in eukaryotes, heritable information is passed to the next generation via processes that include...

23
In eukaryotes, heritable information is passed to the next generation via processes that include meiosis plus fertilization

Upload: lillian-jones

Post on 29-Dec-2015

214 views

Category:

Documents


0 download

TRANSCRIPT

In eukaryotes, heritable information is passed to the

next generation via processes that include meiosis plus

fertilization

Inheritance of Genes

• Genes are the units of heredity, and are made up of segments of DNA

• Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs)

• Each gene has a specific location called a locus on a certain chromosome

• Most DNA is packaged into chromosomes

• One set of chromosomes is inherited from each parent

Sets of Human Chromosomes

• Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes

• A karyotype is an ordered display of the pairs of chromosomes from a cell

• The two chromosomes in each pair are called homologous chromosomes, or homologs

• Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters

Fig. 13-3b

TECHNIQUE

Pair of homologousreplicated chromosomes

Centromere

Sisterchromatids

Metaphasechromosome

5 µm

• The sex chromosomes are called X and Y

• Human females have a homologous pair of X chromosomes (XX)

• Human males have one X and one Y chromosome

• The 22 pairs of chromosomes that do not determine sex are called autosomes

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

• Each pair of homologous chromosomes includes one chromosome from each parent

• The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father

• A diploid cell (2n) has two sets of chromosomes

• For humans, the diploid number is 46 (2n = 46)

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-4

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Centromere

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

• A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)

• For humans, the haploid number is 23 (n = 23)

• Each set of 23 consists of 22 autosomes and a single sex chromosome

• In an unfertilized egg (ovum), the sex chromosome is X

• In a sperm cell, the sex chromosome may be either X or Y

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Chromosome Life Cycle

• Fertilization is the union of gametes (the sperm and the egg)

• The fertilized egg is called a zygote and has one set of chromosomes from each parent

• The zygote produces somatic cells by mitosis and develops into an adult

• At sexual maturity, the ovaries and testes produce haploid gametes

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

• Meiosis results in one set of chromosomes in each gamete

• Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-5Key

Haploid (n)Diploid (2n)

Haploid gametes (n = 23)

Egg (n)

Sperm (n)

MEIOSIS FERTILIZATION

Ovary Testis

Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

Fig. 13-6

Key

Haploid (n)

Diploid (2n)

n nGametes

nn n

Mitosis

MEIOSIS FERTILIZATION

MEIOSIS

2n 2nZygote2n

Mitosis

Diploidmulticellularorganism

(a) Animals

Spores

Diploidmulticellularorganism(sporophyte)

(b) Plants and some algae

2n

Mitosis

Gametes

Mitosisn

nn

Zygote

FERTILIZATION

nn

nMitosis

Zygote

(c) Most fungi and some protists

MEIOSIS FERTILIZATION

2n

Gametes

n

n

Mitosis

Haploid multi-cellular organism(gametophyte)

Haploid unicellular ormulticellular organism

Different Life Cycle

Meiosis

Reduction division followed by fertilization ensures genetic diversity in sexually reproducing organisms

Meiosis ensures that each gamete receives one complete haploid (1N) set of chromosomes

During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles

Meiosis

Separation of the homologous chromosomes ensures each gamete receives a haploid (1N) set of chromosomes composed of both maternal and paternal chromosomes

Fig. 13-7-3Interphase

Homologous pair of chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicated chromosomes

Meiosis I

Homologouschromosomesseparate

1

Haploid cells withreplicated chromosomes

Meiosis II

2 Sister chromatidsseparate

Haploid cells with unreplicated chromosomes

Metaphase I

Fig. 13-8a

Prophase I Anaphase I Telophase I andCytokinesis

Centrosome(with centriole pair)

SisterchromatidsChiasmata

Spindle

Homologouschromosomes

Fragmentsof nuclearenvelope

Centromere(with kinetochore)

Metaphaseplate

Microtubuleattached tokinetochore

Sister chromatidsremain attached

Homologouschromosomesseparate

Cleavagefurrow

Crossing Over

During meiosis (prophase I), homologous chromosomes exchange genetic material via a process called “crossing over” which increases genetic variation in resultant gametes

Area of cross over is called chiasmata

Fig. 13-8d

Prophase II Metaphase II Anaphase II Telophase II andCytokinesis

Sister chromatidsseparate Haploid daughter cells

forming

Meiosis End Products

• At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes

• Each daughter cell is genetically distinct from the others and from the parent cell

Fig. 13-9a

MITOSIS MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Chromosomereplication

Homologouschromosomepair

Chromosomereplication

2n = 6

Parent cell

Prophase

Replicated chromosome

Metaphase Metaphase I

Anaphase ITelophase IHaploid n = 3

Daughter cells ofmeiosis I

MEIOSIS II

Daughter cells of meiosis II

nnnn

2n2n

Daughter cellsof mitosis

AnaphaseTelophase

Fig. 13-9b

SUMMARY

MeiosisMitosisProperty

DNAreplication

Number ofdivisions

Occurs during interphase beforemitosis begins

One, including prophase, metaphase,anaphase, and telophase

Synapsis ofhomologouschromosomes

Does not occur

Number ofdaughter cellsand geneticcomposition

Two, each diploid (2n) and geneticallyidentical to the parent cell

Role in theanimal body

Enables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproduction

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, And telophase

Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion

Four, each haploid (n), containing half as many Chromosomes as the parent cell; genetically different from the parent cell and from each other

Produces gametes; reduces number of chromosomes by halfand introduces genetic variability among the gametes

3 events unique to meiosis

• all three occur in meiosis l:– Synapsis and crossing over in prophase I:

Homologous chromosomes physically connect and exchange genetic information

– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

– At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Fertilization

Involves the fusion of two gametes

Increases genetic variation by providing for new combinations of genetic information in the zygote

Restores the diploid number of chromosomes