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Process of Meiosis
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What is Meiosis? • Form of nuclear division
• Creates FOUR haploid cells from ONE diploid cell
– Diploid=2N=46 chromosomes (somatic cell)
– Haploid=1N=23 chromosomes (sex cells)
• Two rounds of cell division=Meiosis I & Meiosis II
– Each round=four phases, similar to phases in Mitosis
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What is Meiosis? • reduces chromosome number
• creates genetic diversity
• Takes place only at certain times in organism’s life cycle
• Involved in sexual reproduction
– Makes sperm and egg cells
• Because human cells have 23 chromosomes, meiosis may result in 223, or 8,388,608, possible combinations of chromosomes.
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Review!! • This is a
chromosome…
• Each chromosome is a long, continuous thread of DNA
• DNA stores genetic information in all organisms – DNA determines your
physical appearance
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Review!! • DNA plus proteins is called
chromatin.
• One half of a duplicated chromosome is a chromatid.
• Sister chromatids are held together at the centromere.
• Telomeres protect DNA and do NOT include genes.
telomere
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Paired Up…
• Each pair of chromosomes is known as a homologous pair.
• Homologous means “having the same structure”
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Homologous Chromosomes • Two separate
chromosomes
– one from mother, one from father
• Very similar to each other
– Same length
– Carry the same genes
• Are NOT copies of each other
• Divided in Meiosis I
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Chromatids • Each half of a
duplicated chromosome
• Sister chromatids=two chromatids together
• Divided in Meiosis II
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Homologous Chromosomes vs. Sister Chromatids
homologous chromosomes
sister
chromatids
sister
chromatids
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2 Types of Chromosomes
Autosomes
&
Sex Chromosomes
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Autosomes • Pairs 1-22 are your
Autosomes
• These chromosomes contain genes for characteristics that are NOT related to the sex of an organism
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Sex Chromosomes • Sex chromosomes make
up the 23rd pair
• Sex chromosomes control the development of sexual characteristics
– Determine male or female
• Humans have two sex chromosomes: X and Y
• XX=female
• XY=male
• NOT Homologous
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X and Y Chromosomes
• X chromosome
– Larger sex chromosome
– Carries the most genes
– Determines characteristics other than sex of the organism
• Y chromosome
– Smallest sex chromosome
– Carries the fewest genes
– Determines the development of males
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Two Types of Cells
• Your body is made up of Somatic cells and Gametes.
• These are the two major groups of specialized cells in your body.
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Somatic Cells
• Also known as body cells
• Make up most of your body tissues and organs – For example: your spleen, kidneys, and eyeballs
are made entirely of body cells.
• DNA in body cells are NOT passed on to your children.
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Gametes • Sex cells
– Ova, or eggs, in the female
– Spermatozoa, or sperm cells, in the male
• DNA in your gametes CAN be passed on to your children
• Gametes are formed when germ cells (cells found in the ovaries or the testes) undergo meiosis
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Diploid Cells • Somatic, body, cells are
diploid
• Diploid means a cell has two copies of each chromosome: one copy from the mother, and one from the father.
• Diploid cells can be represented as 2n.
• In humans, the diploid chromosome number is 46.
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Haploid Cells
• Gametes are haploid cells
• Haploid means that a cell has only one copy of each chromosome
• Haploid cells are represented as n.
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Meiosis I • Meiosis I occurs after DNA has
been replicated.
• Meiosis I divides homologous
chromosomes in four phases.
–Prophase I
–Metaphase I
–Anaphase I
–Telophase I
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Meiosis I-Prophase I • Nuclear membrane breaks down
• Centrosomes and centrioles move to opposite sides if the cell
• Spindle fibers start to form
• Homologous chromosomes pair up, including sex chromosomes
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Meiosis I-Metaphase I • Spindle fibers randomly align the
homologous chromosomes along the cell equator.
• Meta=Middle – Each side has chromosomes from both parents
• This arrangement mixes up the chromosomal combinations
• Helps create and maintain genetic diversity
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Meiosis I=Anaphase I • Paired homologous chromosomes
separate from each other and move toward opposite sides of the cell.
• Sister chromatids remain attached.
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Meiosis I=Telophase I • Nuclear membrane forms again
• Spindle fibers disassemble
• Cell undergoes cytokinesis=cell completely divides
• Results in two cells that are genetically different – Each have a unique combination of 23 duplicated
chromosomes coming from both parents
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Meiosis II • DNA is not replicated between meiosis I and
meiosis II.
• Meiosis II divides sister chromatids in four phases.
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II
• Process applies to BOTH cells produced in Meiosis I
• Results in undoubled chromosomes
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Meiosis II=Prophase II • Nuclear membrane breaks down
• Centrosomes and centrioles move to opposite sides of the cell
• Spindle fibers assemble
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Meiosis II=Metaphase II • Spindle fibers align the 23
chromosomes at the cell equator.
• Meta=Middle
• Each chromosome still has two sister chromatids at this stage.
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Meiosis II=Anaphase II
•Spindle fibers pull the sister chromatids apart from one another and move them to opposite sides of the cell.
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Meiosis II=Telophase II • Nuclear membranes form again
around the chromosomes
• Spindle fibers break apart
• Cell undergoes cytokinesis
• Results in FOUR haploid cells – Each with a combination of genes from both the
mother and the father
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Next step for haploid cells…
• Haploid cells are the end result of meiosis.
• These cells must undergo changes to become mature gametes before they are ready for fertilization.
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Gametogenesis • Production of gametes
–Gametes=sex cells
• ova, or eggs in the female
• spermatazoa, or sperm cells in males
• Gametogenesis differs between females and males
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Gametogenesis in Males
– Sperm become
streamlined and
motile.
– Sperm primarily
contribute DNA to
an embryo.
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Gametogenesis in Females
– Eggs contribute DNA,
cytoplasm, and
organelles to an embryo.
– During meiosis, the egg
gets most of the
contents; the other cells
form polar bodies=cells
with little more than DNA
that are eventually
broken down.
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Meiosis vs Mitosis Meiosis
• Two cell divisions
• Homologous chromosomes pair up long the equator
• In anaphase I, sister chromatids are still together.
• Results in haploid cells that are genetically different.
Mitosis • One cell division
• Homologous chromosomes NEVER pair up
• In anaphase, sister chromatids separate.
• Results in diploid cells that are genetically identical.