the importance of dna and rna in heredity

The importance ofThe importance ofDNA and RNA in DNA and RNA in

heredityheredity

Central Dogma of Molecular Central Dogma of Molecular BiologyBiology

DNA is self-replicating.DNA is self-replicating.

DNA is transcribed to produce DNA is transcribed to produce mRNA.mRNA.

mRNA is translated to produce mRNA is translated to produce protein.protein.

Revision – What are DNA and Revision – What are DNA and RNARNA

DNA – deoxyribonucleic acidDNA – deoxyribonucleic acid

RNA – ribonucleic acidRNA – ribonucleic acid Both are polynucleotide chains but Both are polynucleotide chains but

they have different functions and they have different functions and structures.structures.

Nucleotides are composed of a base, Nucleotides are composed of a base, a phosphate group and a sugar.a phosphate group and a sugar.

Parts of a NucleotideParts of a Nucleotide BaseBase

Nitrogen containing ring compoundsNitrogen containing ring compounds Two types – purine (two rings) or pyrimidine (one ring)Two types – purine (two rings) or pyrimidine (one ring) Purines – adenine and guanine Purines – adenine and guanine Pyrimidines – thymine, cytosine and uracilPyrimidines – thymine, cytosine and uracil

Phosphate groupPhosphate group Used to form phosphodiester linkage between 5’ and 3’ Used to form phosphodiester linkage between 5’ and 3’

carbons of adjacent nucleotides in order to form polynucleotide carbons of adjacent nucleotides in order to form polynucleotide chains.chains.

SugarSugar Nucleotides contain a 5 carbon sugarNucleotides contain a 5 carbon sugar The sugar used in RNA is The sugar used in RNA is -D-ribose-D-ribose The sugar used in DNA is The sugar used in DNA is -D-deoxyribose-D-deoxyribose There is one less oxygen on the sugar used for DNA hence the There is one less oxygen on the sugar used for DNA hence the

name name deoxydeoxyribonucleic acid.ribonucleic acid.

Subunits of a NucleotideSubunits of a Nucleotide

Nomenclature (naming) of Nomenclature (naming) of NucleotidesNucleotides

Nucleoside refers to base + sugarNucleoside refers to base + sugar Nucleotide refers to base + sugar + Nucleotide refers to base + sugar +

phosphatephosphate

BASEBASE NUCLEOSIDENUCLEOSIDE ABBREVIATIONABBREVIATION

AdenineAdenine AdenosineAdenosine AA

GuanineGuanine GuanosineGuanosine GG

CytosineCytosine CytidineCytidine CC

UracilUracil UridineUridine UU

ThymineThymine ThymidineThymidine TT

Key differences between DNA and Key differences between DNA and RNARNA

DNADNA RNARNA

SugarSugar deoxyribosedeoxyribose riboseribose

BasesBases adenineadenine

guanineguanine

cytosinecytosine

thyminethymine

adenineadenine

guanineguanine

cytosinecytosine

uraciluracil

Structure of Structure of polynucleotide polynucleotide chainchain

double strandeddouble stranded single strandedsingle stranded

Structure of DNAStructure of DNA

Watson and Crick modelled Watson and Crick modelled the structure of DNA in 1953 the structure of DNA in 1953 based on observations of based on observations of other scientists including other scientists including Rosamond FranklinRosamond Franklin

Now accepted that DNA is a Now accepted that DNA is a double-stranded helix (like a double-stranded helix (like a curved staircase) formed by curved staircase) formed by cross-linking of two anti-cross-linking of two anti-parallel nucleotide strands parallel nucleotide strands with complementary with complementary nucleotide sequences.nucleotide sequences.

Why complementary base Why complementary base pairs?pairs?

Chargraff’s chromatography data examining the base Chargraff’s chromatography data examining the base composition of DNA from several different organisms indicated composition of DNA from several different organisms indicated that in all casesthat in all cases Ratio A:T was 1Ratio A:T was 1 Ratio C:G was 1Ratio C:G was 1

Further work showed that adenine forms two hydrogen bonds Further work showed that adenine forms two hydrogen bonds with thymine while guanine forms three bonds with cytosine.with thymine while guanine forms three bonds with cytosine.

There are approximately 10 complementary base pairs per There are approximately 10 complementary base pairs per helical turn in the DNA helix.helical turn in the DNA helix.

A purine (double ring) is always paired with a pyrimidine (single A purine (double ring) is always paired with a pyrimidine (single ring) in order for the helix to fit together properly.ring) in order for the helix to fit together properly.

Uracil is only found in RNA – it is complementary to the DNA Uracil is only found in RNA – it is complementary to the DNA base adenine and replaces thymidine during the transcription base adenine and replaces thymidine during the transcription process.process.

Four Types of RNAFour Types of RNA Messenger RNA (mRNA)Messenger RNA (mRNA)

Copied portion of coding DNACopied portion of coding DNA Carries genetic information from the DNA out of the nucleus Carries genetic information from the DNA out of the nucleus

into the cytoplasminto the cytoplasm

Transfer RNA (tRNA)Transfer RNA (tRNA) Transports amino acids to the ribosome during protein Transports amino acids to the ribosome during protein

synthesissynthesis

Ribosomal RNA (rRNA)Ribosomal RNA (rRNA) Structural component of ribosomesStructural component of ribosomes

snRNA (snRNA)snRNA (snRNA) Involved in splicing of pre-mRNA message in the nucleus to Involved in splicing of pre-mRNA message in the nucleus to

remove intronsremove introns

Nucleotide bases form the Nucleotide bases form the basis of the genetic codebasis of the genetic code

The genetic codes consists of four nucleotides (A, C, The genetic codes consists of four nucleotides (A, C, G, T) and provides the instructions to make each of G, T) and provides the instructions to make each of the 100,000+ proteins in the human bodythe 100,000+ proteins in the human body

The code is read from 5’ to 3’ end of a DNA sequence The code is read from 5’ to 3’ end of a DNA sequence and is usually written from left to rightand is usually written from left to right

A group of three bases codes for one amino acidA group of three bases codes for one amino acid DNA code is copied (transcribed) to produce mRNA, DNA code is copied (transcribed) to produce mRNA,

and the order of amino acids in proteins is determined and the order of amino acids in proteins is determined by the sequence of the three letter codes in mRNAby the sequence of the three letter codes in mRNA

The mRNA sequence reads the same as the 5’ to 3’ The mRNA sequence reads the same as the 5’ to 3’ DNA sequence except for the substitution of U for T.DNA sequence except for the substitution of U for T.

Why do we need both DNA and Why do we need both DNA and RNA?RNA?

DNA holds all the genetic information/instructions for DNA holds all the genetic information/instructions for the proteins produced in a cell so why do we need the proteins produced in a cell so why do we need RNA.RNA.

It is BECAUSE DNA holds all the genetic information, It is BECAUSE DNA holds all the genetic information, this means it is EXTREMELY important.this means it is EXTREMELY important.

If DNA is damaged in any way, the coding sequence If DNA is damaged in any way, the coding sequence can change and a MUTATION will arise that will can change and a MUTATION will arise that will potentially influence the particular protein and potentially influence the particular protein and perhaps the whole cell or organism.perhaps the whole cell or organism.

If DNA ventured into the cytoplasm to give If DNA ventured into the cytoplasm to give instructions for protein synthesis it would be instructions for protein synthesis it would be vulnerable to damage from chemicals, UV radiation vulnerable to damage from chemicals, UV radiation and other mutagens.and other mutagens.

RNA acts as a messenger. Damage to mRNA will not RNA acts as a messenger. Damage to mRNA will not permanently affect function of the cell as the DNA permanently affect function of the cell as the DNA template is undamaged.template is undamaged.

DNA is self-replicatingDNA is self-replicating

In order for genetic information to be passed In order for genetic information to be passed on to new cells, chromosomal DNA must be on to new cells, chromosomal DNA must be replicated prior to cell division.replicated prior to cell division.

Replication of DNA creates the sister Replication of DNA creates the sister chromatids found in chromosomes that are chromatids found in chromosomes that are preparing to divide.preparing to divide.

This process duplicates the whole This process duplicates the whole chromosome, and the sister chromatids are chromosome, and the sister chromatids are then held together by a common centromere then held together by a common centromere until they are separated in the process of cell until they are separated in the process of cell division.division.

Steps in DNA ReplicationSteps in DNA Replication

1. Unwinding the DNA molecule1. Unwinding the DNA molecule

2. Making new DNA strands2. Making new DNA strands

3. Rewinding the DNA molecule3. Rewinding the DNA molecule

Different enzymes are involved in different stages Different enzymes are involved in different stages of DNA replication, and although they are shown of DNA replication, and although they are shown as separate entities in most diagrams, they will as separate entities in most diagrams, they will tend to cluster together forming a ‘replication tend to cluster together forming a ‘replication complex’complex’

Unwinding the DNA Unwinding the DNA MoleculeMolecule

Replication of DNA begins at a sequence of Replication of DNA begins at a sequence of nucleotides called the nucleotides called the origin of replicationorigin of replication..

An enzyme called An enzyme called helicase helicase unwinds the dsDNA unwinds the dsDNA helix and single-stranded binding proteins helix and single-stranded binding proteins (SSBP) react with the ssDNA and stabilize it. (SSBP) react with the ssDNA and stabilize it.

At the same time, At the same time, DNA gyraseDNA gyrase relieves the relieves the strain that unwinding causes on the molecule strain that unwinding causes on the molecule by cutting, winding and rejoining DNA strands.by cutting, winding and rejoining DNA strands.

Under an electron microscope the unwound Under an electron microscope the unwound section looks like a “bubble” and thus is known section looks like a “bubble” and thus is known as the as the replication bubblereplication bubble..

Making New DNA StrandsMaking New DNA Strands DNA polymeraseDNA polymerase III is the major enzyme involved in DNA III is the major enzyme involved in DNA

replication.replication. It adds nucleotides to the 3’ end of a pre-existing chain of It adds nucleotides to the 3’ end of a pre-existing chain of

nucleotides thus generating a new complementary strand of DNA, nucleotides thus generating a new complementary strand of DNA, but it cannot initiate a nucleotide chain.but it cannot initiate a nucleotide chain.

An RNA polymerase called An RNA polymerase called primaseprimase is needed to start a new is needed to start a new nucleotide chain.nucleotide chain.

Primase constructs an RNA primer (sequence of about 10 nucleotides Primase constructs an RNA primer (sequence of about 10 nucleotides complementary to the parent strand) which DNA polymerase III can complementary to the parent strand) which DNA polymerase III can then add nucleotides to.then add nucleotides to.

The unwound DNA exposes two parental strands of DNA which are The unwound DNA exposes two parental strands of DNA which are antiparallel. This means they are orientated in different directions antiparallel. This means they are orientated in different directions and must be replicated by different mechanisms.and must be replicated by different mechanisms.

The The leading strandleading strand elongates towards the replication fork (in the elongates towards the replication fork (in the direction of unwinding) by the simple addition of nucleotides to is 3’ direction of unwinding) by the simple addition of nucleotides to is 3’ end by DNA polymerase III.end by DNA polymerase III.

The The lagging strandlagging strand must elongate away from the replication fork. must elongate away from the replication fork. It is synthesized discontinuously as a series of short segments called It is synthesized discontinuously as a series of short segments called Ozaki fragmentsOzaki fragments. When DNA polymerase III reaches the RNA . When DNA polymerase III reaches the RNA primer on the lagging strand, it is replaced by DNA polymerase I, primer on the lagging strand, it is replaced by DNA polymerase I, which removes the RNA primer and replaces it with DNA. which removes the RNA primer and replaces it with DNA. DNA DNA ligaseligase then attaches and forms phosphodiester bonds. then attaches and forms phosphodiester bonds.

Rewinding the DNA Rewinding the DNA MoleculeMolecule

Since each new strand is complementary to its old Since each new strand is complementary to its old template strand, two identical new copies of the DNA template strand, two identical new copies of the DNA double helix are produced during replication. double helix are produced during replication.

In each new helix, one strand is the old template and In each new helix, one strand is the old template and the other is new synthesised, therefore replication is the other is new synthesised, therefore replication is said to be said to be semi-conservativesemi-conservative..

The two DNA molecules rewind into the double-The two DNA molecules rewind into the double-helices, then each double-helix is coiled around helices, then each double-helix is coiled around histone proteins and further wrapped up to form histone proteins and further wrapped up to form separate chromatids (still joined by a common separate chromatids (still joined by a common centromere).centromere).

The two chromatids will become separated in the cell The two chromatids will become separated in the cell division process to form two separate chromosomes.division process to form two separate chromosomes.

Overview of DNA replicationOverview of DNA replication

Transcription of DNATranscription of DNA Process of transcription begins when a section of DNA (a gene) unwinds Process of transcription begins when a section of DNA (a gene) unwinds

and the bases separate exposing two single strands of DNA with unpaired and the bases separate exposing two single strands of DNA with unpaired bases.bases.

One of these strands act as a template for the formation of an mRNA One of these strands act as a template for the formation of an mRNA molecule (it is transcribed)molecule (it is transcribed)

Individual nucleotides of RNA align with the exposed bases on the DNA Individual nucleotides of RNA align with the exposed bases on the DNA template according to base pairing rules. Nucleotides are added to 3’ end template according to base pairing rules. Nucleotides are added to 3’ end of growing RNA molecule.of growing RNA molecule.

The formation of the mRNA molecule is catalysed by the enzyme RNA The formation of the mRNA molecule is catalysed by the enzyme RNA polymerase.polymerase.

This molecule is actually referred to as pre-mRNA. It is complementary to This molecule is actually referred to as pre-mRNA. It is complementary to the template strand but requires some post-transcriptional modification.the template strand but requires some post-transcriptional modification.

Post-transcriptional modification of pre-mRNA or nuclear mRNA involves Post-transcriptional modification of pre-mRNA or nuclear mRNA involves the removal of introns (non-coding regions within genes) and stitching the removal of introns (non-coding regions within genes) and stitching together of exons (coding regions of genes). This process is known as RNA together of exons (coding regions of genes). This process is known as RNA splicing.splicing.

Following RNA splicing a chemical cap is added to the 5’ end of the Following RNA splicing a chemical cap is added to the 5’ end of the molecule and a poly-A tail (string of A nucleotides) to the 3’ end. The 5’ molecule and a poly-A tail (string of A nucleotides) to the 3’ end. The 5’ cap enables efficient protein synthesis as it is part of the structure cap enables efficient protein synthesis as it is part of the structure recognized by the small ribosomal subunit. The poly-A tail is also recognized by the small ribosomal subunit. The poly-A tail is also important for initiating translation. It also has a role in regulating the important for initiating translation. It also has a role in regulating the degradation of mRNA molecules in the cytoplasm.degradation of mRNA molecules in the cytoplasm.

Comparison of transcription in Comparison of transcription in eukaryotes and prokaryoteseukaryotes and prokaryotes



DNA is transcribed to produce mRNA.DNA is transcribed to produce mRNA.


Translation of mRNATranslation of mRNA mRNA exits the nucleus through nuclear pores and binds to mRNA exits the nucleus through nuclear pores and binds to

ribosomes within the cytoplasm.ribosomes within the cytoplasm. Translation of mRNA begins with the sequence AUG (start Translation of mRNA begins with the sequence AUG (start

codon).codon). Transfer RNAs (tRNA) bring specific amino acids to the Transfer RNAs (tRNA) bring specific amino acids to the

ribosome and these are added to the growing polypeptide ribosome and these are added to the growing polypeptide chain by condensation polymerisation. New amino acids chain by condensation polymerisation. New amino acids are added to the carboxyl (COOH) end of the polypeptide.are added to the carboxyl (COOH) end of the polypeptide.

The tRNA drops away from the mRNA and acquires another The tRNA drops away from the mRNA and acquires another specific amino acid from the pool in the cytoplasm. Each specific amino acid from the pool in the cytoplasm. Each tRNA can only carry one type of amino acid.tRNA can only carry one type of amino acid.

Translation ends when the ribosome reaches a stop codon – Translation ends when the ribosome reaches a stop codon – the tRNA molecules corresponding to the stop codons UAG, the tRNA molecules corresponding to the stop codons UAG, UGA and UAA don’t carry a amino acid.UGA and UAA don’t carry a amino acid.

The mRNA is then released from the ribosome.The mRNA is then released from the ribosome.

Structure of tRNA moleculeStructure of tRNA molecule

Once a tRNA gene is transcribed, the RNA that is produced folds to form the shape of a three-leafed clover. This is a functional tRNA molecule.

This tRNA is charged with the amino acid lysine at the amino acid attachment site. The anticodon UUU will bind to the complementary AAA sequence in the mRNA.

Gene RegulationGene Regulation

All the somatic cells in your body contain the All the somatic cells in your body contain the same chromosomes and therefore the same DNA same chromosomes and therefore the same DNA and same genes. and same genes.

However, these cells are able to have different However, these cells are able to have different shapes and sizes and perform different functions shapes and sizes and perform different functions and change throughout your lifespan.and change throughout your lifespan.

These differences are possible because of These differences are possible because of different mechanisms that control the expression different mechanisms that control the expression of individual genes. These mechanisms are of individual genes. These mechanisms are collectively referred to as mechanisms for collectively referred to as mechanisms for gene gene regulation.regulation.

How are genes regulated?How are genes regulated?

There are many steps involved in the There are many steps involved in the expression of gene, therefore there are many expression of gene, therefore there are many different mechanisms for regulating different mechanisms for regulating expression:expression: The structure of genes variesThe structure of genes varies The rate of transcription can be regulatedThe rate of transcription can be regulated Post-transcriptional modifications can influence Post-transcriptional modifications can influence

which protein is producedwhich protein is produced The rate of translation can be regulatedThe rate of translation can be regulated The activity of the protein product (enzyme) can be The activity of the protein product (enzyme) can be

regulatedregulated

Role of different mechanisms Role of different mechanisms in gene regulationin gene regulation

Gene structureGene structure All genes contain an upstream promoter region. This consists of a All genes contain an upstream promoter region. This consists of a

binding site for RNA polymerase and other base sequences known as binding site for RNA polymerase and other base sequences known as upstream upstream promoter elementspromoter elements (UPEs). UPEs initiate transcription. (UPEs). UPEs initiate transcription. Genes vary in the number and type of UPEs. A gene with only one UPE Genes vary in the number and type of UPEs. A gene with only one UPE will be weakly expressed. A gene with many UPEs is actively will be weakly expressed. A gene with many UPEs is actively transcribed.transcribed.

Other DNA sequences known as Other DNA sequences known as enhancersenhancers increase the rate of increase the rate of transcription.transcription.

Genes which code for the production of essential proteins are often Genes which code for the production of essential proteins are often present as multiple copies.present as multiple copies.

Genes can be permanentlyGenes can be permanently inactivated inactivated in some cells by changes in the in some cells by changes in the chromosome’s structure.chromosome’s structure.

Transcription rateTranscription rate DNA binding proteins called DNA binding proteins called transcription factorstranscription factors, regulate the rate at , regulate the rate at

which a gene is transcribed. These proteins bind with the upstream which a gene is transcribed. These proteins bind with the upstream region of the gene and stimulate transcription.region of the gene and stimulate transcription.

Transcription factors may be activated by hormones.Transcription factors may be activated by hormones.

Role of different mechanisms Role of different mechanisms in gene regulationin gene regulation

Post-transcriptional modificationsPost-transcriptional modifications Some pre-mRNAs can be modified in more than one way. Pre-mRNA may be Some pre-mRNAs can be modified in more than one way. Pre-mRNA may be

spliced differently in different tissues, leading to different protein products.spliced differently in different tissues, leading to different protein products.

TranslationTranslation Cells can regulate the amount of translation which occurs by controlling the Cells can regulate the amount of translation which occurs by controlling the

life-span of mRNA; mRNA may be inactivated after only a short time being life-span of mRNA; mRNA may be inactivated after only a short time being translated, or may survive longer in the cell and be translated many times.translated, or may survive longer in the cell and be translated many times.

Protein activityProtein activity Gene expression may be regulated by controlling the activity of the proteins Gene expression may be regulated by controlling the activity of the proteins

produced in translation. For example, enzyme inhibitors may inactivate an produced in translation. For example, enzyme inhibitors may inactivate an enzyme until it is needed.enzyme until it is needed.

Some proteins may control the production of other proteins – e.g. repressor Some proteins may control the production of other proteins – e.g. repressor proteins can bind to promoter region of DNA and prevent transcription.proteins can bind to promoter region of DNA and prevent transcription.

Other factors in gene Other factors in gene regulationregulation

It is important to realize that the It is important to realize that the environment of a cell can also environment of a cell can also influence the expression of genes. influence the expression of genes. This includes factors such as:This includes factors such as: LightLight TemperatureTemperature IonsIons HormonesHormones

Lac Operon – an example of Lac Operon – an example of gene regulation in E. Coligene regulation in E. Coli

The bacterium The bacterium Escherichia coliEscherichia coli is capable of producing the is capable of producing the enzyme enzyme -galactosidase which splits lactose to produce -galactosidase which splits lactose to produce glucose and galactose.glucose and galactose.

This enzyme is only produced when the bacteria encounters This enzyme is only produced when the bacteria encounters lactose. When lactose is not present, a protein binds to the lactose. When lactose is not present, a protein binds to the promoter region of the promoter region of the -galactosidase gene and prevents -galactosidase gene and prevents transcription (RNA polymerase cannot access the promoter). transcription (RNA polymerase cannot access the promoter). This protein is referred to as a repressor protein. This protein is referred to as a repressor protein.

When lactose is present in the growth medium of the When lactose is present in the growth medium of the bacteria, it enters the cell and binds to the repressor protein bacteria, it enters the cell and binds to the repressor protein causing it to be removed from the DNA and allowing causing it to be removed from the DNA and allowing transcription to occur.transcription to occur.

The gene is ‘on’ or ‘off’ depending on the nutrients available The gene is ‘on’ or ‘off’ depending on the nutrients available to the cell.to the cell.

the importance of dna and rna in heredity

Documents

dna sequence

dna helix

dna base adenine

base composition of

complementary base pairs

base sugarnucleotide

mrnathe mrna sequence

ribosethe sugar