DNA, Genes and the Continuity of Life

Sarah Jones

blog.lgcgroup.com

• Deoxyribonucleic acid (DNA) is a molecule that contains all of the information that determines who you are and what you look like. The “Blueprint of Life”.

• In 1869 Friedrich Miescher isolated chemicals that were later called nucleic acids. This lead to the identifications of DNA as the carrier of inheritance.

bastiaandegoede

• In 1953 James Watson and Francis Crick established the ‘double helix’ shape of DNA.

• The sides of the ladder like structure are a chain of alternating sugar and phosphate molecules.

• The rungs of the ladder are bases that are attached to the sugar molecules.

• Each rung is made up of two chemicals called bases.

– Adenine (A)

– Thymine (T)

– Guanine (G)

– Cytosine (C)

• Base Pairs: A-T and G-C linked form a rung.

• Remember – At The Gold Coast

• Nucleotide – consists of a sugar, phosphate and base.

• Gene – a segment of DNA that codes for a protein, which in turn codes for a trait.

– Skin tone, eye colour etc.

staff.jccc.net

• DNA contains the coding that makes up the building blocks of you body – amino acids.

• Amino acids join together to make proteins and proteins are used to make new cells.

• There are 20 amino acids.

blueprintsforliving.com

• Amino acids are coded by combinations of three bases – AAA and AAG both make the amino acid phenylalanine.

• Triplet codes GGA, GGG, GGT and GGC make the amino acid proline.

www.chemtube3d.com

• Organism

• Cell

• Nucleus

• Chromosome

• DNA

• Genewww.csiro.au

Mitochondria are structures within cells that convert the energy from food into a form that cells can use. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. This genetic material is known as mitochondrial DNA or mtDNA.

dnatestingexpert.com

Mitochondrial DNA contains genes that provide instructions for making enzymes involved in oxidative phosphorylation - a process that uses oxygen and simple sugars to create adenosine triphosphate (ATP), the cell's main energy source. http://ghr.nlm.nih.gov/mitochondrial-dna

en.wikipedia.org

Rosalind Franklin, Maurice Wilkins, James Watson and Francis Crick

The main people responsible for the discovery of DNA are Rosalind Franklin and Maurice Wilkins from a research unit at King's College, London, and James Watson and Francis Crick from Cambridge University, England.

The story of Rosalind Franklin would have been quite different if she were born male. During the middle of the twentieth century, women were not encouraged to study science.

Franklin

Wilkins

Franklin researched X-ray crystallography, a method of determining the structure of crystals based on the use of X-rays.

Rosalind contributed to the development of this technology by pioneering its use in analysing a variety of substances, including DNA.

The Cambridge team of Watson and Crick made a failed model of DNA and were told to stop their research.

Watson

Crick

Franklin mostly worked alone because she and Wilkins could not get along. He assumed she was to assist his work while Franklin assumed she would be the only one working on DNA.

Franklin suspected that all DNA had a helix structure but did not want to announce this finding until she had sufficient evidence. Wilkins was frustrated and showed Franklin's results to Watson without her knowledge or consent.

learn.crystallography.org.uk

The information Watson and Crick received from Wilkins was crucial to the ultimate discovery of DNA. It was principally these X-ray diffraction techniques developed by Franklin that allowed Watson and Crick to suggest the double helix structure for DNA.

The work of Wilkins and Franklin was acknowledged in this paper. The structure so perfectly fit the experimental data that it was accepted almost immediately.

www.lifesciencesfoundation.org

Rosalind Franklin died of cancer in 1958 at the age of 37. In 1962 the Nobel Prize for physiology and medicine went to James Watson, Francis Crick and Maurice Wilkins for their role in the discovery of the structure of DNA.

Chromosomes

• The way living things pass on traits – skin tone, eye colour etc.

• Chromosomes are formed of a single DNA molecule that contains many genes.

Some of our worst diseases result from chromosomes that have failed to function correctly e.g. polycystic kidney disease, Huntington’s disease, sickle cell anaemia, haemophilia.

en.wikipedia.org

Chromosomes

• Humans contain 23 pairs

• Chromosomes determine if you are male or female.

Humans have 46 chromosomes – 23 from each parent.

• Two of the 46 chromosomes are called sex chromosomes and the determine the sex of an individual – XX or XY

• The other 44 chromosomes are referred to as autosomes.

ibbiologyhelp.com

• Sex cells – ova and sperm only have half of the number of chromosomes as the rest of the cells in your body.

• Ova – only X

• Sperm – X or Y therefore, your father determines your sex.

• Identical twins occur when a fertilised egg divides in two – the babies are genetically the same.

• Fraternal twins occur when two eggs are fertilised – the twins are not genetically the same.

petapixel.com

Meiosis

• Mitosis is cell division where a cell reproduces by splitting to form two identical offspring.

• Meiosis (reductive division) is cell divisions that only happens in the reproductive organs – ovaries and testes.

• Meiosis produces cells which have half the number of chromosomes.

• The nucleus of normal human body cells consist of 46 chromosomes or 23 pairs of chromosomes (2 of each chromosome).

• This is referred to as the diploid number for humans (2n).

• Gametes, sex cells, only have one set of chromosomes (23).

• This is referred to as the haploid number for humans (n).

Fertillisation is the fusion of haploid male and female gametes, to form a diploid number of chromosomes in a zygote.

Meiosis I

• Homologous chromosomes pair up. They are called a bivalent.

• Non-sister chromatids cross over at points called chiasmata.

• They may exchange genetic material –crossing over.

• Homologous pairs line up at equator.

• Maternal and paternal chromosomes of each pair line up independently of other pairs –independent assortment.

• Homologous chromosomes separate and move towards opposite poles.

• Two new cells form, each with half the original chromosome number.

Meiosis II

• New spindle apparatus forms.

• Chromosomes line up at the equator in a single line.

• Centromeres divide and sister chromatids move towards opposite poles.

• Each cells divides, resulting in a total of four haploid cells.

• Each cell formed is genetically unique due to crossing over and independent assortment.

https://online.science.psu.edu/biol011_sandbox_7239/node/7288

https://online.science.psu.edu/biol011_sandbox_7239/node/7288

• In prophase I of meiosis I, two things happen that don’t happen in mitosis.

• Diploid (2N) cells contain two copies of each chromosome, one from the mother and one from the father.

• In meiosis I, these pairs of chromosomes find each other in the cell (we don't completely understand the signals used), but it does occur and in most cases without error.

Crossing-Over

www.vce.bioninja.com.au

• After locating each other, these chromosomes interact and pair in a process called synapsis.

• During synapsis, the cells align themselves along their entire length, matching gene for gene.

• Then the cells exchange pieces of their DNA through a process called "crossing over” producing a unique chromosome, genetically distinct from both the maternal and paternal chromosomes (https://online.science.psu.edu).

www.vce.bioninja.com.au

Synapsis

https://online.science.psu.edu/biol011_sandbox_7239/node/7288

Crossing-Over

https://online.science.psu.edu/biol011_sandbox_7239/node/7288

Mitosis

• Produces new cells identical to the original cell.

• How plants and animals grow and replace dead and damaged cells.

• Some organisms reproduce using mitosis –asexual reproduction.

b4fa.org

Binary Fission1. The bacterium before binary fission is when the DNA

is tightly coiled.

2. The DNA of the bacterium has replicated.

3. The DNA is pulled to the separate poles of the bacterium as it increases size to prepare for splitting.

4. The growth of a new cell wall begin.

5. The new cell wall fully develops, resulting in the complete split of the bacterium.

6. The new daughter cells have tightly coiled DNA, ribosomes, and plasmids.

http://en.wikipedia.org/wiki/Fission

DNA Replication

• DNA Replication is semi-conservative.

• After DNA replication, each new DNA double helix molecule contains:

–One old strand.

–One new strand.

en.wikipedia.org

Enzymes Involved in DNA Replication

Two enzymes are involved in the process:

• Helicase unwinds the DNA double helix molecule.

• Helicase then unzips the DNA double helix molecule by breaking the hydrogen bonds between the complementary bases.

• DNA polymerase attaches new nucleotides to the old DNA strands.

www.di.uq.edu.au

Ref: IB Biology, OSC

Ref: Biology for the IB Diploma, Allott

DNA and RNA

• There are TWO types of Nucleic acids – DNA and RNA

• Both consist of chains of nucleotides, each composed of a sugar, a base and a phosphate.

• Three differences between DNA and RNA:

Feature DNA RNA

No. of Strands Two Strands One Strand

Sugar Deoxyribose Ribose

Bases A-T, C-G A-U, C-G

U = Uracil

Protein Synthesis

• The flow of information for organisms is unidirectional.

DNA RNA Protein

• There are two main processes involved in Protein Synthesis:

– Transcription

• Where a molecule of mRNA is formed from the DNA template strand.

– Translation

• Where the mRNA sequence is converted into a polypeptide chain.

learn.genetics.utah.edu

Transcription• DNA RNA.

• Process in which the message written in DNA code is transcribed into a working copy of mRNA (messenger RNA).

www.mun.ca

Transcription Process

The process is:– The RNA polymerase enzymes separate the

two strands of DNA.–One strand of the DNA is used as a

template for mRNA synthesis.–RNA polymerase forms the mRNA molecule

using Uracil instead of Thymine.–When the mRNA molecule is complete, it

breaks away from the DNA and travels through the nuclear pores into the cytoplasm.

– The DNA strands rejoin.

en.wikipedia.org

Translation

• RNA Protein

• Process in which a polypeptide chain is built from a codon sequence on the mRNA molecule.

commons.wikimedia.org

TranslationThe process is:

– The mRNA molecule attaches to a Ribosome.– tRNA molecules bring specific amino acids to

the ribosome according to the codon on the mRNA.

– There is a different tRNA molecule for each of the 61 codons.

– Each tRNA molecule is about 80 nucleotides long and is folded into a clover shape.

–At one end there is an exposed triplet of bases called an anticodon and at the other a specific amino acid.

– The anticodon on the tRNA matches the codon on the mRNA.

Ref Biology for the IB Diploma, Allott

• Environmental variation – differences in characteristics caused by surrounds or living conditions.

• Inherited variation – differences in characteristics that are passed on from parents to offspring.

• Natural selection – passing on inherited characteristics that make living thins best adapted for survival.

• Selective breeding – producing specific offspring that have useful characteristics of both parents.

Inheritance and Selection

Gregor Mendel 1822-1884

• An Austrian monk who worked on his garden at the monastery and noted how the characteristics of plants were passed on from one generation to the next.

• The results of Mendel's research became the foundation of modern genetics.

Pea Plants• One of the main variations Mendel noted was

that some peas were smooth and some wrinkled.

• When he cross bred these peas – using the pollen from the flowers, he noted that all of the offspring were smooth.

www.fws.gov

He then took smooth peas and cross pollinated these plants – he noted that ¾ of the peas were smooth and ¼ wrinkled.

mrjohnston.org

• Mendel realised that the peas had two factors inside of them – one from each parent.

• Smooth peas – SS

• Wrinkled peas – ww

• Mendel deduced that the smooth factor (gene) is dominant and the wrinkled factor is recessive.

rowdy.msudenver.edu

Punnett Square

w w

S Sw Sw

S Sw Sw

When these two peas SS and ww were crossed all of the offspring were smooth because S is dominant.

Second Generation

S w

S SS Sw

w Sw ww

¼ of offspring were wrinkled.

• Genotype – the type of genes e.g. SS and ww the dominant gene is always written as a capital.

• Alleles – the possible genes in the sex cells

e.g. S and w

• Phenotype – how the genotype displays e.g. smooth or wrinkled.

• Homozygous – contains only one type of gene e.g. SS or ww

• Heterozygous – two types of genes e.g. Sw

Guinea Pigs• Short fur is dominant (F) and long fur

recessive (f).

• If a heterozygous guinea pig mated with a homozygous long hair guinea pig we would get the following results.

F f

f Ff ff

f Ff ff

50 % Short fur and 50% Long fur

• Brunette hair is dominant and blonde recessive.

• Therefore, if the parents genes are homozygous brunette and blonde all offspring will be brunette.

B B

b Bb Bb

b Bb Bb

Incomplete Dominance

• Purebred budgies are yellow and blue birds.

• Green budgies result from one parent with blue feathers and one with yellow.

• Both parents are homozygous and the two alleles are not dominant or recessive and result in a blend of characteristics.

animal-world.com

Polygenic Inheritance

• When characteristics are controlled by more than one gene.

• Examples of traits controlled by polygenic inheritance are:

– seed colour in wheat, skin colour and eye colourPolygenic Inheritance of Grain Wheat Colour

www.ib.bioninja.com.au

• There are four ABO blood groups in humans: A, B, AB and O. The genes A and B are both dominant over the gene O, but A and B are co-dominant when both present in the same individual resulting in blood group AB. The genes for the ABO blood groups are not sex-linked.

• The ABO blood groups are named from the antigens that they carry on red blood cells.

Multiple Alleles in Blood Groups

en.wikipedia.org

en.wikipedia.org

knowyourblood.com

• A separate blood group classification system in humans is the rhesus or Rh factor, first found in Rhesus monkeys.

• The Rh blood groups are independent to the ABO blood groups.

• When the Rh factor is present, the blood group is Rh+ when it is absent the blood group is Rh-.

Genetic Pedigrees

Huntington’s Disease

• A neurological condition caused by the inheritance of a defective gene.

• The death of brain cells leads to the gradual loss of cognitive, physical and emotional function.

• There is no cure.

macintosh-genetic-disorders.wikispaces.com

H is the gene for Huntington's which is dominant.

Cystic Fibrosis

• A hereditary disease which affects the entire body, causing progressive disability and often early death.

• a is the affected allele and A not affected

Haemophilia

• Passed from one generation to the next through the X (female) chromosome.

• Disease in which blood does not clot normally.

• Because the blood clots so slowly there is a constant danger of bleeding to death even with a minor injury.

• Women transmit the disease, however, only men exhibit it.

• Women who carry haemophilia have the gene on one of their X chromosomes.

• Since the gene is recessive, they do not have the disorder.

• If a male has the bad gene they will be a haemophiliac because there is no matching gene on his Y chromosome to be dominant to it.

• Females must have the gene on both X chromosomes to have haemophilia.

Queen Victoria was a carrier

of the haemophiliac gene.

Genetic Mutations

• Colour Blindness

• Down Syndrome

• Albinism

• Cystic fibrosis

• Haemophilia

• Tay-Sachs Disease

• Turner Syndrome

Genetic Modifications• In GM organisms the genetic information has

be changed by inserting new genes.

• The new genes are then passed to daughter cells through mitosis.

• GM is used to create desirable

traits in organisms such as

insect resistance and increased

nutrient value.

Canola Modification• Western Australia, Victoria and New South

Wales all have farmers growing GM canola.

• This canola is resistant to herbicides that are commonly used to control weeds.

• Therefore, the farmers can kill the weeds using chemicals and not affect their canola.

www.abc.net.au

Rice Modifications• White rice is a main food source for half the

worlds population, however, it lacks essential minerals and vitamins (Vitamin A deficiency can lead to blindness in children).

• Golden rice-21 is genetically modified using the genes from corn, daffodils and bacteria. The rice contains beta-carotene which the body converts to vitamin A.

www.21stcentech.com