cell division: meiosis

biology 1

• Offspring acquire genes from parents by inheriting chromosomes

• Two general strategies– Sexual reproduction– Asexual reproduction

• Fertilization and meiosis alternate in a sexual lifecycle

• Meiosis reduces chromosome number from diploid to haploid

• Sexual reproduction produces genetic variation, a vital component of evolutionary adaptation

A glossary...• DNA is a nucleic acid composed of four different kinds of

nucleotide in different sequences• Specific sequences of nucleotides that correspond to synthesis

of a specific protein are called genes• Genes are joined together in strands called chromosomes• The lineal location of a gene on a chromosome is called a locus

(plural: loci)• Different expressions of a gene at a particular locus are possible

- these expressions are called alleles• Inheritance is possible because DNA is precisely replicated,

with gene copies being passed onto offspring• Each species has a characteristic chromosome number -

humans have 46

Asexual vs. sexual reproduction

Asexual reproduction Sexual reproduction

Single individual is the sole parent Two parents give rise to offspring

Single parent passes on all its genes to

its offspring

Each parent passes on half its genes to

its offspring

Offspring are genetically identical to the

parent

Offspring have a unique combination of

genes inherited from both parents

Results in a clone, or genetically identical

individual. Rarely, genetic differences

may occur as a result of mutation

Results in greater genetic variation:

offspring vary genetically from their

siblings and parents

Genetic variation in sexual reproduction is a result of meiosis

The sexual life cycle• Human somatic cells contain 46 chromosomes (as determined

by karyotyping)• Closer examination reveals that these 46 can be assigned into

23 pairs– 22 pairs are homologous pairs (ie, per pair, same set of loci). These are

known as autosomes– 1 pair carries different loci - these are sex chromosomes

• Each homologue from a pair is inherited from a specific parent• Thus, a human somatic cell consists of two sets of 23

chromosomes, each set inherited by a specific parent• A cell that possesses both sets is said to be diploid (2n)• A cell that has only one set is said to be haploid (n)

• In a sexual life cycle, meiosis halves the chromosome number from diploid to haploid to create gametes

• In fertilization, gametes fuse to become a single celled zygote which restores the diploid condition

• Depending on species (and Kingdom), different periods of time spent in haploid and diploid phases

• Occasionally, some organisms remain in either haploid or diploid state (although most organisms cycle)

Meiosis• Steps to meiosis in some ways mirror

those in mitosis. However, meiosis consists of two divisions (Meiosis I and Meiosis II)– Produces 4 daughter cells– Each daughter cell is haploid– Meiosis plays a key role in generating

variation

• As in mitosis, replication of DNA occurs unseen while genetic material is uncoiled

Interphase I• Chromosomes replicate as in mitosis• Each duplicated chromosome consists of 2

sister chromatids attached at a centromere• BUT remember that in a diploid cell each

chromosome (pair of chromatids) has a homologue

• Therefore, following duplication, for any one gene, there will be two pairs of two alleles

Prophase I• Chromosomes condense and are visible• Homologues associate as a tetrad in the process of synapsis• During synapsis homologues may join at specific loci termed chiasma• At a chiasma, homologues may exchange a length of DNA (set of

genes). This process is known as crossing over. The joint between the two homologues is known as a synaptonemal complex– In humans, 2-3 chiasmata per chromosone pair

• Cell prepares for first division– Migration of centrosomes

– Dispersion of nuclear membrane

– Formation of meiotic spindle

– Chromosomes begin to migrate

• Prophase I accounts for 90% of the time spent in meiosis

Metaphase I

• Tetrads align along metaphase plate

• Centromeres of homologues point towards opposite poles of cell

• Kinetochore microtubules connect to kinetochore sites in centromere of each homologue

Anaphase I

• Homologues separate and move towards separate poles of the cell, pulled by depolymerization of kinetochore microtubules at kinetochore end

• Sister chromatids remain intact and travel together to either pole

• Homologue separation is not necessarily by parental assignment

Telophase I and Cytokinesis

• Each pole now has a haploid set of chromosomes (homologues are at other pole), each homologue consisting of a chromatid pair

• Cytokinesis separates cell into two daughter cells

• In some cases nuclear membranes reform (interkinesis)

Prophase II

• The goal of meiosis II is to separate sister chromatids

• In prophase II, nuclear envelope disperses (if it reformed)

• Spindle apparatus reforms and chromosomes start to move towards metaphase plate

Metaphase II

• Chromosomes align on the metaphase plate, with each sister chromatid pointing towards a different pole of the cell

• Each sister chromatid is joined to a kinetochore microtubule at the kinetochore

Anaphase II• Centromeres of sister chromatids

separate

• Sister chromatids are pulled apart and move towards separate poles of the cell

Telophase II and cytokinesis• Nuclei form at opposite poles of the cell

• Cytokinesis occurs producing 4 haploid daughter cells

Sexual life cycles produce variation

• Genetic variation is essential for evolutionary adaptation

• In meiosis, variation occurs by– Independent assortment

• Homologues do not assign to different poles of the cell necessarily according to parental designation: 2n possible combinations

– Crossing over• Alleles associated previously with other alleles on the same

chromosome may now associate with different alleles from other homologue

– Random fusion of gametes (2n x 2n)

• Beyond meiosis, variation can also occur through mutation