Somatic Cells Vs. Gametes
• In organisms that undergo sexual reproduction, there are two types of cells– Gametes are the sex cells of an organism.– Somatic cells are the second type of cell
Somatic Cells Vs. Gametes
• Somatic Cells are “body” cells and contain the normal number of chromosomes. This is also known as the diploid number (symbol = 2n).
– Examples would be … skin cells, brain cells, etc.• Gametes are the “sex” cells and contain only ½
the normal number of chromosomes…. called the haploid number (the symbol is n)….. Sperm cells and ova are gametes.
ChromosomesIn our bodies, we have 23 pairs of chromosomes for a total of 46.
Note, pair #23, consists of the sex chromosome pair. The first 22 pairs are known as autosomes
Therefore, how many chromosomes would a diploid cell have?
How about a haploid cell?
Each chromosome pair consists of two chromosomes
One comes from one parent
The other comes from the other parent
Sex Chromosomes
XX chromosome - female XY chromosome - male
In Humans the “Sex Chromosomes” are
the 23rd set
Fertilization• Gametes, or the sperm and the egg are both
haploid and must fuse together to form a diploid cell (and subsequently, offspring)
• The fusion of a sperm and egg to form a zygote, which is a fertilized egg
n=23egg
spermn=23
2n=46zygote
Meiosis• Meiosis is the process by which gametes
are formed. • Another way of looking at meiosis is the
production of haploid cells from a diploid cells
• Only certain cells can undergo meiosis– In males spermatogenesis occurs to create
four haploid sperm cells– In females, oogenesis occurs to create a
haploid egg cell
Human Spermatogenesis
2n=46
humansex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
4 sperm cells are produced from each
primary spermatocyte.
Primary Spermatocyte
Secondary Spermatocyte
Secondary Spermatocyte
Human Oogenesis
*** The polar bodies die… only one ovum (egg) is produced from each
primary oocyte.
Meiosis
• Meiosis consists of two steps, meiosis Iand meiosis II
• Meiosis I is very similar to mitosis while in meiosis II, the gametes (haploid cells) are formed
Meiosis I
Interphase I: Cells duplicate their DNA to form duplicate chromosomes
Prophase I: Each chromosome pairs with its homologous chromosome to form a tetrad. Also, crossing over occurs in this phase
Metaphase I: Spindle fibers attach to the chromosomes
Meiosis I
Anaphase I: Spindle fibers move the homologous chromosomes toward opposite sides of the cell
Telophase I and Cytokinesis: Nuclear membranes form and the cell separates into two cells
To Meiosis II…
Meiosis II
Prophase II: Starts with the two cells formed from meiosis I. Spindle fibers start to reform but chromosomes do not duplicate!
Metaphase II: Chromosomes in each cell line up similar to metaphase of mitosis
Anaphase II: Sister chromatids separate and move toward opposite sides of the cell
Telophase II and Cytokinesis: Results in the formation of four haploid cells
Crossing OverOccurs in prophase I of meiosis. Sections of chromosomes literally exchange places to allow, theoretically, any combination of parental alleles and traits in their offspring
Tetrad(homologous pair ofchromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologouschromosomes at anaphase I
Separation of chromatids atanaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosomeParental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-colorgenes
Eye-colorgenes
Figure 8.15
MITOSIS MEIOSISPARENT CELL
(before chromosome replication) Site ofcrossing over
MEIOSIS I
PROPHASE ITetrad formedby synapsis of homologous chromosomes
PROPHASE
Duplicatedchromosome(two sister chromatids)
METAPHASE
Chromosomereplication
Chromosomereplication
2n = 4
ANAPHASETELOPHASE
Chromosomes align at the metaphase plate
Tetradsalign at theMetaphase plate
METAPHASE I
ANAPHASE ITELOPHASE ISister chromatids
separate duringanaphase
Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together
No further chromosomal replication; sister chromatids separate during anaphase II
2n 2n
Daughter cellsof mitosis
Daughter cells of meiosis II
MEIOSIS IIDaughtercells of
meiosis I
Haploidn = 2
n n n n
