Meiosis and Sex Determination

Organisms reproduce by

two means that each involve different modes of

cellular reproduction

Sexual reproduction is a mode of reproduction where

two parents give rise to

an offspring through the fusion of two gametes• Offspring produced through sexual reproduction are genetically unique from

their parents due to an “equal” contribution of material from both parents

• Offspring produced through sexual reproduction are also genetically unique

from their siblings (except identical twins which are clones)

Gametes are

haploid reproductive cells (ex. eggs and sperm) that are formed

through meiosis or are the descendants of meiotic cells

Gametes unite during

sexual reproduction to form a zygote

Eggs are

female gametes that carry nuclear DNA and extranuclear DNA and

contribute the cytoplasm and all organelles to the offspring

Sperm are

male gametes that only carry genetic information and enough

mitochondria and “food” molecules to allow it to reach an egg

Sex determination is any mechanism by which

the sex of an organism is conferred

Biological sex is based on the

reproductive

phenotype and the gametes that are produced

(also called “physiological sex”)

Male sex (“physiological male”) –

organism that

produces sperm and has reproductive

phenotypes associated with the production of

sperm

Female sex (“physiological female”) –

organism

that produces eggs and has reproductive

phenotypes associated with the production of

eggs

This is different than gender which is based on

individual or cultural behaviors and identity

Hermaphrodites are

organisms that have both male and female

reproductive phenotypes

“True” hermaphrodites produce

both sperm and eggs

Species that are normally hermaphroditic are called

monoecious

• From Greek: “mono” (“one”) + “oikos” (“house”) = “one house”

Species that with individuals that normally only have one

reproductive phenotype are

dioecious

• From Greek: “di-” (“two”) + “oikos” (“house”) = “two houses”

Hermaphroditism is found in

both plants and animals

Genetic sex determination is when

sex is determined by genes at

one or more loci but there are no obvious differences in

chromosome structure or number between males and females

Chromosomal sex determination is sex is determined by the

presence or absence of

sex chromosomes and their related genes

Even if an organism’s sex is “chromosomally determined”, sex is

still determined by

specific genes on those sex chromosomes

Mammals have

XY chromosome sex determination however the

presence of the SRY gene is what actually determines male phenotype,

which is normally located on the Y-chromosome

XX-XO sex determination goes as follows

• Females have two X chromosomes

• Males have one X chromosome and no

other sex chromosomes

• “O” indicates lack of sex chromosome

• During meiosis in females the X

chromosome pair and segregate evenly

into four gametes

• During meiosis in males the X

chromosomes are segregated into two

Gametes

Males are

heterogametic and produce

two different types of gametes

Females are

homogametic and produce

chromosomally identical gametes

ZZ-ZW sex determination goes as follows

• Females have one Z and one W

chromosome and are heterogamous• Produce eggs with either a Z chromosome or W

chromosome• Males have two Z chromosomes and are

homogamous• All sperm have a Z chromosome

There is 1:1 sex ratio in offspring

• ½ will be ZZ and ½ will be ZW

•This is found in

all birds and butterflies,

and some reptiles, amphibians, and fish

XX-XY sex determination goes as follows

• Females have two X chromosomes and are homogamous

• All normal eggs have an X chromosome

• Males have one X chromosome and one Y chromosome and are heterogamous

• Males produce sperm with either an X chromosome or Y chromosome

• There is 1:1 sex ratio

• ½ will be XX and ½ will be XY

• Found in all mammals, some reptiles, arthropods (insects), amphibians, fish, and plants

Because of the difference in size and genetic content, X and Y chromosomes have

pseudoautosomal regions which allow X and Y chromosomes to pair along the

metaphase plate during meiosis

• Pseudoautosomal regions carry the same genes, which allows the chromosomes to

pair during meiosis

In humans the Y chromosome is

acrocentric

Acrocentric means that

the centromere is near one end of the chromosome,

producing a long arm on one end and a much shorter arm on the other

Environmental sex determination is when

sex is determined by

environmental factors, both biotic and abiotic (living and non-living)

Nest or spawning temperature determines sex in

a number of reptiles (esp.

turtles and crocodilians) and fish, as well as a few birds•These organisms may also have chromosomal sex determination but environmental

conditions may also influence sex• Environmental sex determination driven by the presence of other animals(biotic) is important in sex-changing systems

Sequential hermaphroditism is

a sex system in which an organism changes

sex over the course of its lifetime due to either developmental stage (age) or

outside influence

Lifecycles are

the generation-to-generation sequence of stages in the

reproductive history of an organism

Fertilization is

the union of two haploid gametes to produce a diploid

zygote

A zygote is the diploid cell produced by the union of

two gametes during fertilization

(a fertilized egg)

Germ cells are specialized cells present in the reproductive structures of an

organism that produce gametes through

meiosis

Germline refers to

the lineage of these cells

All cells can undergo mitosis but only cells with an even number of

homolog pairs (diploid, tetraploid, or octoploid) can undergo

meiosis

There are three different general types of sexual life cycles:

diploid

dominant, haploid dominant, and alternation of generations

In the diploid dominant reproductive cycle, the multicellular organism is

diploid (or tetraploid or octoploid); Haploid cells are short lived and do not form a multicellular organism on their own

Meiosis occurs in germline cells to produce

gametes• These gametes do not undergo further cell division

Two gametes unite during fertilization, forming a

single diploid (multiploid)

cell• This cell undergoes mitosis to ultimately produce a diploid multicellular organism• This lifecycle is found in almost all animals

In the haploid dominant reproductive cycle, the multicellular organism is

haploid

The zygote formed through

fertilization is the only diploid state of the organism

Two haploid reproductive cells (gametes) are generated through mitosis of

other haploid cells unite to form a diploid zygote• Fungi and some algae reproduce using this lifecycle

In the alternation of generations lifecycle the organism has both

multicellular

diploid and multicellular haploid stages

The sporophyte is the

multicellular diploid stage of the organism

• The germline cells of the sporophyte undergo meiosis and produces haploid spores

• Spores do not unite with other cells

• The spores produced by the sporophyte undergo mitosis to produce a multicellular haploid

stage

The gametophyte is the

multicellular haploid stage of the organism

• Reproductive cells in the gametophyte produce gametes through mitosis

• The gametes eventually unite with other gametes to form a diploid zygote, which ultimately

gives rise to a new sporophyte•Most species of plants and algae reproduce using this lifecycle

Gametophyte-dominant plants include

mosses and hornworts where the “recognizable”

part of the plant is a gametophyte

Vascular plants (everything else) are

sporophyte-dominant where the recognizable parts are

sporophytes and the gametophytes are the structures that produce pollen or ovum

Meiosis is the

cell division process in which gametes are formed and produces

haploid cells from diploid cells

Genetic recombination ultimately occurs as a result of

meiosis, where offspring

are produced with combinations of traits that are different from both parents

The three steps of meiosis are

Each homologous chromosome is duplicated and the homologous chromosomes each have 2 sister chromatids 2. Meiosis I is the first cellular division that splits the diploid cell into two haploidcells with a single set of chromosomes that are duplicated• The haploid cells produced have chromosomes that still have two chromatids3. Meiosis II is the second cellular division that splits the two haploid cells with 2 chromatids of each chromosome into four haploid cells with one chromatid ofeach chromosome

-The phases of meiosis are similar to those in mitosis with

Prophase I

The pairs of homologous chromosomes pair up and form a complex of 4 sister chromatids called a tetrad

;Within each set the sister chromatids exchange genetic information and perform recombination in the form of crossing over

Prometaphase I

Kinetochores from and spindle fibers attach to the centromeres of each chromosome

Metaphase I

Homologous chromosome pairs like up on the metaphase plate

;Each chromosome in the pair is oriented towards opposite sides of the cell

Anaphase I

Homologous chromosomes pairs are separated with two sister chromatids migrating together to opposite sides of the cell

Telophase I and Cytokinesis I occur similar to in mitosis but forms

two haploid daughter cells

Prior to Meiosis II cells enter a phase called

interkinesis where chromosomes

do not duplicate

• Some cells (like mammalian eggs) will spend a majority of their “life” prior to fertilization

in a state of interkinesis

Prophase II and Prometaphase II

Spindle fibers attach to the centromeres of the sister chromatids

Metaphase II

Chromosomes line up on the metaphase plate with each chromatid towards opposite

sides of the cell

Anaphase II

Sister chromatids are separated (like in mitosis) into two cells

Telophase II and Cytokinesis II occur similar to

mitosis and meiosis I

Each of the 4 daughter cells has one chromatid which becomes called a

chromosome• Daughter cells are haploid

The genetic variation produced as a result of

meiosis is ultimately what gives an

evolutionary advantage to organisms that produce sexually (or produce gametes)

Meiosis and fertilization ultimately produces

genetic variation in the individual that

makes them unique from their parents and siblings

There may be some traits that are likely to be inherited together, but there is still a high amount of variation due to certain processes in

meiosis I

Chromosome pairs from each parent can line up differently on the metaphase plate

during

meiosis I

• This produces gametes with a different mix of chromosomes that originate from the maternal

or paternal line of the parents, with a 50% chance of a particular chromosome ending up in a

particular gamete

• This phenomenon is called independent assortment (you’ll learn about it in BIO 22)

Non-sister chromatids can all directly exchange genetic information within their

homologous pairs through

crossing over

The fertilization of an egg by an individual male gamete is also generally

random

Crossing over is the

exchange of corresponding segments of chromosomes across

non-sister chromatids of homologous pairs• Sequences from the chromosome of one parent move to the homologous chromosome fromthe other parent• Occurs during Prophase I when tetrads of homologous pairs are formed• Individual chromatids can end up identical in sequence to those passed down by

parents or as mixed sequences from both parents• Offspring can pass on a sequentially different chromatid from their parents

• Recombinant chromatids (eventually recombinant chromosomes) are chromatids

that have exchanged genetic material with a non-sister chromatid on a homolog

Recombinant chromosomes carry

DNA sequences from chromosomes of different parental

origins

Recombinant chromosomes have a combination of genetic material from both

the

maternal and paternal chromosomes that did not exist prior to crossing over

Linked genes are

genes that are located close enough together that they tend to be

inherited together and crossing over is unlikely to occur between them

• They get inherited as a “unit” because the distance between them is so small that it is hard to

sever the DNA molecules between them without accidentally damaging the genes

Crossing over involves

breaking and rejoining DNA molecules so it occurs in four

steps to minimize errors and maintain the integrity of the molecules

1. Once they form tetrads chromatids and homologs are held together by cohesin

proteins, the molecules are (ideally) held in perfect alignment and sections of

the DNA molecules are uncoiled then severed at identical points in their

sequences

2. The synaptonemal complex forms between the chromatids undergoing

crossing over

• The synaptonemal complex is a zipper-like protein complex that tightly connects

chromatids together to prevent movement when the broken molecules are rearranged

3. Synapsis occurs and the broken DNA is joined to the corresponding segment of

the non-sister chromatid

• Synapsis is the pairing and physical connection between the two chromatids

• Chiasmata are “X” shaped junctions between the chromatids where crossing over has

occurred

4. The synaptonemal complex dissolves and the homologs move apart, but are

still held together through cohesin proteins and the molecules begin to

recondense

Mitosis produces

two daughter cells that are identical to the parent cell

• The daughter cells are diploid (multiploid)

Meiosis ultimately produces

four daughter cells that are genetically unique

from each other and different from the parent cell• The daughter cells are haploid

Genetic variation is driven by

processes that occur during meiosis I:• Crossing over and synapsis• Random alignment of homologous pairs at the metaphase plate facing each pole• Separation of homologs

Genetic variation is driven

by meiosis II due to the random alignment of sister

chromatids (which may have undergone crossing over and are genetically

unique) and the subsequent random sorting into individual gametes

Errors in meiosis can occur when homologous pairs or sister chromatids fail

to separate during meiosis I and meiosis II in a process called

nondisjunction

Nondisjunction occurs when

members of chromosome or chromatid pairs fail to

separate during anaphase I or anaphase II• Nondisjunction results in an abnormal number of chromosomes in the

gametes

When gametes fuse and form a zygote, the zygote will have an abnormal

number of

chromosomes with either more or less than the normal number

• This abnormal number will affect the offspring’s somatic and gametic cells

Nondisjunction happens somewhat frequently and is thought to be the

main reason for

pregnancy loss

If the zygote survives, abnormal numbers of chromosomes will affect how

proteins are produced

Ploidy refers to the

number of chromosomes and complete chromosome

sets in an organism

Organisms that are euploid have the

“correct” or “appropriate” number of

chromosomes and chromosome sets

Organisms that are aneuploid have an

abnormal number of chromosomes

• Aneuploidy stems from an error in chromosome number in a homologous set or

incompletely number of sets

Organisms with monosomy have an

incomplete set of chromosomes

where a chromosome has been lost (2n – 1 chromosomes of a set)

Organisms with trisomy have

an abnormal set of chromosomes where an

extra chromosome has been gained (2n + 1 chromosomes of a set)

Errors can also occur during

meiosis that physically alter the structure of the chromosomes• This can be caused during replication, crossing over, or even chromosome movement if thechromosomes are weak or prone to damage

A deletion occurs when

a chromosome fragment is lost due to a breakage along the

molecule

A duplication occurs when

a chromosome has an abnormal structure where a portion of

the molecule is duplicated• This duplication can occur as a result of errors in DNA synthesis or as a result of a deleted section of onechromosome joining itself to its “normal” homolog

An inversion occurs when

a chromosome has an abnormal structure resulting from the

reattachment of a fragment in a reverse orientation to its original position

A translocation occurs when

a chromosome has a portion of DNA from a non-homologous chromosome

Reciprocal translocations occur when

the non-homologous sequences completely switch between the chromosomes

Nonreciprocal translocations occur

when a chromosome transfers a segment to a non-homolog but

does not receive a segment from it

Typically, an abnormal number or structure of autosomes results in pregnancy

failure in the form of

miscarriages• Some autosomal abnormalities allow the zygote to survive but with atypical

characteristics

Monosomy in autosomes generally leads to a

failure of development or successful

birth because of a lack of essential gene products that are produced by have two

complete sets of chromosomes• This is called underdosage

Trisomy in autosomes generally (especially in small autosomes) leads to

atypical

characteristics that will negatively impact an individual’s ability to survive and

reproduce because of an over-expression of gene products that are produced by

more than two homologous chromosomes• This is called overdosage

Structural abnormalities in autosomes are often

lethal at a young age or produce

long-term and chronic illnesses• This occurs because although the embryo may survive gestation since it may have a majority ofthe genes on an autosome, it is still missing certain genes that may be necessary for furtherdevelopment or long-term function