chapter 9: meiosis pages 207-213. sexual reproduction the union of 2 specialized sex cells (gametes)...
TRANSCRIPT
Chapter 9: Meiosis
Pages 207-213
Sexual Reproduction
the union of 2 specialized sex cells (gametes) to form a single cell called a zygote Gametes usually come from 2 different parents Results in genetic variation among offspring
How is Chromosome Number Kept the Same with Sexual Reproduction?
Homologous Chromosomes - each chromosome in a somatic (body) cell has a partner chromosome Similar in size, shape, and position of centromeres
• 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
• 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)
• 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
Gametes are Haploid
When egg and sperm join at fertilization, each gamete is haploid The diploid number is restored in the fertilized egg
(zygote). When the zygote divides by mitosis to form the
first 2 cells of the embryo, each daughter cell receives the diploid number of chromosomes
Polyploid
3 or more sets of chromosomes Rare in animals Common among plants
Important factor in plant evolution Larger and hardier than diploid Wheat, cotton
Meiosis
Diploid cells -> Haploid Cells Meiosis: 1 cell undergoes 2 divisions –
Meiosis I and II Produces 4 haploid cells Similar to mitosis, except:
2 divisions Only 1 DNA duplication (first division) Genetic information is shuffled
http://www.johnkyrk.com/meiosis.html
Meiosis
• In a cell in which DNA synthesis has occurred, each chromosome is replicated
• Each replicated chromosome consists of two identical sister chromatids
Prophase I Remember – the chromosomes are
duplicated during the S phase (before meiosis)
Homologous chromosomes synapse – line up side by side Form tetrads
Crossing over occurs – genetic material is exchanged between homologous chromatids Produces a new combination of genes Increases the genetic variation among offspring
Metaphase I
Kinetochores are attached by spindle fibers to the poles Both sister chromosomes are attached to the
same pole, the other sister chromosomes to the other pole
Tetrads (4 chromatids) line up on the equatorial plane of the cell
Tetrads are held together at chiasmata – sites of prior crossing-over
Anaphase I
Paired homologous chromosomes separate and move toward opposite poles
Each pole receives a random mixture of maternal and paternal chromosomes Only one member of each homologous pair is
present at each pole. Sister chromatids are united at their
centromere regions.
Telophase I
One of each pair of homologous chromosomes is at each pole.
Each new nucleus contains the haploid number of chromosomes, but each chromosomes is a duplicated chromosome.
Cytokinesis occurs: Often called interkinesis
Video
Meiosis II
Prophase II Metaphase II
Chromosomes condense again
NO crossing over occurs
Chromosomes line up on equatorial planes of their cells
Chromatids are in groups of two
Anaphase II Telophase II
Sister chromatids separate
chromosomes move to opposite poles
Nuclei form at opposite poles of each cell
Cytokinesis occurs
Results of Meiosis
Four n cells with unduplicated chromosomes Animals – production of gametes
“gametogenesis” Spermatogenesis or oogenesis
Plants – production of spores
Tutorial
Oogenesis
Human females have about 700,000 primary oocytes at birth
Suspended in Prophase I until puberty Usually 1 egg is ovulated per cycle ~ 28 days
Oocyte fully matures by finishing meiosis
Human female ovulates ~ 400 times/lifetime Meiosis II only finishes if fertilization occurs