Chapter 13 – Meiosis and life cycles - PowerPoint Presentation

Slide1

Chapter 13 – Meiosis and life cycles

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Introductory Vocabulary

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Heredity



t

he

transmission of traits from one generation to the next

Variation

 when o

ffspring

differ somewhat from their parents and

siblings

Genetics



the scientific study of heredity and hereditary

variation

Genes

 segments of DNA that code for individual traits/proteins;

p

arents

endow their offspring with

this coded information (genes); genetic

information is transmitted as specific sequences of the four nucleotides in DNA.

Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism’s inherited traits.

Gametes

 reproductive cells; haploid (one copy of each chromosome); egg and sperm; made during meiosis in the gonads (ovary/testes)

Fertilization

 fusion of egg and sperm

After

fertilization

genes

from both parents are present in the nucleus of the fertilized egg, or

zygote

.Slide3

Asexual vs. Sexual Reproduction

ASEXUAL (MITOSIS)

→

Makes SOMATIC cells

one diploid to two

diploid cells

identical to parent

ONE parent donates genetic information

SEXUAL (MEIOSIS)

→

- makes GAMETESOne diploid to four haploid cellsAll new cells are GENETICALLY DIFFERENT from each parent and each otherTWO parents contribute genetic information

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Haploid vs. Diploid

Diploid

 TWO sets of each chromosome pairHumans have a diploid number of 46; therefore each somatic cell (body cell) has 46 chromosomes

This is represented by 2n

These chromosome pairs are called homologous chromosomes (see next slide)

Cells made by MITOSIS are diploid

Haploid

 ONE set of each chromosome pair

Humans have a haploid number of 23; therefore each gamete (sperm/egg) have 23 chromosomesThis is represented by nCells made by MEIOSIS are haploid

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Homologous Chromosomes

Diploid organisms have homologous chromosomes. These chromosomes are the same size, shape, carry genes for the same traits, and have the centromere in the same spot. However, the genes can be different!

For example, the maternal chromosome (the one you got from mom) can have a brown eye gene and the paternal chromosome can have a blue eye gene

The EXCEPTION

to the homologous chromosome pattern are the

sex chromosomes

, X and Y

Females = XX Males = XY

The other 22 pairs of chromosomes are called

autosomes

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It is crucial to understand the differences among homologous chromosomes, sister chromatids,

nonsister

chromatids, and chromosome

sets.Slide6

Karyotypes

A

karyotype

is a picture of all the chromosomes of an organism. The chromosomes are usually ordered by size and can give clues about genetic disorders. They can be done on babies before they are born by amniocentesis or CVS – chorionic villi sampling.

This can tell doctors if the person has an extra chromosome or is missing one, or if they have some other type of chromosomal disorder (deletion, duplication,

etc

).

Down Syndrome Karyotype

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Beginning of

the Life Cycle

The human life cycle begins when a haploid sperm cell fuses with a haploid ovum (egg), this joining of cells is called

syngamy. The union of these gametes, culminating in the fusion of their nuclei, is

fertilization.

(in other words,

syngamy

precedes fertilization)

The fertilized egg (

zygote

) is diploid because it contains two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.As a person develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body.Gametes develop from specialized cells called germ cells in the

gonads

.

Gametes undergo the process of

meiosis

, in which the chromosome number is halved.Fertilization restores the diploid condition

by combining two haploid sets of chromosomes.

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Life Cycle Type 1

: Humans and other Animals

Gametes (HAP) → Fertilization → Zygote (DIP) → Mitosis and Development → Meiosis in Gonads → Gametes (HAP)

Adult form is

Diploid

…meiosis is done only to create haploid gametes

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Life Cycle

 generation to generation sequence of stages in the reproductive history of an organism

Fertilization

and

meiosis

alternate in all sexual life cycles, in plants, fungi,

protists

, and

animals.Slide9

Life Cycle Type 2

: Plants and some algae (protists)

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- This

life cycle includes

two multicellular stages

, one

haploid

and one

diploid

.- The

multicellular diploid stage

is called the

sporophyte

.

- Meiosis in the

sporophyte

produces haploid

spores

.

- Unlike

a gamete, a haploid

spore

doesn’t fuse with another cell but rather

divides by mitosis to form a multicellular haploid

gametophyte

stage.

-

Gametes

produced via mitosis by the gametophyte

fuse to form the zygote

, which grows into the sporophyte by mitosis.

- In

this type of life cycle, the

sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next sporophyte generation

.

Alternation of GenerationsSlide10

Life Cycle Type 3

:Most fungi and Protists

“Alternation of Generations”

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Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental result:

genetic variation among offspringSlide11

Meiosis

Many steps of meiosis resemble steps in mitosis.

Both meiosis and mitosis are preceded by the

duplication of chromosomes. (during interphase)In meiosis, there are two consecutive cell divisions

,

meiosis I

and

meiosis II

,

resulting in

four daughter cells.The first division, meiosis I, separates homologous chromosomes.The second division, meiosis II, separates sister chromatids.The four daughter cells at the end of meiosis have only HALF as many chromosomes as the original parent cell.11Slide12

Meiosis I is preceded by

interphase

, in which the chromosomes are duplicated to form sister chromatids.

Two genetically identical sister chromatids make up one duplicated chromosome.

The sister chromatids are closely associated all along their length. This association is called

sister chromatid cohesion

.

(Recall this from Chapter 12!)

In contrast,

the two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents

.Homologous chromosomes appear to be alike, but they may have different versions of genes, each called an allele, at corresponding loci.

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Synapsis

= Formation of Tetrads

When the homologous chromosomes come together they are held together by the synaptonemal complex

; this process is called synapsisCrossing over also happens at this stageOccurs during Prophase I

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Prophase 1 – Synapsis

 Ignore the poor English…Slide14

Prophase 1 – Crossing over

Crossing over

occurs during

Prophase I

(only Meiosis I – not meiosis II because tetrads are not present in Meiosis II) of meiosis. The homologous chromosomes of the tetrads can switch positions and the genes can be exchanged. This is one of the processes that

leads to genetic variation

.

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Crossing over occurs between

NON-SISTER CHROMATIDS

. The location where crossing over happens is called the

chiasmata

. Crossing over can happen 2-3 times per chromosome. Slide15

Meiosis I

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Meiosis II

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Meiosis I

→ homologous chromosomes are separated

Meiosis II

→ sister chromatids are separated

Meiosis

= Two divisions; creates 4 new haploid cells that are genetically different from the parent cell.

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BOTH preceded by Interphase

Meiosis HALVES the chromosome number

Mitosis CONSERVES the chromosome numberMeiosis makes gametes (egg/sperm)

Mitosis makes somatic cells (body cells)Meiosis yields daughter cells that are all genetically differentMitosis yields daughter cells that are all identical

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Comparing Mitosis to MeiosisSlide19

Processes that are UNIQUE to MEIOSIS

Synapsis and Crossing Over

Making the tetrads (synapsis) and crossing over between non-sister chromatids; occurs during Prophase I

Tetrads (homologs) on the metaphase plate

In mitosis this step is just the individual chromosomes on the metaphase plate, not the tetrads

Separation of the tetrads (homologs)

In Anaphase I, the tetrads separate; in mitosis the sister chromatids separate during anaphase

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Although

MUTATIONS

are the ORIGINAL SOURCE of genetic diversity, there are three factors that contribute to increasing genetic variation during meiosis:

- Independent Assortment

- Crossing Over

- Random Fertilization

Genetic Variation Slide21

Independent Assortment

Independent assortment

means that the maternal chromosomes and the paternal chromosomes can line up any way possible. This contributes to genetic variation.

To determine how many possibilities there are, use the following equation:

N = haploid number

2

N

= number of combinations

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If

n = 3, there are 2

3

= 8 possible combinations.

For humans with

n = 23, there are 2

23, or about 8.4 million possible combinations of chromosomes.Slide22

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Crossing Over

Produces

RECOMBINANT CHROMOSOMES

. This process combines genes inherited from each parent. Recall: the location of crossing over is called chiasmataSlide23

Evolutionary Adaptation depends on a population’s genetic variation

Charles Darwin recognized the importance of genetic variation in evolution.

As the environment changes, the population may survive if some members can cope effectively with the new conditions.

Mutations are the original source of different alleles, which are then mixed and matched during meiosis.

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