meiosis – the basis of sexual reproduction section 6.1
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Meiosis – the basis of sexual reproductionSection 6.1
6.1 Part 1 Asexual reproduction – involves
cell division (mitosis)
Sexual reproduction – involves sex cells joining to form a zygote
Zygote – first cell created when two sex cells join (egg and sperm)
Organisms that reproduce sexually have two types of cells: Somatic cells
◦Body cells (muscles, skin, bone etc.)◦Reproduce by cell division (mitosis) (grow
and replace dead cells)◦ In humans 23 pairs of chromosomes (46
total)Gametes
◦Sex cells◦½ the chromosomes of the parent cell (23
total in humans)◦Sperm: male gametes Ova: Female gametes
Gametes: name for sex cells
Sex is basically the joining of genetic material from two individuals.
In humans – we receive one set of 23 chromosomes from our mother and one set of 23 chromosomes from our father.
In our somatic cells we have 23 pairs of chromosomes – how are the chromosome numbers kept down if there is genetic information from both parents? How do the egg and sperm only have one set?
MeiosisChromosomes need to be reduced
so offspring have the correct number of chromosomes.
Meiois – reduces the number of chromosomes in sex cells◦produces gametes◦½ the chromosomes of the parent cell◦only occurs in cells that produce
gametes (germ cells)◦“reductive” division
Cells with half the chromosomes are called haploid (n) – (gametes)
Cells with the full amount of chromosomes are called diploid (2n)
In our body cells we have pairs of chromosomes called homologous pairs. Half of the chromosomes come from the mother, half from the father
In our body cells 22 chromosomes have a matching chromosome from the other set. The two chromosomes that form the 23rd pair are called the sex chromosomes.
For females – the sex chromosomes appear identical and have an X shape. This pair is referred to as XX.
In Males – the chromosomes are not homologous and do not have the same shape. One appears to have an X shape, and the other smaller chromosome is called the Y chromosome. This pair is called XY.
Remember mitosis produces two identical daughter cells from one parent cell – they have exactly the same genetic information as the parent cells.
Meiosis is a different form of cell division – it is the basis for sexual reproduction.
Meiosis ensures variation within a species.
Meiosis produces gametes.
Do Questions pg 203 #2, 3, 4, 17
Start Vocabulary Assignment
6.1 Part 2 MeiosisReminders of terms:Gamete – sex cellsDiploid – cells with a full set of
chromosomes (2 sets of 23 in humans)
Haploid – cells with a half set of chromosomes (1 set of 23 in humans)
Male sex cell = spermFemale sex cell = egg (ova)
Phases of Meiosis:
Interphase: Before Meiosis begins◦each homologous pair of
chromosomes is replicated
Meiosis I ◦Homologous chromosomes separate
into two cellsMeiosis II
◦Meiosis continues with a second division
◦Chromosomes do not replicate◦Sister chromatids separate◦Outcome is four cells with haploid
set of chromosomes in each cell
Ensuring variationAn important step in meiosis 1 is
called Crossing Over.Non –sister chromatids cross over
and exchange DNA segments – each chromosome now has new genetic information.
Multiple crossovers can occur between chromosomes
Crossing Over
Independent AssortmentIn Meiosis pairs of homologous
chromosomes are divided in half – to form haploid cells
This separation is random.Not all maternal chromosomes end
up in one cell, and not all paternal chromosomes end up in another. There is a mixture of genetic material in cells from both the parents.
Meiosis IProphase 1
◦Chromosomes condense◦spindle fibres forms◦centrioles move to opposite poles of
cell◦nuclear envelope disappears◦homologous chromosomes pair up◦Crossing over occurs
Meiosis 1 continuesMetaphase 1
◦Paired homologous chromosomes move to the equator of cell
◦spindle fibers are connected to centromeres of one of each homologous pair
Anaphase 1 ◦spindle fibers move homologous
chromosomes away from each other to opposite poles
◦sister chromatids are attached◦each side of cell only has half the
chromosomes
Meiosis 1 continuesTelophase 1
◦spindle fibers disappear◦nuclear envelope appears briefly◦equator of cell pinches in to create
new cells
Cytokinesis occurs – two cells formed◦Chromosomes are still duplicated
Meiosis 2 beginsProphase 2
◦similar to mitosis◦nuclear envelope disappears◦spindle fibers appear from centrioles at
opposite poles◦chromosomes condense
Metaphase 2◦non homologous chromosomes move to
equator forming a line◦spindle fibers are connected to each sister
chromatid
Meiosis 2 continuesAnaphase 2
◦spindle fibers pull◦sister chromatids separate and move to
opposite ends of cell
Telophase 2◦chromatids (now chromosomes) at each pole
lengthen and decondense◦Nuclear membrane reappears around
chromatin◦Four new cells are created with different
genetic components
Meiosis Video
http://www.youtube.com/watch?v=rqPMp0U0HOA
6.1 Part 3 Comparing Mitosis and Meiosis, Gamete formation and Mutations
The following diagram shows how meiosis and mitosis fit in to the life cycle of a sexually reproducing organism
Mitosis and Meiosis
Comparing Meiosis and Mitosis
Meiosis Mitosis Keeps the number of chromosomes from doubling each generation by producing haploid sperm or egg cells
Is responsible for growth, tissue repair and some forms of asexual reproduction
Ensure variation in a species because chromosomes from each parent are combined
Ensures all cells produced are identical
Has two cell divisions Has one cell division
Makes the diploid sex cell become haploid
Makes two exact copies (daughter cells) of a parent cell
Gamete formation
Four sperm are produced from one “germ” cell in males, whereas only one egg cell is produced in females for every female germ cell.
Germ cells are produced in the gonads of organisms – testicles in males and ovary in females.
Chromosome mutationsMutations to chromosomes can
occur in meiosis.◦pieces of chromosomes may be lost◦chromosomes may be duplicated◦gene sequences may be moved
within chromosomes or to other chromosomes
Mutations may be caused by problems in meiosis or can be caused by mutagens, such as radiation or chemicals
In Meiosis 1 , whole chromosomes may not separate
In Meiosis 2, sister chromatids may not separate
One gamete may have two copies of a chromosome and the other none.
KaryotypesKaryotype: is the number and visual
appearance of chromosomes in the cell nuclei of a species
Karyotypes can be analyzed to determine genetic abnormalities, or to compare relatedness.
Karyotypes are prepared by staining cells in stages of cell division and basically a picture is taken of chromosomes. They are then compiled into one image.
Normal Male Karyotype
Note the small Y chromosme
Karyotypes of Males with genetic disorders
Downs Syndrome – extra copy of chromosome 21
Other genetic disorder chromsome mutation locations
Answer Questions pg 203: 6, 7, 8, 10, 16, 18
Meiosis video
Ameba sisters – see handout
http://www.youtube.com/watch?v=toWK0fIyFlY