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

Adenine

Purines

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

Chromosome Structure of Eukaryotes

Nucleosome 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.

Replication fork

DNA Replication

New strand Original strand DNA polymerase DNA polymerase

Growth Growth

Replication fork Nitrogenous bases New strand Original 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.

Types Of RNA

RNA can be Ribosomal RNA Messenger RNA Transfer RNA also called mRNA which functions to Carry instructions from DNA to Ribosome also called rRNA which functions to Combine with proteins to make up Ribosomes also called tRNA which functions to 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) DNA strand Start transcription 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.