DNA and RNA

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DNA and RNA
DNA
• To understand genetics, biologist had to learn the
chemical makeup of the gene. Scientist discovered that
genes are made of DNA.
• Scientists also found that DNA stores and transmits the
genetic information from one generation of an organism
to the next.
• Scientists began studying DNA structure to find out how it
carries information, decides traits, and replicates itself.
DNA
• DNA: The molecule of heredity
– The genetic information that is held in the molecules of
DNA ultimately determines an organism’s traits.
– DNA achieves its control by producing proteins
– Within the structure of DNA is the information for life –
the complete instructions for manufacturing all the
proteins for an organism.
– DNA is a polymer made of repeating subunits called
nucleotides.
– Nucleotides have 3 parts: a simple sugar (deoxyribose),
a phosphate group, and a nitrogen base.
DNA Nucleotides And Base Pairing
• In DNA there are 4 possible nitrogen bases: adenine (A),
guanine (G), cytosine (C), and thymine (T).
• Adenine and guanine are double-ring bases called purines.
• Thymine and cytosine are smaller, single-ring bases called
pyrimidines.
• In DNA: adenine = thymine and guanine = cytosine
• In each chain of nucleotides, the sugar of one nucleotide is
joined to the phosphate group of the next nucleotide by a
covalent bond.
DNA Nucleotides
Purines
Adenine
Guanine
Pyrimidines
Cytosine
Thymine
Phosphate group
Deoxyribose
BASE PAIRING
DNA Double Helix
• In 1953, James Watson and Francis Crick made a 3-D model of
DNA. Their model was a double helix, in which two strands
were wound around each other.
– A double helix is like a twisted ladder.
– Sugars and phosphates make up the sides of the ladder.
– Hydrogen bonds between the bases hold the strands
together.
– Bonds form only between certain base pairs: between
adenine and thymine, and between guanine and cytosine.
This is called base pairing.
Structure of DNA
Chromosomes and DNA Replication
• Most prokaryotes have one large DNA molecule in their
cytoplasm.
• Eukaryotes have DNA in chromosomes in their nuclei.
Chromosome Structure
• Eukaryotic chromosomes contain both DNA and protein,
tightly packed together to form a substance called
chromatin.
• Chromatin consists of DNA that is tightly coiled around
proteins called histones.
• Together, the DNA and histone molecules form a beadlike
structure called a nucleosome. Nucleosomes pack with
one another to form a thick fiber, which is shortened by a
system of loops and coils
Prokaryotic Chromosome Structure
E. coli bacterium
Chromosome Structure
of Eukaryotes
Nucleosome
Chromosome
DNA
double
helix
Coils
Supercoils
Histones
DNA Replication
• Before a cell divides, it copies its DNA in a process called
replication. During DNA replication,
– The DNA molecule separates into two strands. Each new
strand of the DNA molecule serves as a model for the new
strand.
– Following the rules of basic pairing, new bases are added to
each strand. For example, if the base on the original strand
is adenine, thymine is added to the newly forming strand.
Likewise cytosine is always added to guanine.
– The end result is two identical strands.
How DNA Replication Occurs
• DNA replication is carried out by a series of enzymes. These
enzymes “unzip” a molecule of DNA.
• The unzipping occurs when the hydrogen bonds between
the base pairs are broken and the two strands of the
molecule unwind.
• Each strand serves as a template for the attachment of
complementary bases.
• DNA polymerase is the principal enzyme involved in DNA
replication, because it joins individual nucleotides to
produce a DNA molecule.
DNA Replication
New strand
Original
strand
DNA
polymerase
Growth
DNA polymerase
Growth
Replication fork
Replication fork
New strand
Original
strand
Nitrogenous bases
DNA Replication Animation
RNA And Protein Synthesis
• Structure of RNA
• For a gene to work, the genetic instructions in the DNA
molecule must be decoded.
• The first step is to copy the DNA sequence into RNA. RNA
is a molecule which contains instructions for making
proteins.
• RNA is similar to DNA, except for 3 differences
– The sugar in RNA is ribose instead of deoxyribose.
– RNA is single-stranded.
– RNA has uracil in place of thymine.
RNA Vs. DNA
Types Of RNA
• Most RNA molecules are involved in making proteins.
There are three main kinds of RNA.
– Messenger RNA has the instructions for joining amino
acids to make proteins.
– Proteins are assembled on ribosomes. Ribosomes are
made up of proteins and ribosomal RNA.
– Transfer RNA carries each amino acids to the ribosome
according to the coded message in messenger RNA.
Types Of RNA
RNA
can be
Messenger RNA
also called
Ribosomal RNA
which functions to
mRNA
Carry instructions
also called
which functions to
rRNA
Combine
with proteins
from
to
to make up
DNA
Ribosome
Ribosomes
Transfer RNA
also called
which functions to
tRNA
Bring
amino acids to
ribosome
Transcription
• RNA is copied from DNA in a process called transcription.
• During Transcription
– The enzyme RNA polymerase binds to DNA and
separates the 2 DNA strands.
– RNA polymerase builds a strand of RNA using one
strand of DNA as the template.
– The DNA is transcribed into RNA following base-pairing
rules except that uracil binds to adenine.
Transcription
Transcription Animation
The Genetic Code
• The directions for making proteins are in the order of the four
nitrogenous bases.
• This code is read three letters at a time.
• Each codon, or group of three nucleotides, stands for an
amino acid.
• Some amino acids are specified by more than one codon.
• One codon is a start signal for translation.
• Three codons signal the end of a protein.
The Genetic Code
RNA Translation
• Translation is the process in which the cell uses
information from messenger RNA to make proteins.
Translation takes place on ribosomes.
– Before translation can begin, messenger RNA is
transcribed from DNA
– The messenger RNA moves into the cytoplasm and
attaches to a ribosome.
– As each codon of the messenger RNA moves through
the ribosome, the proper amino acid is brought into
the ribosome by transfer RNA. The ribosome joins
together each amino acid. In this way, the protein
chain grows.
Translation
• When the ribosome reaches a stop codon, it releases the
newly formed polypepetide and the process of translation
is complete.
TRANSLATION
PROTEIN SYNTHESIS
Mutations
• Mutations are mistakes made when cells copy their own DNA.
• Mutations are changes in the genetic material of a cell.
• 2 types of mutations (gene and chromosome mutations)
Gene Mutations
• Gene mutations are changes in a single gene.
– A point mutation occurs at a single point in the DNA
sequence of a gene. When a point mutation causes one
base to replace another, only one amino acid is affected.
– If a nucleotide is added or removed, it causes a frameshift
mutation. All the groupings of codons are changed. This
can cause the gene to make a completely different protein.
Gene Mutations: Substitution, Insertion,
and Deletion
Substitution
Insertion
Deletion
Chromosomal Mutations
• In a chromosomal mutation, there is a change in the
number of the structure of chromosomes. There are four
kinds of chromosomal mutations.
– Deletions: involve the loss of all or part of a
chromosome
– Duplications: produce extra copies of parts of a
chromosome
– Inversions: reverse the direction of parts of
chromosomes.
– Translocations: occur when part of one chromosome
breaks off and attaches to another
Chromosomal Mutations
Deletion
Duplication
Inversion
Translocation
Gene Regulation
• Genes can be turned on and off as different proteins are
needed.
• In prokaryotes, some genes are turned on and off by a
chromosome section called an operon. An operon is a group
of genes that work, or operate, together.
– Ex. In bacteria, one operon controls whether the organism
can use the sugar lactose as food. It is called the lac
operon. The lac genes are turned off by repressors and
turned on by the presence of lactose.
Gene Regulation
• Operators and promoters are DNA sequences in the
operon that control when genes are turned on and off.
– When the cell needs a certain protein, RNA polymerase
attaches to the promoter and makes a messenger RNA
that is translated into the needed protein.
– When the cell no longer needs the protein, it makes
another protein called the repressor. The repressor
attaches to the operator. This blocks the promoter so
RNA polymerase cannot attach to it. This turns the
genes of the operon off.
Gene Regulation
• Most eukaryotic genes are controlled individually and have regulatory
sequences that are much more complex than those of the lac operon.
• In eukaryotes, genes are regulated by enhancer sequences located before
the point at which transcription begins
• Some proteins can bind directly to these DNA sequences.
• Ways in which these proteins affect transcription include:
– Increasing the transcription of certain genes
– attracting RNA polymerase
– blocking access to genes
Typical Gene Structure
Regulatory
sites
Promoter
(RNA polymerase
binding site)
Start transcription
DNA strand
Stop transcription
Cell Differentiation
• Differentiation: process in which cells become specialized in
structure and function. (takes place during embryonic
development)
• Hox Genes: series of genes that controls the differentiation of
cells and tissues in an embryo.

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