GeneticsCell Division / Cycle

Cell DivisionDifference between Mitosis and MeiosisThe science of Genetics

1. Growth requires production of new cells.2. As cell size increases, cells become less efficient ( distances between plasma membrane and

central organelles also increases making transport of essentials molecules in and out more difficult ).

3. “Old” cells wear out. die, and are continually replaced by new ones.

4. On the average, wbc live for 2 days, rbc for 4 months, brain cells for 60 years or more, while intestinal cells live for only 36 hours.

Cell division(The process by which a cell divides to form two daughter cells. Upon completion of the process, each daughter cell contains the same genetic material as the original cell

Cell cycle

In recent years, the way in which cell division is controlled has been clarified. Inside each cell there is a cell cycle ‘clock’ which determines whether or not a cell should divide. The clock is an executive decision maker and integrates the regulatory signals received by the cell with the current state of health of the cell

Cell Cycle

The cell cycle consists of four steps. In the gap 1 (G1) phase, the cell grows in size and checks the status of its internal systems. If everything is functioning normally, and any damage to the DNA has been corrected, the cell moves on through the cycle. If something is wrong and cannot be corrected, the cell halts its progression through the cycle and may initiate apoptosis and close down. R marks the point where restriction of the cycle can occur.

In the following synthetic (S) phase, the cell replicates its store of DNA in the chromosomes. Following this there is a period of preparation for division called the G2 phase. Then the cell divides - the mitotic (M) phase. The two new daughter cells then enter the G1 phase of their own cell cycle.

This sequence of events involves interactions between many different proteins, some of which are capable of halting the process if conditions are unfavourable. p53 is a tumour suppressor protein that binds to specific DNA sequences. It is thought of as the "guardian of the genome" and controls the cell cycle to enable the repair of damaged DNA

The control of cell proliferation is intimately connected to apoptosis - a process by which cells methodically close down their metabolic activities and die when they have irreparable damage to their DNA or have no further role in the body. Normal p53 suppresses tumour growth by arresting cells in G1 phase or triggering apoptosis. The p53 gene is mutated in a wide range of tumours, for example: skin cancer and colorectal cancer, with the result that in the affected cells the cell cycle clock spins out of control and the cells divide without restraint.

Meiosis and Mitosis describe cell division in eukaryotic cells when the chromosome separates.

Mitosis – separation of chromosomes into two identical sets of daughter cells

Meiosis- reductional cell division and the number of chromosomes is divided into half; it is essential for sexual reproduction

Differences between Mitosis and Meiosis

Mitosis

DNA replication

MITOSIS

2 diploid cells

1. Interface -where cell prepares for cell division and it also includes three other phases such as G1 (growth), S (synthesis), and G2 (second gap

2. Prophase – formation of centrosomes, condensation of chromatin

3. Prometaphase- degradation of the nuclear membrane, attachment of microtubules to kinetochores

4. Metaphase- alignment of chromosomes at the metaphase plate

5. Early anaphase- shortening of kinetochore microtubules

6. Telophase- decondensation of chromosomes and surrounded by nuclear membranes, formation of cleavage furrow.

7. Cytokinesis- division of cytoplasm

Stages in Mitosis

Meiosis is a reductional cell division where the number of chromosomes is divided into half.

Gametes formations occur in animal cell and meiosis is necessary for sexual reproduction which occurs in eukaryotes.

Meiosis influence stable sexual reproduction by halving of ploidy or chromosome count. Without meiosis the fertilization would result in zygote with twice the number of the parent.

MEIOSIS

MEIOSIS

INTERPHASE

Meiosis I

Meiosis II

1. Mitosis produces the same number of chromosomes in the parent cell, while in meiosis, only half the number.

2. In mitosis, daughter cells are genetically identical to parent cell, while in meiosis, the daughter cells are genetically different from the parent cell.

3. In mitosis, cell division takes place only once, while in meiosis, cell division takes place twice.

What is Genetics?

- a discipline of Biology- the science of heredity and variations in living organisms

A monk with a scientific streak, Mendel made botanical discoveries which became the basis of modern genetics. His careful cross-breeding of thousands of pea plants led Mendel to key insights, now called Mendel's Laws of Heredity, about how inherited traits are passed on from generation to generation.

Born: 22 July 1822 Birthplace: Czech RepublicDied: 6 January 1884 Best Known As: The founding father of modern genetics

Between 1856 and 1863 he catalogued successive generations of pea plants with statistical precision, looking for clues to how traits like color and shape were reproduced. Among his findings were the law of segregation (which includes the notion of dominant and recessive genes) and the law of independent assortment (which says that an organism's individual traits are passed on independently of one another). Mendel published Experiments in Plant Hybridization in 1865, but his theories were not widely embraced until the 1900s.

Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics.  For instance, the pea flowers are either purple or white--intermediate colors do not appear in the offspring of cross-pollinated pea plants.  Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms:

1. flower color is purple or white 2. flower position is axil or terminal        3. stem length is long or short 4. seed shape is round or wrinkled  5. seed color is yellow or green 6. pod shape is inflated or constricted 7. pod color is yellow or green

Mendel picked common garden pea plants for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated.  Pea plants have both male and female reproductive organs.  As a result, they can either self-pollinate themselves or cross-pollinate with another plant.  In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations.  This was the basis for his conclusions about the nature of genetic inheritance.

Parent generation

F 1

F 2

F 3

3:1 ratio

1.  that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged   (these units are now called genes )

2. that an individual inherits one such unit from each parent for each trait

3. that a trait may not show up in an individual but can still be passed on to the next generation.

Mendel came to three important conclusions based from experimental results:

1.  Principle of segregation - for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring.  Which allele in a parent's pair of alleles is inherited is a matter of chance. 

2. Principle of independent assortment- different pairs of alleles are passed to offspring independently of each other.  The result is that new combinations of genes present in neither parent are possible. Genes for independently assorted traits are located on different chromosomes

Principles /Laws of Heredity

1.Trait – characteristic passed from parent to offspring2. Purebred – an organism receiving the same genetic

traits from both parents 3. Hybrid – an organism receiving different forms of

genetic traits from each parent4. Dominant trait – prominent trait in the F 1generation5. Recessive trait – trait that appeared least in a

generation6. Gene – part of the chromosomes that codes for a trait,

controls the inherited trait7. Chromosomes - thread-like structures located inside

the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

Terminologies:

8. Allele - an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome

9. Genotype – genetic make-up of an organism10. Phenotype – outward expression of the trait of

an organism11. Homozygous – organism with 2 identical

alleles for a trait12. Heterozygous – organism with 2 different

genes for a trait13. Punnett square – grid for organizing genetic

information14. Probability – use of ratios or fractions to

predict the likelihood of an event to occur

15. Testcross – a cross to determine whether an individual is heterozygous or homozygous for a certain trait; if unknown genotype is heterozygous, about half of the offspring should show the recessive phenotype while if the unknown is homozygous dominant, all the offspring will show the dominant phenotype

One problem that must be solved is how to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes.Plants could self-fertilize. Homozygous plants are pure breeding but heterozygous plants will give 75% dominant and 25% recessive phenotypes in their offspring.

Animals are not usually hermaphroditic so they cannot self-fertilize.Animals require a test cross to be carried out (though these days a gene probe is more likely to be used).

ExampleCharacter: Coat colour in hamsters

Allele Genotype Phenotype

Grey G GG Grey

White g Gg Grey

gg White

ExampleCharacter: Coat colour in hamster

Grey mice hamster have one of two different genotypes, GG or Gg.If they are crossed with a white mouse (gg) these genotypes will give two different results.

A test cross is also used to determine the proportions of gametes carrying different alleles.

Homozygous

  WhiteGrey   v

Phenotypes

GG ggGenotypes

Gametes G G gg

Genotypes

Grey White100% 0%

Phenotypes

Proportions

Heterozygous

G

Grey     v     White

Gg gg

g g g

Grey White

50% 50%

16. Incomplete dominance – occurs when a heterozygous organism shows a phenotype that is intermediate between two homozygous parents ( RED X WHITE=PINK)

17. Codominance – occurs when both alleles in the heterozygous organism express themselves fully (human A and B blood groups are codominant of each other)

18. Multiple allele – presence of three or more alleles for a certain trait in a certain population: blood types: AA,BB,OO

19. Polygenic trait – a trait controlled by 2 or more gene pairs; e.g. Human eye color

20. Pleiotropy – ability of a single gene to have multiple effects ( gene that controls fur pigmentation in Siamese cats also influences the connection between a cat’s pair of eyes and its brain; tigers with abnormal pigmentation also tend to be crossed-eyed)

21. Phenotype – the product of the complex interaction between an organism’s genetic makeup and its environment. An individual is locked into its inherited genotype.

22. Epistasis – mode of inheritance whereby one gene interferes with the expression of another gene that is independently inherited

P – purple flowers in sweet peas PP or Pp- white flower if pea plant is

homozygous for a recessive allele c of another gene PpCc – purple flower Ppcc – white flower23. Family pedigree – family tree showing the

interrelationships of parents and children across generations

24. Albinism – a recessive inherited disorder in humans expressed due to lack of skin pigmention

25. Dominant inherited disorders: Achondroplasis ( a form of dwarfism ) - incidence of 1:10.000 occurrence Alzheimer’s disease – degeneration of brain parts usually due to aging Manic depression – psychiatric disorder characterized by extreme mood

swings as a result of the presence of an abnormal dominant allele

26. Genetic screening: Fetal testing ( amniocentesis) technique to determine whether developing fetus has a genetic disorder ( 14th to 16th weeks of pregnancy) Karyotyping - technique for identifying chromosomal defects using cultured cells from fetal cells that had been sloughed off into the amniotic fluid Chorionic villi sampling – technique to detect genetic -disorder during the 8th- 10th weeks of pregnancy

Ultrasound – technique for detection of genetic abnormalities using sound waves producing an image of the fetus (non-invasive procedure)

27. Linked genes – genes that are on the same chromosomes

XY (heterogametic sex genes) – male XX (homogametic sex genes) – female note: The sex of an individual is determined by

the kind of sperm cell, X-bearing or Y-bearing that fertilizes the egg

Monoecious organism – organism having both sperms and eggs (e.g. earthworms)

Dioecious – organisms with separate sexes

Types of mutations: Germ mutation - occur in gametes. Somatic mutation - occur in body cells. Chromosome mutations

Mutagen: anything that causes a mutation.

Some well known environmental examples:Ultraviolet radiation Tars from tobaccoAsbestosTeratogen

Some examples: •Arsenic•Benzene•Caffeine•Cannabis•Ethyl alcohol•Tobacco

anything that can cause malformations of an embryo or fetus.

Note: Fathers pass X-linked alleles to all their daughters but to none of their sons Mothers can pass sex-linked alleles to both sons and daughters 1. Color- blindness (inability to distinguish colors) A color-blind daughter may be born to a color- blind father and carrier mother 2. Hemophilia (excessive bleeding when injured) detected among the royal families of England and other European countries

Sex-linked traits

1. Down syndrome – presence of extra chromosome #21( 47 instead of the normal 46) – trisomy

1:800 probability of occurrence2. Patau syndrome (harelip and cleft palate) presence of extra chromosome #133. Edward’s syndrome – organ system

malfunctioning; presence of extra chromosome 184. Klinefelter’s syndrome (XXY) – w/ male sex organs

but testes are abnormally small, sterile, enlarged breast w/ feminine body contours (1:1000)

5. Turner’s syndrome (45XO) – female, sterile, immature sex organs (1:2500)

Chromosomal mutations (either by deletion or duplication)

Down SyndromeBy:John Langdon Down (presence of an extra chromosome 21 )

RELATIONSHIP OF DOWN SYNDROME INCIDENCE TO MOTHERS' AGE

Mothers AgeIncidence of Down Syndrome

Under 30 Less than 1 in 1,000

30 1 in 900

35 1 in 400

36 1 in 300

37 1 in 230

38 1 in 180

39 1 in 135

40 1 in 105

42 1 in 60

44 1 in 35

46 1 in 20

48 1 in 16

49 1 in 12

Source: Hook, E.G., Lindsjo, A. Down Syndrome in Live Births by Single Year Maternal Age.

Symptoms1. Extra fingers or toes (polydactyly) 2. Deformed feet, known as rocker-bottom feet 3. Neurological problems such as small head

(microcephaly), failure of the brain to divide into halves during gestation (holoprosencephaly), severe mental deficiency

4. Facial defects such as small eyes (microphthalmia), absent or malformed nose, cleft lip and/or cleft palate

5. Heart defects (80% of individuals) 6. Kidney defects - associated with increased age of the mother. It may

affect individuals of all ethnic backgrounds - affect females more than males

Patau Syndrome (Trisomy 13)cause: Extra copy of chromosome 13 (1:10,000)proven by: Dr. Klaus Patau in 1960

Features and characteristics 1. kidney malformations 2. structural heart defects at birth 3. small head (microcephaly ) 4. prominent back portion of the head (occiput ) 5. low-set, malformed ears 6. abnormally small jaw (micrognathia ) 7. cleft lip/cleft palate 8. narrow eyelid folds, drooping upper upper eyelids 9. underdeveloped thumbs and or nails10. undescended testicles

Edwards syndromediscovered by: J ohn Hilton Edwards ( 1960)cause: presence of an extra copy of genetic material on the 18th chromosome

Signs and symptoms1. low testosterone

level 2. infertile males3. small testicles4. lanky5. increased breast

tissues6. no signs of

affectedness

Klinefelter's syndrome(discovered by: Dr. Harry Klinefelter (1942 )cause: extra X chromosome ( 47XXY)

Girl with Turnersyndrome before andimmediately after heroperation for neck-webbing

Turner’s syndrome

Abnormalities: 1. non-working ovaries: amenorrhea, sterile 2. short stature 3. swelling of extremities 4. broad chest 5. low hairline 6. webbed neck

Complications 1. congenital heart disease 2. hypothyroidism 3. diabetes

Discovered: by Harry Hubert Turner (1938) Bonnevie-Ullrich

Turner syndrome group

Ullrich case

cause: Chromosome dysfunction