Reproduction in the cellsMitosis and Meiosis

Definitions• Genome is the entirety of an organism's hereditary

information. It is encoded either in DNA or, for many types of virus, in RNA.

• Diploid (indicated by 2x) cells have two homologous copies of each chromosome, usually one from the mother and one from the father.

• Haploid (n) is the number of chromosomes in a gamete of an individual, and this is distinct from the monoploid number (x) which is the number of unique chromosomes in a single complete set.

• Gametes is a cell that fuses with another gamete during fertilization (conception)—called an ovum (or egg)—and a male produces the smaller tadpole-like type—called a sperm.

Mitosis and Meiosis•Mitosis and meiosis are an integral part

of cell division. •Meiosis The resultant number of cells is

four times the number of original cells. This results in cells with half the number of chromosomes present in the parent cell.

•A diploid cell duplicates itself, then undergoes two divisions (tetraploid to diploid to haploid), in the process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II.

Mitosis vs Meiosis

•Mitosis The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the daughter cells is the same as that of the parent cell.

•Mitosis occurs in somatic cells, while meiosis occurs in gametes.

Prokaryotes Have a Simple Cell Cycle• Cell division in prokaryotes takes place

in two stages, which together make up a simple cell cycle

1. copy the DNA this process is called replication

2. split the cell in two to form daughter cells this process is called binary fission

Prokaryotes Have a Simple Cell Cycle•The hereditary information in a prokaryote

is stored in DNA▫the prokaryotic chromosome is a single circle of

DNA▫DNA replication begins with the unzipping of the

double-stranded DNA at a point called the origin of replication

▫a new double helix is formed by adding complementary nucleotides to the exposed DNA strands that have been unzipped

▫the end result of replication is that the cell possess two complete copies of the hereditary information

Eukaryotes Have a Complex Cell Cycle•Eukaryotic cells undergo two different

mechanisms to divide up the DNA▫mitosis is a cell division mechanism that

occurs in nonreproductive cells these cells are called somatic cells

▫meiosis is a cell division mechanism that occurs in cells that participate in sexual reproduction these cells are called germ cells

Eukaryotes Have a Complex Cell Cycle•Mitosis (M phase)

▫a microtubular apparatus binds to the chromosomes and moves them apart

•Cytokinesis (C phase)▫the cytoplasm divides, creating two

daughter cells

How the cell cycle works

Mitosis and Meiosis

•Mitosis:-division of somatic (body) cells

•Meiosis-division of gametes (sex cells)

Chromosomes•Chromosome number varies among

organisms▫most eukaryotes have between 10 and 50

chromosomes in their somatic cells

•Chromosomes are paired in somatic cells▫these pairs are called homologous

chromosomes, or homologues▫homologues contain information about the same

traits but the information may vary▫cells that have two of each type of chromosome

are called diploid cells one chromosome of each pair is inherited from the

mother and the other is inherited from the father

Chromosomes•Prior to cell division, each of the

homologous chromosomes replicates, forming two identical copies called sister chromatids▫the sister chromatids are joined together

by a structure called a centromere▫humans have 23 pairs of homologous

chromosomes when each chromosome in the pair is

replicated, this makes for a total of 92 chromatids

The difference between homologous chromosomes and sister chromatids

Chromosomes

• A karyotype is an arrangement of chromosomes

• Chromosomes can be compared based on size, shape, and centromere location

• The karyotype at right shows the 23 pairs of human chromosomes

The chromosomes of a human

Chromosomes

•Chromosomes are comprised of chromatin, a complex of DNA and protein▫there is also some RNA associated with

chromosomes▫the DNA in a chromosome is one very long

double-stranded fiber that extends unbroken for the length of the chromosome

▫the DNA is coiled in order to allow it to fit into a small space despite being very long

Chromosomes•DNA is coiled around proteins called

histones▫ the histones have positive charges to counteract

the negative charges associated with the phosphate groups of the DNA

•The DNA coils around a core of eight histone proteins to form a complex called a nucleosome▫ the nucleosomes in turn can be coiled together

further to ultimately form a compact chromosome

WHY MEIOSIS?•MITOSIS – RESULTS IN GENETICALLY

IDENTICAL OFFSPRING – INCLUDING THE # CHROMOSOMES

•WHAT WOULD HAPPEN IF THE EGG AND SPERM HAD THE SAME # OF CHROMOSOMES AS THE BODY CELLS?

EGG = 46 CHROMOSOMES SPERM = 46 CHROM.EGG = 46 CHROMOSOMES SPERM = 46 CHROM.

ZYGOTE = 46 + 46 = 92 CHROMOSOMES = ZYGOTE = 46 + 46 = 92 CHROMOSOMES =

NOT HUMANNOT HUMAN

MEIOSIS•A TYPE OF CELL DIVISION WHICH

PRODUCES GAMETES CONTAING HALF THE NUMBER OF CHROMOSOMES AS THE BODY CELLS

•2 STAGES – MEIOSIS I & MEIOSIS II•START W/ 1 DIPLOID CELL, END UP W/ 4

HAPLOID CELLS (GAMETES)•4 DAUGHTER CELLS ARE GENETICALLY

DIFFERENT FROM EACH OTHER AND MOTHER CELL

MEIOSIS

•SPERM – MALE GAMETE (n)•EGG – FEMALE GAMETE (n)•FERTILIZATION PRODUCES A ZYGOTE

(2n)•THIS TYPE OF REPRODUCTION IS

CALLED SEXUAL REPRODUCTION

STAGES OF MEIOSIS

•MEIOSIS I▫PROPHASE I, METAPHASE I, ANAPHASE

I, TELOPHASE I (PMAT)•MEIOSIS II

▫PROPHASE II, METAPHASE II, ANAPHASE II, TELOPHASE II (PMAT)

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Interphase

• Interesting things happen!1. Cell preparing to divide2. Genetic material doubles

Prophase

•Chromosome pair up!•Chromosomes thicken and shorten• -become visible• -2 chromatids joined by a

centromere•Centrioles move to the opposite sides of

the nucleus•Nucleolus disappears•Nuclear membrane disintegrate

Metaphase

•Chromosomes meet in the middle!•Chromosomes arrange at equator of cell•Become attached to spindle fibres by

centromeres•Homologous chromosomes do not

associate

Anaphase

•Chromosomes get pulled apart•Spindle fibres contract pulling chromatids

to the opposite poles of the cell

Telophase

•Now there are two!•Chromosomes uncoil•Spindle fibres disintegrate•Centrioles replicate•Nucleur membrane forms•Cell divides

Before Meiosis begins…

•As with mitosis, before meiosis begins, the DNA in the original cell is replicated during S-phase of the cell cycle.

•Two cell divisions separate the replicated chromosomes into four haploid gametes or spores.

Meiosis

•During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells.

Meiosis•Because the chromosomes

of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.

Meiosis

•4 daughter cells produced•Each daughter cell has half the

chromosomes of the parent•2 sets of cell division involved

Problems with Meiosis• The normal separation of chromosomes in

meiosis I or sister chromatids in meiosis II is termed disjunction.

• When the separation is not normal, it is called nondisjunction.

• This results in the production of gametes which have either too many or too few of a particular chromosome, and is a cause genetic problems.

• Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.

Problems with Meiosis• Down Syndrome - trisomy of chromosome 21

(abnormalities)• Patau Syndrome - trisomy of chromosome 13

(abnormalities)• Edward Syndrome - trisomy of chromosome 18

(95% die in utero) less than 1% live to the age of 10. On average they only live 5-10 days.

• Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY, XXXXY (infertile)

• Turner Syndrome - lacking of one X chromosome in females - ie XO (Occurring in 1 out of every 2500 girls)

• Triple X syndrome - an extra X chromosome in females (once in every 1,000 births) can be mistaken for behavior problems

• XYY Syndrome - an extra Y chromosome in males (once in every 1,000 births)

Stages of Mitosis

•Interphase•Preprophase •Prophase •Prometaphase •Metaphase •Anaphase •Telophase •Cytokinesis

Interphase• The mitotic phase is a relatively

short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for cell division.

• Interphase is therefore not part of mitosis. Interphase is divided into three phases, G1 (first gap), S (synthesis), and G2 (second gap).

• During all three phases, the cell grows by producing proteins and cytoplasmic organelles.

Phases of Meiosis

•Meiosis I•Meiosis I separates homologous

chromosomes, producing two haploid cells (23 chromosomes, N in humans), so meiosis I is referred to as a reductional division.

Phases of Meiosis

•Prophase I•During prophase I, DNA is exchanged

between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover.

Phases of Meiosis•Metaphase I•Homologous pairs move together along the

metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes

Phases of Meiosis

•Anaphase I•Kinetochore microtubules shorten,

severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome has only one functional unit of a pair of kinetochores[3], whole chromosomes are pulled toward opposing poles, forming two haploid sets.

Phases of Meiosis

•Telophase I•The last meiotic division effectively ends

when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids.

Phases of Meiosis•Meiosis II•Meiosis II is the second part of the meiotic

process. Much of the process is similar to mitosis. The end result is production of four haploid cells (23 chromosomes, 1N in humans) from the two haploid cells (23 chromosomes, 1N * each of the chromosomes consisting of two sister chromatids) produced in meiosis I. The four main steps of Meiosis II are: Prophase II, Metaphase II, Anaphase II, and Telophase II.

Phases of Meiosis

•Prophase II takes an inversely proportional time compared to prophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.

•In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.

•This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.

•The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle.

•Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.

•Meiosis is now complete and ends up with four new daughter cells.

Stages of Meiosis

Significance of Meiosis• Meiosis facilitates stable sexual reproduction. • Without the halving of ploidy, or chromosome

count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation.

• Successive generations would have an exponential increase in chromosome count. In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities

Significance of Meiosis

•Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring.

Cancer

•Cancer is a growth disorder of cells•It starts when an apparently normal cell

begins to grow in an uncontrolled way. •The result is a cluster of cells, called

tumor. •Benign tumors are completely enclosed by

normal tissue and are said to be encapsulated.

Cancer

•Tumors that grow in size eventully begins to shed cells that enter the bloodstream through the lymph system.

•This can form new tumors at distant sites call metastases.

US Caner Stats

•One in every two Americans born in 2006 will be diagnosed with some form of cancer during their lifetime

•The US top 3 deadliest human cancers▫Lung▫Colon/Rectum▫Breast

How is cancer caused?

•Cancer can be caused by▫Cigarette smoke▫Environmental factors▫UV rays (damage DNA)▫Viruses (circumvent the cells normal

growth)

How is cancer caused?

•Cancer results from damaged genes failing to control cell division

•Damage to DNA, such as damage to genes, is called mutation

•One gene seems to be a KEY regulator of the cell cycle called the p53.

2 classes of growth factors genes•Proto-oncogenes: encode proteins that

stimulate cell division. Mutation to these genes can cause cells to divide excessively. ▫Mutated proto-oncogenes become cancer-

causing genes call oncogenes•Tumor-suppressor genes: cell division is

turned off by protiens encoded by tumor-suppressor genes. Mutations to these genes essentially “release the breaks” allowing the cell containing the mutated gene to divide uncontrolled.

•It plays a key role in the G1 checkpoint of the cell division.

P53 gene•Monitors the integrity of DNA, checking that

it has been successfully replicated and is undamaged.

• If the p53 protein detects damaged DNA, it halts cell division and stimulates the activity of special enzymes to repair the damages.

• In cases where the DNA cannot be repaired, the p53 then directs the cell to kill itself, activating an apoptosis (cell suicide).

P53 gene•Abnormal p53 does not stop cell division and

the damaged strands is replicated, which results in multiplied damaged cells---lead to cancer

•In 50% of cancers, the p53 cancer defense malfunctions because of damage by chemicals or radiation so that the protein code no longer functions properly.

• In the other 50% defects like in other genes because of damage that inhibit the p53 gene

Curing cancer worksheet