Meiosis and Sexual Life Cycles Flashcards
Genes
- Genes are the units of heredity
- Genes are segments of DNA
- Each gene has a specific locus on a certain
chromosome - One set of chromosomes is inherited from each
parent - Reproductive cells called gametes (sperm and
eggs) unite, passing genes to the next generation
Comparison of Asexual and Sexual Reproduction
In asexual reproduction, one parent produces
genetically identical offspring by mitosis
* In sexual reproduction, two parents give rise to
offspring that have unique combinations of genes
inherited from the two parents
Sexual Life Cycles
A life cycle is the generation-to-generation
sequence of stages in the reproductive history of
an organism
Sets of Chromosomes in Human Cells
- Each human somatic cell (any cell other than a
gamete) has 46 chromosomes arranged in pairs - 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 homologues - Both chromosomes in a pair carry genes
controlling the same inherited characteristics
Sets of Chromosomes in Human Cells (2)
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
Sets of Chromosomes in Human Cells
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
* The number of chromosomes in a single set is
represented by n
* A cell with two sets is called diploid (2n)
* For humans, the diploid number is 46 (2n = 46)
* In a cell in which DNA synthesis has occurred,
each chromosome is replicated
* Each replicated chromosome consists of two
identical sister chromatids
Gametes
Gametes are haploid cells, containing only one set
of chromosomes
* 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
Behavior of Chromosome Sets in the Human Life Cycle
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, the fusing of gametes, restores the
diploid condition, forming a zygote
* The diploid zygote develops into an adult
The Variety of Sexual Life Cycles
- The alternation of meiosis and fertilization is
common to all organisms that reproduce sexually - The three main types of sexual life cycles differ in
the timing of meiosis and fertilization - In animals, meiosis produces gametes, which
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
Alternation of Generations
- Plants and some algae exhibit an alternation of
generations - This life cycle includes two multicellular
generations or stages: one diploid and one haploid - The diploid organism, 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
Fungi and Protists
- 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
- Depending on the type of life cycle, either haploid
or diploid cells can divide by mitosis - However, only diploid cells can undergo meiosis
- In all three life cycles, chromosome halving and
doubling contribute to genetic variation in offspring
Concept 13.3: Meiosis reduces the number of
chromosome sets from diploid to haploid
Like mitosis, meiosis is preceded by the
replication of chromosomes
* Meiosis takes place in two sets of cell divisions,
called meiosis I and meiosis II
* The two cell divisions result in four daughter cells,
rather than the two daughter cells in mitosis
* Each daughter cell has only half as many
chromosomes as the parent cell
The Stages of Meiosis
- In the first cell division (meiosis I), homologous
chromosomes separate - Meiosis I results in two haploid daughter cells with
replicated chromosomes - In the second cell division (meiosis II), sister
chromatids separate - Meiosis II results in four haploid daughter cells
with unreplicated chromosomes
Meiosis 1
Meiosis I is preceded by interphase, in which
chromosomes are replicated to form sister
chromatids
* The sister chromatids are genetically identical and
joined at the centromere
* The single centrosome replicates, forming two
centrosomes
- Division in meiosis I occurs in four phases:
Prophase I
Metaphase I
Anaphase I
Telophase I
