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