Meiosis 8.10-8.23

In general, the absence of a Y chromosome yields:

"femaleness"

In general, a single Y chromosome is enough to produce:

"maleness," even in combination with several X chromosomes

how many possible combinations of chromosomes for humans?

(n=23) there are 2^23, or roughly 8 million possible chromosome combinations for humans.

The two divisions of meiosis

1. each chromosome is duplicated during interphase 2. Meiosis I: separated the two chromosomes in a homologous pair, packaging them in separate haploid daughter cells. 3. Meiosis II: separates the sister chromatids. Each of the four daughter cells is haploid and contains only one chromosome from each homologous pair

so, the three sources of genetic variability in sexually reproducing organisms:

1. independent orientation of chromosomes at metaphase I 2. random fertilization 3. crossing over during prophase I

Eggs produced by Women

1/4 cells produced by meiosis in women are eggs. 3 out of 4 are polar bodies that just disintegrate.

Each gamete contains

22 autosomes and a sex chromosome

In humans, somatic cells have how many chromosomes? how many pairs of homologous chromosomes?

46 chromosomes forming 23 pairs of homologous chromosomes.

What is the chance that a particular daughter cell will get the maternal chromosome of a certain homologous pair?

50% chance. (there is a 50% chance for paternal as well)

What kingdom does Asexual Reproduction?

ALL kingdoms have organisms that undergo asexual reproduction.

The incidence of down syndrome increases with

Age

Telophase I and Cytokinesis

Chromosomes arrive at opposite poles, and each pole has a haploid chromosome set, although sister chromatids are still together. then cytokinesis occurs, the cytoplasm splits and two haploid daughter cells are formed

A person with trisomy 21 has a condition called

Down syndrome

Why is it called down syndrome?

Dr. John Langdon Down, a doctor who described the syndrome in 1866

Independent orientation at metaphase I?

Each tetrad independently aligns at the cell equator. There is an equal probability of the maternal or paternal chromosome facing a given pole. The number of combinations for chromosomes packaged into gametes is 2n, where n = haploid number of chromosomes.

In a Karyotype, how are the cells treated?

First a blood sample is treated with a chemical that stimulates mitosis. After several days, the cells are treated with another chemical to arrest mitosis at metaphase

After meiosis II, what is produced?

Four daughter cells, each with the haploid number of (single) chromatids.

Prophase I

Homologous pairs form Tetrads in the process called synapis. During synapis, crossing over occurs which rearranges genetic information. Spindle forms, and nuclear envelope breaks. The chromosome tetrads move toward the center of the cell.

How many possibilities are there when a gamete from one individual unites with a gamete from another individual in fertilization?

In humans, the random fusion of a single sperm with a single egg during fertilization will produce a zygote with any of about 64 trillion combinations of chromosomes (8 million x 8 million)

How many cross over events occur per chromosome pair in humans?

In meiosis in humans, an average of one to three crossover events occur per chromosome pair.

Where does meiosis occur?

In the reproductive organs

How common is Trisomy 21?

It is the most common chromosome number abnormality, affecting one out of every 700 kids. It is the most serious birth defect in the US.

How do the end products of mitosis and meiosis differ?

Mitosis produces two genetically identical diploid somatic daughter cells. Meiosis produces four genetically unique haploid gametes.

What happens when nondisjunction occurs?

Mitosis will transmit the mistake to all embryonic cells. If the organism survived, it would most likely display a syndrome of disorders caused by the abnormal number of genes.

How do the X and Y differ?

Most of the genes carried on the X do not appear on Y, and the Y has chromosomes that don't appear of the X

So, what is ultimately responsible for the genetic diversity in living organisms?

Mutations

chiasma

Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over.

Are chromosome X and chromosome Y homologous?

Only small parts of X and Y are homologous.

Synapis

The process in which homologous chromosomes come together to form a tetrad. During synapis, crossing over occurs.

homologous chromosomes

The two chromosomes of a matching pair in which each chromosome carries genes controlling the same inherited characteristics. Two chromosomes of a homologous pair may have different versions of the same gene.

Human females have a homologous pair of

X chromosomes (XX)

Tetrad

a structure consisting of four chromatids aligned gene by gene.

somatic cell

a typical body cell

Humans and many animals and plants are diploid, because

all somatic cells contain pairs of homologous chromosomes.

In most cases, an abnormal number of chromosomes is so harmful to development that...

an affected embryo is spontaneously aborted (miscarried) long before birth.

Crossing over

an exchange of corresponding segments between nonsister chromatids of homologous chromosomes

Following telophase I in some organisms, is

an interphase before meiosis II

karyotype

an ordered display of magnified images of an individual's chromosomes arranged in pairs.

Sex chromosome abnormalities seem to upset the genetic balance less than...

an unusual number of autosomes. This may be because the small size of the Y chromosome or X chromosome inactivation

Gametes

are eggs and sperm

aside from the sex chromosomes, the other 22 pairs of chromosomes are

autosomes with the same size and genetic composition

Why are X and Y called sex chromosomes?

because they determine an individual's sex

meiosis is followed by two consecutive

cell divisions (meiosis I and meiosis 2)

Down syndrome has a characteristic set of symptoms including:

characteristic facial features, short stature, heart defects, susceptibility to respiratory infections, leukemia, and Alzheimer's disease, and varying degrees of developmental disabilities.

Meiosis reduces the

chromosome number by half

Interphase

chromosomes duplicate.

autosomes

chromosomes other than sex chromosomes

diploid

contains homologous pairs

what important event can occur during prophase I?

crossing over, in which nonsister chromatids may trade segments, contributing to the genetic variability that results from sexual reproduction.

men with XXYY, XXXY, or XXXXY are more likely to have:

developmental disabilities than XY or XXY

is a zygote haploid or diploid?

diploid

most animals and plants are said to be

diploid because their somatic cells contain pairs of homologous chromosomes.

Mitosis and meiosis both begin with:

diploid parent cells that have chromosomes duplicated during the previous interphase.

Ovum

egg

Since homologous chromosomes move to opposite poles during anaphase I, gametes will receive:

either the maternal or paternal version of the gene.

The human life cycle begins when a haploid sperm fuses with a haploid egg in

fertilization

Separation of homologous chromosomes during meiosis can lead to genetic differences between

gametes

The only cell not produced by mitosis are the

gametes, which are produced by meiosis

Are gametes haploid or diploid?

haploid because each cell has a single set of chromosomes.

Meiosis is a type of cell division that produces

haploid gametes in diploid organisms

haploid

has only one member of each homologous pair

Karyotypes allow for the observation of...

homologous chromosome pairs, chromosome number, and chromosome structure.

Trisomy 21

involves the inheritance of three copies of chromosome 21 (47 chromosomes total) and is the most common human chromosome abnormality.

Homologous chromosomes are matched in

length, centromere position, and staining pattern.

To prepare a karyotype, scientists often use...

lymphocytes, a type of white blood cell

The sex chromosomes of males?

males have one X chromosome and one Y chromosome (XY)

Klinefelter syndrome:

males with XXY (sterile individual, small testes, often female body characteristics)

When can nondisjunction occur?

meiosis I, if both members of a homologous pair go to one pole, or meiosis II, if both sister chromatids go to one pole.

Meiosis II is essentially the same as mitosis except...

meiosis II starts with a haploid cell

Gametes are made by

meiosis in the ovaries and testes

Karyotypes are often produced from dividing cells arrested at:

metaphase of mitosis, chromosomes condensed and doubled

The different versions of genes that homologous chromosomes may have at each locus originally arise from:

mutations (changes in the sequence of DNA)

the haploid number

n

At least half of all species of flowering plants are:

polyploid (including wheat, potatoes, and cotton)

Errors in mitosis or meiosis may produce...

polyploid species meaning they have more than two chromosome sets.

When does crossing over begin?

prophase I

Chromosome breakage can lead to:

rearrangements that can produce genetic disorders or, if changes occur in somatic cells, cancer.

Germ cell

sex cell (egg/sperm)

nondisjunction can result in abnormal numbers of:

sex chromosomes

People with down syndrome usually have a life span...

shorter than normal.

The human sex chromosomes X and Y differ in

size and genetic composition

zygote

sperm+egg, formed by fertilization

The incidence of down syndrome increases with

the age of the mother

Genetic variation in gametes results from:

the arrangement of homologous chromosomes at metaphase I and random fertilization.

Metaphase I

the chromosome tetrads are aligned on the metaphase plate. Spindle microtubules are attached to kinetochores at the centrosomes. Chromosomes continue with crossing over.

Random fertilization means that

the combination of each unique sperm with each unique egg increases genetic variability.

Nondisjunction

the failure of chromosomes or chromatids to separate normally during meiosis.

Anaphase I

the migration of chromosomes of a homologous pair being pulled apart towards opposite poles

A locus (plural, loci) is

the position of a gene

Genetic recombination

the production of new combinations of genes due to crossing over

An organism's life cycle is

the sequence of stages leading from the adults of one generation to the adults of the next.

life cycle

the sequence of stages leading from the adults of one generation to the adults of the next.

The diploid number

the total number of chromosomes (2n)

The formation of polyploid species is...

widely observed in many plant species but less frequently found in animals

Fertilization after nondisjunction yields...

zygotes with altered numbers of chromosomes.