GENÉTICA MOLECULARLOS ÁCIDOS NUCLEICOS Y SU

EXPRESIÓN

1. Conocer y comparar los distintos tipos de ácidos nucleicos:a) Composición.b) Función.

2. El ADN como portador de la información genética:a) Qué es un gen.

3. Conocer como se expresa la información genética.a) El código genético.b) Los procesos de transcripción y traducción.

4. La conservación de la información genética:a) La replicación del ADN.b) Las mutaciones, generación de variabilidad, soluciones (evolución) y

problemas.

OBJETIVOS:

1. Los ácidos nucleicos: ADN y ARN

3. El ADN y la genética molecular:a) El concepto de gen.b) La replicación del ADN.c) Las mutaciones genéticas y sus tipos.

2. Expresión de la información genética:a) El código genético.b) Transcripción: de ADN a ARNc) Traducción: de ARN a proteínas

CONTENIDOS:

Nucleic acidsA few key events

DNA as an acidic substance present in nucleus was first identified by Friedrich Meischer in 1868

In 1953, James Watson and Francis Crick, described a very simple but famous Double Helix model for the structure of DNA.

She study X-ray diffraction of DNAThe diffraction pattern she obtained suggested

several structural features of DNA

Genetics molecules

• Genetics molecules has the ability to carry instructions.

• Genetics molecules should have the ability to be self-copied perfectly, over and over again.

Nucleic acids struture• Nucleics acids are polymers of small repeating

units calles nucleotides.• Components of a nucleotide are:

- Base.

- Phosphate - Sugar

Nucleic acids struture• Nucleics acids are polymers of small repeating

units calles nucleotides.• Components of a nucleotide are:

- Base.

• There are five kinds of nucleotides• There are two mind kinds of nucleics acids

DNA RNA

- Phosphate - Sugar

The Nitrogenous Bases

THEY ARE DIVIDED INTO TWO GROUPS• Pyrimidines and purines

Purines• Adenine• Guanine

Pyrimidines• Thymine• Cytosine

The rings are not only made of carbon

Nitrogenous bases of DNA & RNA

NucleotideNucleoside

Base

Phosphate

SugarX=H: DNAX=OH: RNA

Base always attached here

Phosphates are attached here

HH

OH

OCH2

Base

Phosphate

RiboseOH

5′4′ 1′

3′ 2′

O

O

P

O–

O

HH

A, G, C or U

HH

H

OCH2

Base

Phosphate

Deoxyribose

5′

OH

4′ 1′

3′ 2′

OO

O

P

O–HH

DNA Nucleotide RNA Nucleotide

A, G, C or T

Nucleotides

Nucleotides are linked together by covalent bonds called phosphodiester linkage.

SugarBase

P

SugarBase

P

1

23

4

5

1

23

4

5

A chemical bond that involves sharing a pair of electrons between atoms in a molecule.

DNA. Deoxyribonucleic acid

NH2

N O

N

O

N O

N

Adenine (A)

Guanine (G)

Thymine (T)

BasesBackbone

Cytosine (C)

O

HH

H

H

HH

OOO

O–

P CH2

O–

HH

H

H

H

HH

OOO

O

P CH2

O–

NH2

N

N

H

N

N

HH

H

HH

OOO

O

P CH2

O–

NH2

HN

N

N

H

N

HH

HOH

HH

OOO

O

P CH2

O–Singlenucleotide

Phosphodiesterlinkage

Sugar (deoxyribose)

Phosphate

3′

5′

5′4′ 1′

2′3′

5′4′ 1′

2′3′

5′4′ 1′

2′3′

5′4′ 1′

2′3′

CH3

The primary structure of DNA is the sequence of linked nucleotides

5’ end

3’ end

5’

3’

A shorthand notation for this sequence is ACGTA

The secondary structure of DNA is the anti-parallel double helix

Salient features of the Double-helix structure of DNA: It is made of two polynucleotide chains, where the backbone

is constituted by sugar-phosphate, and the bases project inside.

The two chains have anti-parallel polarity. It means, if one chain has the polarity 5’3’, the other has 3’5’

G

T

C

A

5’ 3’C

A

T

G

The bases in two strands are paired through hydrogen bond (H-bonds)forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds.

Hydrogen bond: A chemical bond consisting of a hydrogen atom between two electronegative atoms (e.g., oxygen or nitrogen) with one side be a covalent bond and the other being an ionic bond.

Based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine; and Guanine and Cytosine are constant and equals one.

DNA Double Helix & Hydrogen bonding

3 Hydrogen bonds 2 Hydrogen bonds

A T U C G

30%

Ratios between nucleotides in a double strand of DNA

Ratios between nucleotides in a double strand of DNA

30% 0% 20% 20%

A T U C G

28% 28% 0% 22% 22%

ADNA T

G C

ARNA U

G C

*Chargaff´s rule

Experimental level Conceptual level

For each type of cell, extract the chromosomal material. This can bedone in a variety of ways, including theuse of high salt, detergent, or mild alkalitreatment. Note: The chromosomescontain both DNA and protein.

1.

Remove the protein. This can be done in several ways, including treament withprotease.

2.

Hydrolyze the DNA to release the bases from the DNA strands. A common wayto do this is by strong acid treatment.

3.

Separate the bases by chromatography. Paper chromatography provides an easyway to separate the four types of bases.(The technique of chromatography isdescribed in the Appendix.)

4.

Extract bands from paper into solutions and determine the amounts of each baseby spectroscopy. Each base will absorblight at a particular wavelength. Byexamining the absorption profile of asample of base, it is then possible tocalculate the amount of the base.(Spectroscopy is described in theAppendix.)

5.

Compare the base content in the DNA from different organisms.

6.

Solution of chromosomalextract

DNA

DNA + proteins

Individual bases

Origin

Protease

Acid AA

AAA A AA

CC C CC CC

GG GG G GG

TT T T TT

T

T

GG

G

C

CC

yErwin Chargaff’s Experiment

By Himanshu DevVMMC & SJH

The Data

By Himanshu DevVMMC & SJH

Major groove

Minor groove

20 Å

34 Å

The secondary structure of DNA

Nucleotides are linked together by covalent bonds called phosphodiester linkage.

Sugar Base

P

Sugar Base

P

RNA. Ribonucleic acid

Usually single stranded and helical in structure.

But double stranded also present in some viruses.

Nucleotides are:• Guanine, adenine.• Cytosine, uracil.

RNA VS DNA

Types of RNA In all prokaryotic and eukaryotic organisms, three main classes of RNA

molecules exist:1) Messenger RNA(m RNA)2) Transfer RNA (t RNA)3) Ribosomal RNA (r RNA)

The other are: small nuclear RNA (SnRNA), micro RNA(mi RNA) and small interfering RNA(Si RNA) and heterogeneous nuclear RNA (hnRNA).

Messenger RNA (m-RNA)All members of the class function as messengers carrying the information in a gene to the protein synthesizing machinery

The coding region or sequence encodes for the synthesis of a protein.A gene is a locus (or region) of DNA which contains information to build a protein.A gene is the basic physical and functional unit of heredity.

The central dogma of molecular biology has been described as "DNA makes RNA and RNA makes protein”

Transfer RNA are the smallest of three major species of RNA molecules

They transfer the amino acids from cytoplasm to the protein synthesizing machinery, hence the name tRNA.

There are at least 20 species of tRNA one corresponding to each of the 20 amino acids required for protein synthesis.

Transfer RNA (t-RNA)

1) Primary structure- The nucleotide sequence.

2) Secondary structure- Each single t- RNA shows extensive internal base pairing and acquires a clover leaf like structure. The structure is stabilized by hydrogen bonding between the bases.

3) Tertiary structure

Structure

Ribosomal RNA (rRNA) Ribosomal ribonucleic acid (rRNA) is the RNA component of the ribosome, and is essential for protein synthesis in all living organisms. rRNA component performs the peptidyl transferase (peptide bond formation) activity and thus is an enzyme (a ribozyme)

The genes expressionTranscription and translation

DNA

Protein

mRNA

Location Process

Nucleus

Cytoplasme o RER

Transcription

Translation

Transcription

1. Initiation: Promotor, contains the transcription start point.

2. Elongation: Transcription factors and RNA polymerasa IIGrowing RNA strand, from 5’ to 3’

3. Terminatio: DNA duplex reforms

A gene is the stretch of DNA, or transcription unit, that is transcribed to RNA

Transcript (newly synthesized mRNA)

Splicing, 5´Cap & Poly A tail

Translation

In translation, messenger RNA (mRNA) - produced by transcription from DNA - is decoded by a ribosome to produce a specific amino acid chain, or polypeptide.

Is the process in which cellular ribosomes create proteins.

Protein

mRNA

Cytoplasme o RER

TranslationThe genetic code

A. The genetic code is redundant.• More than one codon can specify

the same amino acid.B. The genetic code is degenerate.

• In most instances the third base is the degenerate base.

C. The genetic code is nearly universal.• Living beings has the same code,

only a few rare exceptions have been noted.

• Encode the same 20 amino acid with the same 64 triplets.

There are 20 amino acid, but 64 bases combinations or 64 tRNAs

The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells.

So codons and amino acids are linked with by the genetic code

1. Initiantio:Ribosomes assembles

around mRNA (Firts tRNAmet)

2. Elongation:tRNA transfers amino acids.The ribosome moves to the

next codon

3. Termination:When a stop codon is

reached, the ribosome realease the protein

Translation have directionality, begins at 5’ end of mRNA. 5’ 3’

SUMMARIZING

ReplicationStep one:

- An enzyme called Helicase breaks the hydrogen bonds between the bases of the two antiparallel strands forming a replication fork. - DNA Gyrase (also called Topoisomerase) relieves tension that builds up as a result of unwinding. - Single strand binding proteins (SSBs) help to stabilise the single stranded DNA.

ReplicationStep two:

- RNA polymerase (also known as RNA Primase) synthesizes short RNA nucleotides sequences that act as primers (starters). These essentially provide a starting point for DNA replication.

ReplicationStep three:

- DNA Polymerase III can now start synthesising the new DNA strand using free DNA nucleotides. However, DNA polymerase can only read the original template (parent strand) in the 3’ → 5’ direction (making DNA 5’ → 3’). This is not a problem on the leading strand.- On the lagging strand DNA polymerase synthesise short fragments of DNA in-between each of the RNA primers.

ReplicationStep four:

- DNA Polymerase I now removes the RNA primers and replaces them with DNA - DNA Ligase joins the DNA fragments of the lagging strand together to form one continuous length of DNA.

Replication windows

Replication fork Replication fork

ADN / DNA ARN / RNANucleotide / Nucleotido

Nucleoside / NucleosidoDeoxirribose / Desoxirribosa

+Nitrogenous bases

(A, G, C, T)

Ribose / Ribosa+

Nitrogenous bases(A, G, C, U)

+

Fosfato ℗Double helix mRNA, rRNA, tRNA

The central dogma of molecular biology has been modified

Virus ARNReverse

transcription

Protein

PrionRibozymes

RNA

MUTATIONSTypes of mutations – 1.Genetics:There are many different ways that DNA can be changed, resulting in different types of mutation.

SubstitutionA substitution is a mutation that exchanges one base for another. Such a substitution could:

InsertionInsertions are mutations in which extra base pairs are inserted into a new place in the DNA

DeletionDeletions are mutations in which a section of DNA is lost.

1. Change a codon to one that encodes a different amino acid and cause a small change in the protein produced.

2. Change a codon to one that encodes the same amino acid and causes no change in the protein produced.

3. Change an amino-acid-coding codon to a single "stop" codon and cause an incomplete protein.

A mutation is a change in DNA: genes, chromosomes or genomes

MUTATIONSTypes of mutations – 2. Chromosomals:A chromosome mutation is an change that occurs in a chromosome. These changes are most often brought on by problems that occur during meiosis or by mutagens (chemicals, radiation, etc.).Chromosome mutations can result in changes in the number of chromosomes in a cell or changes in the structure of a chromosome.

Chromosome Structure

a) Translocation: The joining of a piece of a chromosome to a non-homologous chromosome.

b) Deletion: This mutation results from the breakage of a chromosome in which the genetic material becomes lost during cell division.

c) Duplication: Duplications are produced when extra copies of genes are generated on a chromosome.

a) Inversion: In an inversion, the broken chromosome segment is reversed and inserted back into the chromosome.

MUTATIONSTypes of mutations – 3.Genomics:

a) Aneuploidy (Aneuploidía): Is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. Monosomy: refers to lack of one chromosome of the normal

complement. Monosomy of the sex chromosomes (45,X) causes Turner syndrome.

Trisomy: Down syndrome, trisomy 21. Edwards syndrome, trisomy 18. Patau syndromed, trisomy 13. Klinefelter syndrome, sexual trisomy XXY

Moploidy or haploidy, is the state where all cells have just one set of chromosomes beyond the basic set.

Poliploidy (Poliploidía), is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3 or more. Specific terms are triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets)…

b) Euploidy (Euploidía), is the state of a cell or organism having one or more than one set of the same set of chromosomes

Chromosome Number Changes

The causes of mutationsMutations happen for several reasons:

1. DNA fails to copy accurately.

1. External influences: Mutations can also be caused by exposure to specific chemicals or radiation

Cells employ an arsenal of editing mechanisms to correct mistakes made during DNA replication.

The effects of mutations - EvolutionSomatic mutations, occur in non-reproductive cells and won't be passed onto offspring.

Germ line mutations, occur in reproductive cells like eggs and sperm. The only mutations that matter to large-scale evolution are those that can be passed on to offsprin.

http://evolution.berkeley.edu/evolibrary/article/0_0_0/mutations_05

Some regions of DNA control other genes, determining when and where other genes are turned "on". Mutations in these parts of the genome can substantially change the way the organism work or is built. A mutation in a gene "conductor" can cause a cascade of effects in the behavior of genes under its control.

Little mutations with big effects: Mutations to control genes