide(s) is
added to the sequence
• Deletion= When 1 (or a few) nucleotide(s) is
deleted from the sequence.
Indicate how the following point mutation
would affect the resulting protein.
• DNA sequence of sense strand:
(promoter)TACGTTCATGGGATT (term. signal)
• What is the mRNA?
• What is the amino acid sequence?
TACGTTCATGGGATT
• What is the amino acid sequence if the
last “G” was deleted?
•
DNA code
•
mRNA codon
•
tRNA anticodon _____________
________________
•
Amino acid
______________
TGA
_____________
____________
______________
_______________
UCU
_______________
_______________
tryptophan
•
DNA code
•
mRNA codon
•
tRNA anticodon _____________
________________
•
Amino acid
______________
CAT
_____________
____________
______________
_______________
GUU
_______________
_______________
methionine