Download Nucleic acids and protein synthesis

THE FUNCTIONS OF DNA
Nucleic acids and
protein synthesis

DNA has three roles/purposes/functions:

1. Storing information – genes are segments
of DNA that carry messages to make proteins.
2. Copying information – so that when cells
divide, all cells get a complete copy of the
genetic material.
3. Transmitting information – DNA is passed
from parents to offspring.


THE STRUCTURE OF DNA


THE STRUCTURE OF DNA

Nucleotides are the
building blocks of DNA.
Each nucleotide contains a
phosphate group, a five
carbon sugar, and a
nitrogen base.



The five carbon sugar is
called deoxyribose.
Covalent bonds hold the
sugar of one nucleotide to the
phosphate group of another
nucleotide to form chains.


Adenine and Guanine are called
purines. Purines have 2 rings of
carbon and nitrogen atoms.
Thymine and Cytosine are called
pyrimidines. Pyrimidines have a
single ring of carbon and nitrogen
atoms.
The four nitrogen bases that
make up DNA are adenine,
thymine, guanine, and
cytosine.
CHARGAFF’S RULE
THE STRUCTURE OF DNA

The full name of DNA is
deoxyribonucleic acid.
Every nucleotide has the
same sugar molecule and
phosphate group, but each
nucleotide contains one of
four nitrogen bases.

1949-Erwin Chargaff showed that in
DNA, the number of adenines equal
the number of thymines, AND the
number of cytosines equal the
number of guanines. However, the
amount of each nucleotide was not the
same among different organisms.

Base-pairing / Chargaff’s rule:
 Adenine will always pair with Thymine; A
and T are complimentary.
 Cytosine will always pair with Guanine;
C and G are complimentary.
1
THE DNA MOLECULE IS A
DOUBLE HELIX


THE DNA MOLECULE IS A
DOUBLE HELIX

DNA is a twisted ladder
with alternating patterns
of phosphates and
sugars making the sides
of the ladder.

Each rung is a
purine/pyrimidine pair
held together by
hydrogen bonds.
THE DNA MOLECULE IS A
DOUBLE HELIX



The base pair rules tell us what the rungs can
be, A and T or G and C.
Each strand of the double helix is
complementary to each other; the sequence
of 1 strand determines the sequence of the
other.
The two strands of DNA in the double-helix
are antiparallel – they run in opposite
directions.
HOW DNA IS COPIED

DNA is double stranded – base pairing allows for
easy copying; one strand serves as a template for
a new strand.

DNA replication – the process of making a new
DNA strand.
 Occurs before cells divide.
 Ensures that when cells divide, each cell
produced has an entire copy of the organism’s
DNA.
Rosalind Franklin and Maurice Wilkins
used X ray diffraction to take first picture of
DNA. Determined a two dimensional
picture of DNA’s structure.
James Watson and Francis Crick – 3-D
shape of DNA being 2 strands of
nucleotides that form a spiral staircase or
double helix.
HOW DNA IS COPIED


DNA double helix is unzipped by an enzyme
called a helicase. Helicase breaks hydrogen
bonds linking the nitrogen bases.
 Occurs at the replication forks of the double
helix.
At the replication fork; an enzyme called DNA
polymerase moves along the strands, reading
the nitrogen base of each nucleotide, and
adding the complementary nucleotide to the
new strand.
 Remember that A and T are complimentary; C
and G are also complimentary.
2
HOW DNA IS COPIED

HOW DNA IS COPIED
Telomeres – tips of chromosomes where DNA
is hard to replicate.

Replication in prokaryotes– single circular
chromosome; replication begins and proceeds
from ONE location on the chromosome.
Telomerase – enzyme that adds repeated DNA
sequences to the ends of the chromosomes to
prevent loss of the telomere DNA during DNA
replication.
Replication in eukaryotes – many linear
chromosomes; replication begins and proceeds
from hundreds of locations on each chromosome.
THE PATH OF GENETIC
INFORMATION

Gene – segment of DNA
that codes for a protein.

Cells transfer the information found within the
genes on DNA into a set of working instructions
for use in building proteins.
This working set of instructions of the gene is
called ribonucleic acid or RNA.

THE PATH OF GENETIC
INFORMATION



RNA is a single strand of nucleotides; DNA
is double stranded.
The sugar in RNA is a 5 Carbon sugar called
ribose; DNA’s sugar is deoxyribose.
RNA does not contain Thymine, but has
replaced Thymine with the base Uracil.
RNA is a nucleic acid made of chains of
nucleotides, just like DNA.
DNA compared to RNA
DNA
RNA
2
1
How many
strands?
Nucleotide
subunit
Phosphate
Group
Deoxyribose
Sugar
Nitro
-gen
Base
Phosphate
Nitro
-gen
Base
Ribose
Sugar
Group
Deoxyribose sugar
Bases
Thymine (T)
Adenine (A)
Guanine (G)
Cytosine (C)
T–A
G–C
Ribose sugar
Uracil (U)
Adenine (A)
Guanine (G)
Cytosine (C)
U–A
G–C
3
THE PATH OF GENETIC
INFORMATION


Three forms of RNA are messenger RNA
(mRNA), transfer RNA (tRNA), and
ribosomal RNA (rRNA).
All 3 RNA’s are responsible for processing
the information in a gene into protein. The
process of transferring the information in
genes to proteins is called gene expression.
THE PATH OF GENETIC
INFORMATION
THREE TYPES OF RNA



mRNA – used as a blueprint or template for
a protein; carries DNA’s information from the
nucleus to site of translation (ribosomes in
cytoplasm).
tRNA – decodes mRNA into amino acid
sequences.
rRNA – RNA part of a ribosome’s structure
(the other component of ribosomes is
protein).



TRANSCRIPTION: MAKING
RNA


Transcription takes place inside the
nucleus.
Transcription begins when RNA
polymerase binds to the beginning of a
gene on a region of DNA.
Gene expression occurs in 2 stages.
The first, transcription is where DNA is
transferred to mRNA.
The second stage; translation, is when the
information in mRNA is used to make protein.
TRANSCRIPTION: MAKING
RNA



The region of DNA to which RNA
polymerase binds is called a promoter.
Promoters are sequences of DNA that act
as a start signal.
The RNA polymerase begins to unzip and
separate the double helix.
The polymerase uses only one of the DNA
strands as a template for mRNA – The noncoding or complimentary strand is the
template for mRNA synthesis.
4
TRANSCRIPTION: MAKING
RNA


TRANSCRIPTION: MAKING
RNA
Follows the same base pairing rules as
replication except Uracil is used in place of
Thymine.
RNA nucleotides are added one at a time in
the active site of the RNA polymerase.



DNA reforms the double helix following the
RNA Polymerase.
Transcription occurs at about 60
nucleotides per second.
Terminator - the stop signal in the
sequence of DNA – RNA Polymerase
detaches here.
Post –transcriptional mRNA processing

Coding DNA: CTC TTG ATC ATG

Non-coding/complimentary DNA:
GAG AAC TAG TAC

RNA:

RNA editing - mRNA must be processed, or
prepared for the next phase of gene expression,
translation.

CUC UUG AUC AUG


Introns – non-coding gene regions that are cut out of the
RNA molecule in the nucleus.
Exons – expressed sequences of the RNA molecule;
codes for a protein.
The introns are cut out and the exons are spliced
together to make a final mRNA.
http://207.207.4.198/pub/flash/26/transmenu_s.swf
THE GENETIC CODE
Post –transcriptional mRNA processing

A protective cap and tail are then added
before the mRNA leaves the nucleus through
one of the pores and heads to the cytoplasm
where translation will occur on ribosomes.


Instructions on mRNA are written as a series
of three nucleotide sequences called a
codon.
Each codon (set of three nucleotides)
corresponds to a certain amino acid or a
stop signal.


64 possible codon combinations.
Genetic code – collection of codons of mRNA,
each of which directs the incorporation of a
particular amino acid during protein synthesis.
5
START AND STOP CODONS
Codon Table

Start codon or initiator codon – AUG; cues
the start of translation by inserting a
methionine.


Translation proceeds until a stop codon is
reached.

TRANSLATION: MAKING
PROTEINS


Thus, all proteins begin with the amino acid
methionine.
Stop codon – triggers the end of translation.
TRANSLATION: MAKING
PROTEINS
Translation – when ribosomes in the
cytoplasm use the sequence of codons in
mRNA to assemble amino acids into
polypeptide chains.
tRNA is a single stranded RNA folded into a
compact shape with three loops.


One loop has a three nucleotide sequence
(called an anticodon) that is complementary to
one of the 64 codons.
Each tRNA carries one amino acid.
TRANSLATION: MAKING
PROTEINS

tRNA bonds with mRNA at the
codon/anticodon site by hydrogen bonds.



Every tRNA carries a particular amino acid that
corresponds to the particular codon.
Once the amino acid has been added to the
growing polypeptide chain, the tRNA is
released.
Many amino acids link to form peptides –
once a peptide is folded into its proper shape
it is considered a protein.
6
Regulation of gene expression
- Prokaryotes
Central Dogma

Central dogma of molecular biology –
information is transferred from DNA to RNA
to protein.


Prokaryotic gene expression is regulated by
DNA binding proteins.

This allows gene expression, or DNA, RNA, and
proteins working together to put the genetic
information contained in cells into action.

These regulatory proteins help switch genes on
and off.
Operon – group of prokaryotic genes regulated
together.


Regulation of gene expression
- Eukaryotes

Eukaryotic gene expression is also regulated
by DNA binding proteins, however,
eukaryotes typically regulate individual
genes, not groups of them.

Regulation of gene expression
- Eukaryotes

TATA box – short sequence of DNA that marks
the beginning of a gene; used to help position
RNA polymerase.
Regulation of gene expression
- Eukaryotes

Not all genes are expressed in all eukaryotic cells.


Cell specialization – all cells in multicellular organisms
contain all of the organism’s DNA, yet they only
transcribe and translate part of it.
 Ex. Liver cells only transcribe and translate liver
specific genes while skin cells only transcribe and
translate skin specific genes.
RNA interference (RNAi) – used to regulate gene
expression in eukaryotes.
 During RNAi, microRNAs (miRNAs) bind to
transcribed mRNA to block it from being translated
into protein.
Promoter – DNA sequence where RNA polymerase
binds to begin transcription.
Operator – DNA sequence where regulatory proteins
can bind to repress transcription.
Transcription factors – proteins
that help regulate gene
expression by binding DNA
promoter/enhancer sequences
and blocking or activating
transcription.
 Promoter/enhancer –
sequences of DNA with
binding sites for multiple
transcription factors.
Regulation of gene expression
- Eukaryotes

Differentiation – when gene regulation allows
eukaryotic cells to become specialized in structure
and function.
 Occurs during embryonic development.
 Homeotic genes aka master control genes –
specific group of genes that controls the identity
of body parts in embryos.

Homeobox genes – code for transcription factors
that activate other genes important for development
and differentiation.


In flies, these are called Hox genes.
The environment also plays a role in the regulation of
prokaryotic and eukaryotic gene expression.
7
8