Cell Divison: Meiosis

Chromosomes are __

• Passed on from parents to offspring

• Contain __

• Located in the nucleus

made of DNA and protein

genes that code for traits

Homologous chromosomes are __

• One set of chromosomes comes from each parent

• Similar but not identical

• Carries the same genes in the same order

• Alleles for each trait may not be the same

two corresponding chromosomes that carry the same genes.

Meiosis is the process by where a single cell divides twice to produce four daughter cells that contain half the amount of chromosomes of the parent cell.

• __

• Male gametes are sperm

• Female gametes are eggs

Meiosis creates four (4) sex cells called gametes

Non-reproductive body cells are called somatic cells.

• __

• Represented by __

• One set comes from each parent

• Humans diploid cells contain __ chromosomes

• Somatic cells are diploid because they contain 2 sets of chromosomes

• 2N

• 46

Gametes are __

• Haploid cells are represented by the letter __

• Human’s gametes each contain __ chromosomes

• haploid, meaning that they contain half the number of chromosomes of a body cell.

• N

• 23

• Two haploid gametes fuse together during fertilization to create a diploid cell with two complete sets of chromosomes

• When an egg cell is fertilized, it becomes diploid and is called a __

• zygote

• Meiosis is a __

• In humans, the starting cell has 46 chromosomes, and the ending cell has 23 chromosomes.

• Chromosomes are reduced by division

• reduction division process

Fill in the blanks

• 50

• 24

• 8

• 80

Meiosis creates__ with half the number of chromosomes as the parent cell.

• Sexual reproduction

• Makes sex cells (gametes

• Makes haploid (N) cells

• __ to make 4 haploid cells from 1 diploid cell

• Meiosis I and Meiosis II

• genetically unique gametes

• Divides twice

Like in mitosis, interphase must occur before meiosis takes place.

During the S phase of interphase __

• Each duplicated chromosome consists of two identical sister chromatids attached at the centromere

• Centrioles also replicate

• DNA is replicated

Meiosis takes place in two stages of division: Meiosis 1 and Meiosis 2

• PMAT occurs twice

__

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

__

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II

• Meiosis I

• Meiosis II

In prophase I, each __

• Forms a tetrad, containing 4 chromatids

• Spindle fibers form

• Longest stage of meiosis (90%)

• nuclear envelope starts to dissolve

• Homologous chromosomes complete crossing-over (reason for extended duration)

• Crossing over is __

• replicated chromosome pairs up with its homologous chromosome

• the physical exchange of genetic information

Synapsis holds the chromosomes together at a location called a chiasma.

Crossing over produces new combinations of genes

• __

• accounts for genetic diversity

What is the name of the first, second, and third and fourth image (stage/phase)?

• Homologous pair

• Synapsis

• After Crossing-over

During metaphase I, paired homologous chromosomes __ (middle) of the cell.

• Spindle fibers extend from the centriole and attach to the centromere

• Hint* M for Metaphase = M for Middle

• line up across the equator

During anaphase I, spindle fibers pull each homologous chromosome pair towards the opposite ends of the cell.

• Homologous pairs are __

• Hint* A for Anaphase + A for moving Away

• moved away from one another

During Telophase I, the __ around each cluster of chromosomes.

• nuclear membrane reforms

Cytokinesis __

• Splits the elongated cell into two distinct cells

• Each new cell is called a daughter cell

• Chromosomes unravel and decondense

• separates the cytoplasm

Each new cell is genetically different from the other and the parent cell

• Begins with 1 diploid parent cell and __

• ends with 2 haploid daughter cells

Prophase II

As cells enter prophase II, their chromosomes consist of __

• Chromosomes become visible

• Do not pair to become tetrads

• No longer homologous pairs

• Nuclear membrane breaks down

• No crossing-over takes place

• Meiosis II begins with 2 haploid cells

• two chromatids

Metaphase II is very similar to metaphase I

• Chromosomes line up in the __ of the cells

• center

Anaphase II is very similar to anaphase I

• Paired chromatids are separated by the spindle fibers

• Chromatids move to the __ of the cells

• poles

The nuclear membrane reforms and cytokinesis occurs

• The end result of meiosis II is __. Cleavage furrow, where cytoplasm pulls off in the center.

• four genetically different haploid cells called gametes

Haploid cells produced by meiosis II are the gametes important to heredity.

• After an egg is fertilized, the __ to eventually form a new organism.

• diploid zygote cell will undergo by mitosis

asexual

In this type of reproduction, the parent passes ALL of its DNA down to the offspring.

homologous chromosomes

Name given to a pair of structurally similar chromosomes that possess genes for the same characteristics at the same loci (location).

18

If a sexually reproducing organism has a chromosome number of 2N=18, how many chromosomes will be found in the body cells of this organism?

9

If a sexually reproducing organism has a chromosome number of 2N=18, how many chromosomes will be found in the sex cells of this organism?

meiosis

What type of cell division results in cells that have half the number of chromosomes as the original cell?

sexual

In this type of reproduction, the parent passes down only half of its DNA to the offspring

haploid

Term that means that a cell has one of each kind of chromosome

prophase I

During which stage of meiosis are tetrads formed?

• mating is not necessary

• quick reproduction

• positive genes pass on to descendants

List three advantages to asexual reproduction

lack of genetic variety

What is the disadvantage of asexual reproduction

gametes

What term is used to describe sex cells (egg and sperm cells)

metaphase I

During which stage of meiosis do tetrads line up at the center of the cell

prophase II

During which stage of meiosis do sister chromatids line up at the center of the cell

homologues

During anaphase I __ are pulled apart

sister chromatids

During anaphase II __ are pulled apart

genetic variation

What is the advantage of sexual reproduction

diploid

Term that means that a cell has two of each kind of chromosome

2N or 2X

What is the symbol for diploid

N

What is the symbol for haploid

crossing over

The exchange of genes between segments of homologous chromosomes during meiosis.

fertilization

The union of sperm and egg

zygote

A fertilized egg

they are diploid

At the end of meiosis I, are the daughter cells haploid of diploid

sperm cells

What is the result of meiosis in males

egg cells

What is the result of meiosis in females

Fill in the blanks

• Body cells/gametes (sex cells)

• 2/4

• Yes-Same/Different-No

• Diploid/Haploid

• No/Yes

• No/Yes

• One/Two

• One/One

• double the mother’s/half the mother’s

What is crossing over? What is the importance of this event?

Crossing over is the exchange of genes in segments of homologous chromosomes in order to achieve genetic diversity. The point/importance of crossing over is to help increase the genetic diversity of the offspring.

Explain why egg and sperm cells must have half the number of chromosomes as the other cells in the body?

Egg and sperm cells (gametes) must have half the number of chromosomes as body cells because without the reduction of the number of chromosomes, the offspring(s) would have twice the normal number of chromosomes (92 instead of 46).

What are tetrads?

A tetrad is a pair of homologous chromosomes directly next to each other.