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.

Purines Pyrimidines

Adenine Guanine Cytosine Thymine

Phosphate groupDeoxyribose

DNA Nucleotides

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

E. coli bacterium

Prokaryotic Chromosome Structure

Chromosome Structure of Eukaryotes

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

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

Growth

Growth

Replication fork

DNA polymerase

New strand

Original strand DNA

polymerase

Nitrogenous basesReplication fork

Original strand

New strand

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.

from to to make up

Types Of RNA

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can be

RNA

Messenger RNA Ribosomal RNA Transfer RNA

mRNA Carry instructions rRNACombine

with proteins tRNABring

amino acids toribosome

DNA Ribosome Ribosomes

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.

Substitution InsertionDeletion

Gene Mutations: Substitution, Insertion, and 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

Deletion

Duplication

Inversion

Translocation

Chromosomal Mutations

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

Regulatory sites

Promoter(RNA polymerase binding site)

Start transcription

DNA strand

Stop transcription

Typical Gene Structure

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.