Biology: Ch 12 Meiosis

What is the purpose of meiosis?

sexual reproduction

What is the process called when a sperm and an egg unite to form a new individual?

fertilization

Where are sperm cells made?

testicles

Where are egg cells made?

ovaries

_____________ is the nuclear division that precedes the formation of gametes (egg & sperm) and results in a halving of chromosome number.

meiosis

How many distinct chromosomes does a human have?

23

How many total chromosomes does a human have at the end of fertilization?

46

Another term for reproductive organs?

Gonads

Each daughter cell (sperm or egg) are genetically ____________ from the parent cell.

different; not identical

What happens in meiosis?

process of mitosis happening twice; duplicate genes separate into daughter cells, those daughter cells separate again

What does haploid mean?

“one” or n

What is a karyotype?

the number and types of chromosomes present in an organism; “chromosome spread”

What do sex chromosomes determine?

sex of the individual

Chromosomes 1-22 are _____________.

autosomes

Chromosomes of the same type (or code for the same gene) are called _______________ ______________ or _______________.

homologous chromosomes or homologs

What do homologs carry?

the same genes in the same locations; may contain different versions or alleles

What is an allele?

different versions of a specific gene

What does the haploid number (n) indicate?

number of distinct types of chromosomes present

What does the ploidy (coefficient to n) indicate?

number of each type of chromosome present

What is the haploid number?

n = 12

What is the ploidy number?

= 2

what is the human haploid (n) number?

n = 23

What is the ploidy number?

2

Organisms whose cells contain just one of each type of chromosome are called __________ (n)

haploid

Those whose cells contain two versions of each type of chromosome are termed _________ (2n)

diploid

What do diploid cells have?

one paternal chromosome and one maternal chromosome

Organisms with three or more versions of each type of chromosome are called ___________ (3n, 4n, etc.)

polyploid

What is the product of a diploid organism undergoing meiosis?

haploids

Meiosis reduces chromosome number by _________

half

What happens before a diploid undergoes meiosis?

each chromosome in the diploid parent cell is replicated; when replication is complete, each chromosome consists of two identical sister chromatids attached at the centromere

What is meiosis I associated with?

separation of homologous chromosomes

what is meiosis II associated with?

separation of sister chromatids

Meiosis II is similar to what other process?

mitosis

What occurs in early prophase I?

Homolog pairs come together in a pairing process called synopsis

What is the term for the structure that results from synapsis?

tetrad: consisting of two homologs (4 sister chromatids)

The chromatids of the homologs are called _____________

non-sister chromatids

What occurs in late prophase I?

crossing over between homologous non-sister chromatids occurs where chiasmata are formed

What occurs in metaphase I?

the tetrads line up at the metaphase plate

what occurs in anaphase I?

paired homologs separate and begin to migrate to opposite ends of the cell

what occurs in telophase I?

homologs finish migrating to the poles of the cell and then the cell divides in the process of cytokinesis

What is the end result of meiosis I?

• one chromosome of each homologous pair is distributed to a different daughter cell

• a reduction division: daughter cells of meiosis I are haploid and are still in the form of sister chromatids (duplicated)

How many distinct phases does meiosis I have?

5

How many distinct phases does meiosis II have?

4

What occurs in prophase II?

spindle apparatus forms and one spindle fiber attaches to the centromere of each sister chromatid

what occurs in metaphase II?

replicated chromosomes line up at the metaphase plate

what occurs in anaphase II?

sister chromatids separate; resulting in daughter chromosomes begin moving to opposite sides of the cell

what occurs in telophase II?

• chromosomes arrive at the opposite sides of the cell

• nuclear envelope forms around each haploid set of chromosomes

• each cell undergoes cytokinesis

what is the result of meiosis II?

• four haploid cells, each with one of each type of chromosome

  • one diploid cell with replicated chromosomes gives rise to four haploid cells with unreplicated chromosomes

What is considered the genetically distinct from the parental diploid cell?

the 4 unreplicated haploid cells

what causes diversity?

• crossing over

• mutations

• fertilization randomness

What is nondisjunction?

• uneven separation that typically occurs in anaphase I

  • both homologs or sister chromatids move to the same pole of the parent cell

What is considered an aneuploid zygote?

those with too few or many chromosomes

What condition is caused by a defect on chromosome 21 or the sex chromosomes?

down syndrome

What can sex chromosome aneuploidy cause?

• Klinefelter syndrome: XXY males

• Trisomy X

  • (karyotype XXX)

• Turner syndrome: monosomy

  • karyotype is XO (lacking a secondary X)

What gamete determines the gender of a zygote?

sperm; carries either a “x” or “y”

What happens if nondisjunction occurs in meiosis I?

• two gametes will have an extra copy of a chromosome (causing a condition called trisomy)

• two gametes will lack that chromosome (monosomy)