Chpt. 16 DNA & RNA

(Deoxyribonucleic Acid) DNA (Deoxyribonucleic Acid) DNA was discovered by Watson and Crick (1953) It is found in the nucleus . DNA is coiled and folded in order to fit into nucleus. Proteins responsible for holding DNA in its folded state.

Structure of DNA DNA has two strands. Strands are linked by pairs of chemicals called bases. Each pair of bases form a rung on the DNA molecule.

Structure of DNA Only four different bases used in DNA: Adenine - A Thymine - T Guanine - G Cytosine - C A joins with T only and G joins with C only

Structure of DNA Pairs of bases A/T and G/C are said to be complementary. A T G C

Structure of DNA DNA structure can be thought of as a ladder twisted to form a spiral – called Double Helix Side Strand DNA Double Helix Pairs of Bases

This structure is important as The DNA is copied exactly when ever the cell divides The DNA provides a code for protein synthesis

Chromosomes and Genes Chromosome: consists of many base pairs arranged into a double helix. Gene: is a sequence of many bases. Genetic Code: is the precise sequence of bases.

The Genetic Code Gene: are pieces of inherited information that code for the production of proteins. Protein: made up of combination of amino acids. up to 20 different types of amino acids used. Genetic Code: To allow correct amino acids to be assembled to make protein DNA carries a genetic code.

The Genetic Code The genetic code is made up of 3 DNA bases in sequence to form a triplet or a codon. Each triplet is the code for an amino acid. A sequence of codons that produce a protein is called a GENE

Genetic Code Examples: DNA triplet CAA code for amino acid Valine DNA triplet CGA code for amino acid Alanine

Genetic Code The genetic code is like another language and can be compared with the English language as follows: Note: Genome = all the genes in the cell Genetic Code English Equivalent Base Letter Triplet or codon Word Gene Paragraph Genome Book

Genetic Code Non-coding DNA: Approximately 97% DNA in human nucleus does not carry the code for the production of proteins Junk DNA. Junk DNA has no known function. Located in two places: between genes within genes Sequence of bases in non-coding DNA varies greatly from person to person – used in preparing DNA profiles.

Non-Coding DNA

DNA Replication occurs in nucleus of cell during INTERPHASE, which is just before cell division (mitosis) occurs. results in the single-stranded chromosome forming two identical strands (identical genes) held together at the centromeres. Double-stranded chromosome Single-stranded chromosome Interphase – phase in cell cycle when the cell is not dividing . It is longest phase can be up to 90% of cell cycle. DNA in each strand is identical

DNA Replication in Detail: The double helix unwinds. Enzyme breaks bonds between base pairs and the DNA strands separate. New DNA bases (nucleotides), present in the cytoplasm, enter nucleus and attach to their complementary bases on the exposed strands. Each new strand is: a) half new DNA and half old DNA b) identical to original DNA strand and to other new partner strand. Each new piece of DNA rewinds to form double helix.

DNA Replication in Detail:

Significance of DNA Replication: DNA is able to produce exact copies of itself. The same genetic information is passed on from one generation of cells to the next.

DNA Replication Theory DNA Replication Diagram Step 1: Step 2: Step 3: Step 4: Step 5: DNA Replication Diagram

(DNA/Genetic Fingerprinting) DNA Profiling (DNA/Genetic Fingerprinting) DNA Profiling: is a method of making a unique pattern of bands from the DNA of a person, which can then be used to distinguish that DNA from other DNA.

DNA Profiling Preparing a DNA Profile: Involves four steps: 1) Release DNA from cells 2) Cut the DNA into fragments 3) Separate the fragments 4) Compare patterns Release DNA from cells: - Cells are broken down to release their DNA (see activity 15) e.g. DNA from blood, hair, semen.

DNA Profiling 2. Cut the isolated DNA into fragments: - special enzymes (restriction enzymes) are used to cut the isolated DNA into pieces. - different enzymes cut DNA at specific base sequences e.g.: i) One restriction enzyme will always cut DNA at the base sequence: GAATTC. ii) Another restriction enzyme only cuts at the sequence: GATC. - sections of DNA cut out = Restriction Fragments

DNA Profiling 3. Separate the Fragments: - DNA fragments are separated according to their length. - They are separated by a process called gel electrophoresis: i) fragments are placed in a gel and an electric current passed through the gel. ii) small fragments move faster through the gel than large ones. iii) Photographic copy of final pattern of DNA bands is obtained. Note: Each DNA profile looks like a bar code and is different for each person except in the case of identical twins.

This band represents longer segments of DNA DNA Profiling This band represents longer segments of DNA Short segments DNA Profile

DNA Profiling 4. Compare Patterns: - Highly unlikely that any two people will have the same DNA profile (exception: identical twins). - if pattern of bands from two different DNA samples is the same then two samples must have come from same person.

DNA Profiling Step 1: Step 2: Step 3: Step 4:

Applications of DNA Profiles (DNA Fingerprints) A. Crime: DNA profiles may be used to link somebody to a crime or to the scene of a crime. If some biological tissue (saliva on a cigarette butt, semen stain or a hair) is found at a crime scene, its DNA profile is compared with one taken from a suspect. If patterns match, suspect is associated with the crime scene. Note: See Figure 16.18 pg. 155 – comparing DNA profiles

Applications of DNA Profiles B. Medical: DNA profiles can be used to determine whether a particular person is, or is not, the parent of a child i.e. establishing the paternity or maternity of a child. Paternity cases are important in immigration, inheritance and rape cases. Note: See figure 16.19 and 16.21(a) (b) pg 155 – how to use DNA profiles to determine the father of a child.

Genetic Screening Remember: DNA Replication is when DNA makes exact copies of itself during interphase. Genetic Screening: means testing a person’s DNA for the presence of abnormal or altered DNA. the presence of abnormal or altered DNA is an indication that a particular gene is mutated. this may have a severe effect on a person who inherits such genes – examples of genetic disorders include: - albinism, cystic fibrosis, haemochromatosis, sickle cell anaemia.

Genetic Screening Two main forms Adult Screening Foetal Screening - even though they may not have a genetic disorder adults may be screened to see if they carry the defective gene for the disorder. - such people are known as carriers for the condition. - such tests give people information regarding the chances of them having a child with the disorder.

Genetic Screening Foetal Screening - cells are removed from the placenta or the fluid around the foetus. - these cells can then be tested to detect if the child has a genetic disorder.

Genetic Screening Ethics of genetic screening: Genetic screening may cause ethical problems such as: Abortion. Unfair treatment Should a person be told they have a disorder that will develop later in life and lead to death? Discrimination from insurance.

RNA ( Ribonucleic Acid) DNA and RNA are both nucleic acids however they differ inn several ways: DNA RNA Bases: contains the base thymine Bases: contains URACIL (U) instead of thymine Double Stranded Single Stranded DNA always stays in the nucleus RNA can move out of the nucleus into the cytoplasm. Only one type Three types mRNA, tRNA, rRNA Note: in RNA the bases A and U are complementary.

RNA (Ribonucleic acid) Note: RNA bases are complementary to DNA bases e.g. - If DNA has the base sequence GGAATC, the RNA complementary sequence will be CCUUAG.

Protein Synthesis (Ordinary Level) Genes control cells by producing proteins most of which are enzymes. Proteins are composed of amino acids, to produce correct protein amino acids must assemble in the correct sequence. Genes control the order of amino acids, each group of three bases (codon) code for a particular amino acid. Must know/understand how genes work (are expressed) i.e. how DNA (genes) makes proteins.

Protein Synthesis (Ordinary Level) DNA RNA PROTEIN Involves the genetic code in DNA being transcribed to mRNA and this code being translated into the correct sequence of amino acids. Occurs in stages: DNA RNA PROTEIN Transcription Nucleus Translation Ribosomes (cytoplasm)

How proteins are made --- Protein Synthesis In the nucleus the DNA strands separate TRANSCRIPTION– rewriting of the DNA code to RNA RNA bases attach to the exposed bases of the DNA forming messenger RNA( mRNA)

The mRNA detaches and enter a ribosome which is present in the cytoplasm.

TRANSLATION – As the mRNA passes through the ribosome, each group of 3 bases cause amino acids to be made this is called TRANSLATION The amino acids are linked together forming a protein 6. The protein folds into the correct shape which allows the protein to carry out it’s function

DNA Structure – Higher Level DNA is made up of many nucleotides which are arranged into chains called polynucleotides. Nucleotides are made up of: - a 5 carbon sugar deoxyribose - a phosphate group (PO4) - a nitrogen containing base ( A, T ,G , C) Sugar (deoxyribose) Nitrogen Base Phosphate

DNA Structure – Higher Level There are four nitrogenous bases: Adenine (A) Guanine (G) Thymine (T) Cytosine (C) Purines Pyrimidines

DNA Structure – Higher Level In the double helix Adenine & Thymine are held together by double hydrogen bonds Cytosine & Guanine are held together by triple hydrogen bonds Note: - each base pair has a purine and a pyrimidine - nucleotides join together with a bond between phosphate group of one and sugar group of the next – polynucleotide. T A G C

DNA Structure – Higher Level

DNA Structure – Higher Level DNA consists of two spiral chains of polynucleotides. Outside strands – deoxyribose and phosphate Rungs of molecule – are base pairs on the inside

Protein Synthesis – Higher Level Initiation: Occurs in nucleus Double helix unwinds at the site of the gene that is to form a protein. Transcription: (information is copied from DNA to RNA) Occurs in nucleus Complementary RNA bases attach to the exposed DNA bases The RNA bases join together by the enzyme RNA polymerase to form messenger RNA (mRNA) Each group of three bases on mRNA represents a start codon, an amino acid sequence or a stop codon. mRNA moves from the nucleus into the cytoplasm

Translation: (is the making of an amino acid sequence from RNA) Occurs in the ribosome Ribosomes - made up of rRNA and protein - has two sub units: large sub-unit & small sub-unit mRNA attaches to a ribosome Transfer RNA (tRNA) which are found free in the cytoplasm, enter the large sub unit of the ribosome two at a time Each tRNA carries: - a anticodon - a particular amino acid Note: Every tRNA codes for a specific amino acid

Each anticodon on a tRNA is complementary to a codon on the mRNA Just after the start codon the first tRNA molecule attaches to the mRNA (tRNA molecules attach two at a time) Adjacent amino acids are detached from the tRNA and bonded together by the ribosome forming part of new protein. tRNA continues to enter the ribosome until a stop codon is reached At this point: - mRNA code sequence complete - new protein produced.

Protein Synthesis

Protein Synthesis

Protein Synthesis Initiation: Transcription: Translation:

Protein Synthesis