Meiosis & Sexual Reproduction

Chapter 13

Human Life Cycle Human Life Cycle:

1) 23 pairs of homologous chromosomes

2) 1 pair of sex chromsomes – #23

3) 22 pairs of autosomes – #1-22

4) Karyotype: picture of chromosomes

5) Gametes are haploid (n)6) All other cells are diploid (2n)7) Fertilization (syngamy)

results in a zygote

Meiosis: cell division to produce haploid (n) gametes

Alternative Life Cycles Fungi/Some Algae:

**Meiosis produces n cells that divide by mitosis to make n adults (gametes by mitosis) Plants/Some Algae:

*Alternation of Generations: 2n sporophyte, by meiosis, makes n spores; spore divides by mitosis to generate a n gametophyte; gametes are formed by mitosis, which then fertilize into 2n sporophyte

Meiosis

Preceded by

DNA/chromosome

replication, but is

followed by 2 cell

divisions (Meiosis I &

Meiosis II) Produces 4 daughter

cells with ½ the number of

chromosomes (2n n) Creates variation

Animation of Meiosis

Meiosis Animation Homologous chromosomes separate in

Meiosis I Chromosomes separate in Meiosis II Reduction in chromosome # occurs in Meiosis I End Result: 4 cells genetically different

from each other and from the parent cell with half the number of chromosomes as the parent cell

Crossing over in Prophase I creates variation

Mitosis vs. Meiosis

Synapsis/tetrad/chiasmata (Prophase I) Xing over Homologous vs. individual chromosomes (Metaphase I) Sister chromatids do not separate (Anaphase I) Meiosis I separates homologous pairs of chromosomes, not

sister chromatids of individual chromosomes

Genetic Variation: Part I

Independent Assortment:Homologous pairs of

chromosomes position & orient randomly

(metaphase I) Combinations Possible

2n; with n being the

haploid # of the organism

Genetic Variation: Part II

Crossing Over:

The reciprocal exchange

of genetic material b/w

non-sister chromatids

during synapsis of

Meiosis I (Prophase I) Maternal & paternal

chromosomes get “mixed” up

Genetic Variation: Part III

Random Fertilization:

1 sperm (of 8 million) X 1 egg (of 8 million)

64 trillion diploid combinations

Summary of Meiosis

Makes sex cells or gametes Reduces the chromosome # from 2n n DNA Replication occurs before meiosis Two cell divisions Synapsis of homologous chromosomes crossing

over (Prophase I) Makes 4 genetically different cells Introduces genetic variation Associated with sexual reproduction Unique Features of Meiosis

Spermatogenesis

Occurs in the

seminiferous tubules

of the testes Makes 4 sperm per

cell undergoing

meiosis Continuous process!

Oogenesis Occurs in the ovary; about every 28 days Makes 1 egg/ovum per cell undergoing meiosis Through prophase I occurs in female embryo At puberty completes Meiosis I Ovulation Meiosis II occurs if egg is fertilized!

Spermatogenesis vs. Oogenesis

Spermatogenesis

1) Cytokinesis is equal

2) Occurs thorughout a male’s entire life

3) Uninterrupted sequence of meiosis

Oogenesis

1) Cytokinesis is unequal

2) Occurs through menopause and stops. At birth, an ovary has all the primary oocytes it will ever have.

3) Long periods of rest; interrupted sequence of meiosis

Abnormal # of Chromosomes in Gametes Nondisjunction: chromosomes do not separately correctly

in meiosis during Meiosis I (homologous chromosomes) or Meiosis II (sister chromatids) Result may be gametes with 22 or 24 chromosomes

Picture of Nondisjunction

Nondisjunction

TRISOMY Three copies of 1

chromosome Total of 47

chromosomes Down Syndrome –

#21 XXX Klinefelter Syndrome–

#23 XXY

MONOSOMY One copy of a

chromosome Total of 45

chromosomes Turner Syndrome –

#23 X0

Nondisjunction

High frequency in humans Most embryos are spontaneously aborted Alterations in chromosome # are too

detrimental Developmental problems may result from

imbalance of chromosomes

Down Syndrome

Trisomy 21 1 in 700 children born in

U.S. Frequency increases as

mother’s age increases

Down Syndrome Karyotype

Klinefelter Syndrome

XXY Male 1 in every 2000 live births Male sex organs present, but are sterile May have feminine characteristics Normal intelligence Tall

Klinefelter Syndrome Karyotype

Turner Syndrome

X0 Female Monosomy of #23 1 in every 5000 live births Webbed neck Short stature Immature sterile females

Turner Syndrome Karyotype

Jacob’s Syndrome Male

XYY Male 1 in 1000 live male births Extra Y chromosome More active Slight learning disabilities Delayed emotional immaturity Normal intelligence Normal sexual development Also called criminal karyotype