Meiosis

What accounts for family resemblance

  • offspring resemble their parents more than unrelated individuals

Heredity

  • living organisms are distinguished by their ability to reproduce their own kind

Genetics

  • scientific study of heredity and variation

Heredity

  • the transmission of traits form one generation to the next

Variation

  • demonstrated by the differences in appearance that offspring show from parents and siblings

Inheritance of genes

  • genes are units of heredity     

    • 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

    • loci = plural

  • most DNA is packaged into chromosomes

  • one set of chromosomes is inherited from each parent

gametes (sperm and ovum)

  • alleles = different versions of a gene that can exist at a special locus on a chromosome

    • can influence various traits

      • eye color, blood types, etc.

  • each individual inherites two alleles for each gene

    • one from each parents

    • determine the organism’s genotype

  • in a typical mendal cross

    • an allele can be dominant (capital) or recessive (lower)

Homologous chromosomes

  • pair of chromosomes

  • one chromosome from the mother and one from the father

  • after DNA replication, each homologous chromosome is made of a pair of sister chromatids

Sister chromatids

  • two identical copies of a single chromosome that are connected by a structure called the centromere

haploid

  • n or 1n

  • a cell having a single set of chromosomes not having one from each parent

Diploid

  • 2n

  • a cell containing two complete sets of chromosomes, one from each parent

  • n stands for number of full sets of chromosomes

  • 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 mother and one from father

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

  • for humans, diploid number = 46 (2n=46)

Sets of chromosomes in human cells

  • 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

  • sort based on size

  • one pair of sex chromosomes

    • called X and Y

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

  • males have one X and one Y chromosome

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

  • in a cell in which DNA synthesis has occurred

    • each chromosome is replicated

  • each replicated chromosome consists of two identical sister chromatids

Mitosis

  • process by which a single cell divides into two identical daughter cells

  • essential for growth and repair in multicellular organisms

  • essential for asexual reproduction in some single-celled organisms

(make more cells, make cells bigger)

Meiosis

  • process by which a single cell divides into four daughter cells

    • each with half the number of chromosomes of the parent cell

  • critical for sexual production   

    • allows for the production of gametes

Asexual reproduction

  • one parent produces genetically identical offspring by mitosis

  • a clone is a group of genetically identical individuals from the same parent

Sexual reproduction

  • two parents give rise to offspring that have unique combinations of genes inherited from the two parents

Fertilization and meiosis alternate in sexual life cycles

  • a life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism

  • meiosis is the process in which the gametes are formed in animals or spores in plants, fungi, and some protists to create haploid cells

  • fertilization is the process in which two haploid cells (gametes) fuse to form a single diploid cell

    • called zygote (or sometimes a diploid stage)

The variety of Sexual Life cycles

  • the alternation of meiosis and fertilizaton is common to all organisms that reproduce sexually

  • the three main types of sexual life cycles differ in the timing of meiosis and fertilization

a. haploid life cycle

b. diploid life cycle

c. alternation of generations

a) Haploid life cycle

  • in most fungi and some protists

    • the only diploid stage is the single-celled zygote

    • there is no multicellular diploid stage

  • the zygote produces haploid cells by meiosis

  • each haploid cell grows by mitosis into a haploid multicellular organism

  • the haploid adult produces gametes by mitosis

  • most of the organism’s life is haploid (n)

    • adult organism has one set of chromosomes

    • multicellular but haploid

  • fertilization creates a zygote (2n)

    • two haploid gametes fuse

    • forms a single diploid cell (zygote)

    • only diploid stage

  • zygote does meiosis

    • turns zygote into haploid cells

  • haploid cells grow by mitosis

    • form a haploid multicellular organism

  • haploid adults make gametes by mitosis

b) Diploid life cycle

  • in animals

  • meiosis produces gametes

    • undergo no further cell division before fertilization

  • gametes are the only haploid cells in animals

  • gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

c) alternative of generations

  • plants and some algae exhibit an alternation of generation

  • this life cycles includes

    • both a diploid and haploid multicellular stage

  • the diploid organism, called the sporophyte

    • makes haploid spores by meiosis

  • each spore grows by mitosis into a haploid organism called a gametophyte

  • a gametophyte makes haploid gametes by mitosis

  • fertilization of gametes results in a diploid sporophyte

  • plants alternative between two different multicellular bodies

    • a diploid generation (sporophyte)

    • a haploid generation (gametophyte)

  • sporophyte makes spores by meiosis

    • reduces chromosome number (2n→ n)

  • spores grow into gametophytes

    • haploid spore divides by mitosis

    • becomes a haploid multicellular organism (gametophyte)

  • gametophyte makes gametes by mitosis

    • continues to be haploid

  • two gametes fuse

    • forms diploid zygote

    • zygote grows by mitosis into sporophyte

  • meiosis shares some characteristics with mitosis

    • however it produces haploid cells, which introduces some differences

  • this process involves the “PPMAT” phases

    • occurs twice to guarantee that the resulting cells are haploid

  • these two stages are referred to as Meiosis I and Meiosis II

Interphase

G1 Phase

  • cell grows and produces proteins

S Phase

  • DNA replication occurs

G2 Phase

  • cell grows further and prepares for division

  • even though there is twice the DNA as before replication

    • the cell is still considered diploid

    • we count chromosomes by the number of centromeres

  • sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I

  • protein complexes called cohesins are responsible for this cohesion

  • in mitosis    

    • cohesins are cleaved 劈裂 at the end of metaphase

  • in meiosis

    • cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)

  • explains how sister chromatids stay together and then separate at the correct time in mitosis vs meiosis

    • key player = cohesin proteins

  • sister chromatid cohesion = physical holding together of sister chromatids

    • done by cohesin protein complexes

In mitosis

  • all cohesins are cleaved 劈裂

  • sister chromatids separate

Result

  • each daughter cell gets one chromatid

  • chromatids become individual chromosomes

In meiosis

  • cohesins cleaved along the chromosome arms in anaphase I

    • allows homologous chromosome to separate while sisters stay attached

  • centromeres in anaphase II

    • sister chromatids finally separate

Why matters?

  • meiosis I reduces chromosome number (diploid → haploid)

  • meiosis II separates sister chromatids correctly

IF goes wrong → nondisjunction (extra/ missing chromosomes)

Memory trick

  • mitosis

    • cut all cohesins at once

  • meiosis I

    • cut arms only

  • meiosis II

    • cut centromeres

Prophase I

  • chromatin condenses into the chromsomes

  • homologous chromsomes will fuse together in a process called synapsis

  • then, crossing over takes place

    • genetic material is exchanged between homologous chromosome

During cross over

  • recombination occur

    • process where homologous chromosomes pair up and exchange genetic material between each other

    • called recombinant chromosomes

  • each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

Prometaphase I

  • nuclear membrane dissolves

  • mitotic spindle starts to form and attach to the chromosomes at their centromeres

Metaphase I

  • homologous chromosomes align in pairs in center of cell

  • random assortment occurs

    • chromosomes pairs line up randomly

Anaphase I

  • homologous chromosomes pairs separate and move to opposite ends of the cell

  • as homologous pairs are separated

    • each end of the cell will contain only one version of each chromosome

    • ensures that each cell contains half the number of chromosomes and is haploid

Telophase I & Cytokinesis I

  • two new nuclei form around each set of chromosomes

  • cytoplasm splits and two (haploid) daughter cells are formed

  • cleavage furrow forms

  • cell plate forms in plant cells

Prophase II

  • Meiosis II happens to allow 4 haploid gametes to be created

    • no interphase II

      • interkinesis

      • centrosomes replicate, nucleus reforms around the chromosomes

    • DNA is not replicated between meiosis I and II

Prometaphase II

  • nuclear membrane dissolves

  • mitotic spindle begins to form and attach to chromosomes at their centromeres

Metaphase II

  • chromosomes align in a single file in the center of the cell

  • occurs to ensure sister chromatids separate in the next phase

Anaphase II

  • sister chromatids separate and move to opposite ends of the cell

  • chromosomes on each end of the cell will result in haploid daughter cells

    • each will have one version of each chromosome

Telophase II

  • four new nuclei form around each set of chromosomes

Cytokinesis II

  • cytoplasm splits and four (haploid) daughter cells are formed