Chapter 13 - Meiosis and Sexual Life Cycles

Heredity

Transmission of traits from one generation to the next

Variation

Any difference between individuals of the same species.

Genetics

Scientific study of heredity and variation.

Genes

Units of heredity made up of DNA.

Gametes

Reproductive cells (sperm and egg)

How are genes passed down?

Through gametes

DNA is packaged into...

Chromosomes

How many chromosomes do humans have?

46 (23 pairs)

FILL: Mitosis creates __________ cells, Meiosis creates _____ cells

Somatic, reproductive

Somatic

Pertaining to the body

Locus

A gene's specific position along a chromosome

Asexual reproductive

One parent produces offspring without needing a mate. The offspring are genetically identical to the parent.

Mitosis in unicellular organisms...

Reproduction

Clone

A group of genetically identical individuals from the same parent

Sexual reproduction

Two parents contribute genetic material to create offspring with genetic variation.

Life cycle

The generation-to-generation sequence of stages in the reproductive history of an organism.

Cell cycle

The process a cell goes through to grow, copy its DNA, and divide into two new cells.

Karyotype

An ordered display of the pairs of chromosomes from a cell

Homologous chromosomes

Two copies of each autosome

"Homologs"

Homologous chromosomes

FILL: Chromosomes in a homologous pair are the same _____ and ________ and carry genes controlling the same ____________ characters.

Length, shape, inherited

Sex chromosomes

X and Y chromosomes.

Autosomes

Any chromosome that is not a sex chromosome (1-22)

Male chromosomes

One X and one Y

Female chromosomes

Two X chromosomes (XX)

Diploid cell

A cell containing TWO sets of chromosomes (2n), one set inherited from each parent.

Maternal

Of or like a mother

Haploid cell

A cell containing only ONE set of chromosomes (n).

Ovum

Egg

Chromosome does an egg always have?

Egg always carries an X chromosome.

Fertilization

The union of gametes (the sperm and the egg)

Zygote

Fertilized egg

How many sets of chromosomes are in a zygote?

_________ are the only types of human cells produced by meiosis, rather than mitosis

Gametes.

Human Life Cycle

1. Gametes (sperm and egg) have 23 chromosomes.

2. Fertilization creates a zygote with 46 chromosomes.

3. The zygote grows into an adult.

4. Adults produce new gametes.

Two phases of Meiosis

Meiosis I and Meiosis II

Meiosis I and Meiosis II result in

four daughter cells, each half as many chromosomes as the parent cell

Sister chromatic cohesion

Sister chromatids are glued together

Meiosis I

1. Homologs chromosomes separate

2. Haploid cells with duplicated chromosomes

Centromere

Area where the chromatids of a chromosome are attached

Meiosis II

Sister chromatids separate

Meiosis II is similar to mitosis in that

They are similar because sister chromatids line up and separate

Nonsister chromatids

One maternal and one paternal chromatid of a homologous pair

Stages of Meiosis I

Prophase I

Metaphase I

Anaphase I

Telophase I and cytokinesis

Stages of Meiosis II

Prophase II, Metaphase II, Anaphase II, Telophase II

Prophase I

1. Duplicated condensed DNA

2. Homologous pairs

3. Nuclear membrane disappears

4. Crossing over

Spindle forms

Chiasmata

X-shaped regions where crossing over occurred.

PMAT 2

meiosis

Metaphase I

1. The pairs of homologs line up in middle.

2. The centromere of each chromatid pair attaches to one spindle fiber.

Anaphase I

Spindle fibers move the homologs away from the middle

Telophase I

1. Spindle fibers disappears

2. New nuclear membranes form around each chromosome

Cytokinesis I

The cytoplasm divides, resulting in two daughter cells, each with half the original number of chromosomes.

Cytokinesis

Division of the cytoplasm during cell division

In animal cells ______ _____; in plant cells, _________ ______

Cleavage furrow, cell plate

Why doesn't DNA replication occur during Meiosis I or Meiosis II?

Because it already did in Interphase before meiosis starts.

Stages of Meiosis II

Prophase II, Metaphase II, Anaphase II, Telophase II and cytokinesis

Which meiosis is most similar to Mitosis?

Meiosis II

Prophase II

1. Duplicated, condensed DNA

2. Nuclear membrane disappears

3. Spindle forms

Metaphase II

1. Single chromosomes (with sister chromatids) line up in the middle.

2. Spindle fibers attach to chromatids.

Anaphase II

Sister chromatids separate

Telophase II

1. Nuclear membranes form around the separated chromosomes

2. Chromosomes begin to be uncondensed

Cytokinesis II

1. The cytoplasm divides, makes four haploid cells

2. Each cell has half the number of chromosomes as the original.

Each daughter cell is

genetically distinct from the others and from the parent cell

Synaptonemal complex

A zipper-like protein structure that holds the homologs together tightly

Cohesins

Proteins that hold sister chromatids together

Synapsis

Homologs (one from each parent) pair up and align closely together

FILL: Mitosis produces genetically ________ cells to the parent cell; Meiosis ______ produces cells are genetically ___________ from each other and from the parent cell

Identical, Different

What are the three unique events that occur in Meiosis I?

1. Homologous chromosomes pair up (Prophase I).

2. Crossing over (DNA exchange) happens (Prophase I).

3. Homologous chromosomes separate to different sides (Anaphase I).

Cohesins are cleaved in mitosis

At the end of metaphase

Cohesins are cleaved in meiosis

Along the chromosomes arms in anaphase I and at the centromeres in anaphase II

Mutations

Changes in an organism's DNA

What is the original source of genetic diversity?

Mutations

Alleles

Different versions of genes

What produces genetic variation?

Reshuffling of alleles during sexual reproduction

Three primary drivers of genetic variation

1. Independent assortment of chromosomes

2. Crossing over

3. Random fertilization

What is responsible for most of the variation that arises in each generation?

The behavior of chromosomes during meiosis and fertilization

2^n

Number of combinations possible when chromosomes assort independently into gametes

2^23

Number of possible combinations of chromosomes in humans.

Independent assortment of chromosomes

Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

Crossing over

Exchange of genetic material between homologous chromosomes.

How many crossovers happen on average per chromosome in humans during meiosis?

On average, 1 to 3

Recombinant chromosomes

A chromosome created when crossing over combines the DNA from two parents into a single chromosome.

Random Fertilization

Any sperm can fertilize any egg

FILL: The fusion of _____ gametes produces a zygote any of about 70 trillion ________ combinations

Gametes, diploid

TRUE or FALSE: All animals reproduce sexually

FALSE; Animals that reproduce asexually are rare

Accumulation of genetic variations favored by the environment results in...

Natural selection

Bdelloid rotifer

An organism that doesn't need to reproduce sexually. Gets genetic diversity by taking in DNA from other organisms

Tetrads

Pairs of homologous pairs (X-X)