5B Sexual Reproduction and Meiotic Cell Division

Eukaryotic Chromosomes

• DNA binds to proteins called histones• DNA and histones condense to form chromatin

Eukaryotic Chromosomes (cont.)

• Chromatin is condensed completely to form chromosomes after the DNA has replicated and the cell is ready for division.

• Chromosomes make it possible to separate DNA precisely during cell division.

Chromosomes can be seen in two forms:

1. Single-Arm: are composed of a single chromatid 2. Double- Arm (Replicated Form, Duplicated form): are made up of paired, genetically identical chromatids, called sister chromatids. • Sister chromatids are joined at the centromere.• Because the sister chromatids are formed during

replication of DNA, they are identical right down to the nucleotide sequences!

Chromosome Number• Organisms have 2 sets of chromosomes:

– One set from the female parent– One set from the male parent

• Example: The fruit fly (Drosophila)– 8 chromosomes total– How many from mom?– How many from dad?

• The corresponding chromosomes from mom and dad are called homologous.

Homologous Chromosomes

• same size• same centromere position• same banding pattern• same genes in the same locations on each pair• BUT, the nucleotide sequences at the same

gene location may or may not be the same

Diploid and Haploid• Diploid- cells containing both homologous

chromosomes– Diploid number of chromosomes is represent by “2N”

where “N” is the number of chromosomes in a single set– Example: Drosophila have 8 total chromosomes so 2N=8

(N= ______)• Haploid- cells containing just one set of

chromosomes– Gametes are haploid– “N”– Example: In Drosophila, N = _______

Diploid or Haploid?

Human Chromosome Number

• Humans have 23 pairs of chromosomes for a total of 46.

• 1 of each pair came from mom, and 1 of each pair came from dad

• A Karyotype shows a complete diploid set of chromosomes, grouped in pairs, arranged in size order.

Human Karyotype

Somatic Cells and Sex Cells

• Somatic cells- body cells which contain all of the chromosomes.

• Sex cells (gametes)- sperm and egg cell which contain half the number of chromosomes.– One of each sex cell is needed during sexual

reproduction to produce an offspring

Reproduction

• Reproduction- the making of new individuals• Asexual Reproduction-

– Requires only one “parent” cell– Offspring are genetically identical to the parent

Sexual Reproduction

• Involves fusion of two separate parent cells– Genetic information is inherited from both parents– Most animals and plants reproduce this way

Comparing Asexual and Sexual Reproduction

• Species survive by reproduction• Species better suited to their environment survive

better, and reproduce more, passing on their genes

Asexual Reproduction: Fast! - All organisms identical. - This is an advantage if conditions are favorable and a disadvantage of they are not.

Sexual Reproduction: Requires more time- Provides genetic diversity because DNA comes from both parent cells- Advantageous in varying environments.

Asexual Sexual

Number of Parents

Are sex cells needed? (yes/no)

Genetic Variability of Offspring (high/low)

Advantage in what kind of environment? (Changing/non-changing)

Mitosis vs. MeiosisMitosis –a step in cell division in which the nucleus divides into two, genetically identical nuclei

- Occurs during asexual reproduction, growth and repair of cells (somatic cells)

Meiosis –division of nuclear material that produces sex cells (egg & sperm) with half the number of chromosomes

- Occurs during sexual reproduction to make sex cells

Review of Mitosis

• Interphase– G1, S, G2

• Mitosis– PMAT

• Cytokinesis

Meiosis

• The process of producing gametes (sex cells)• In this process the number of chromosomes is

cut in half • Why is it important for gametes to have only

half the number of chromosomes?

+ =

Fertilization Mitosis

Mitosis

Mitosis

Meiosis

• Meiosis only occurs in cells that will produce sex cells– In mammals, these cells are located in the ovary

or the testes• The rest of the body cells that make up an

organism divide normally by mitosis

Meiosis Involves 2 Separate Divisions

1. Meiosis I: Similar to MitosisA. During S-phase of Interphase

I the chromosomes duplicateB. During Prophase I, the

homologous chromosomes pair up into tetrads

C. Crossing over (the exchange of parts of the homologous chromosomes) occurs at this time

What will crossing over do to the daughter cells that will be produced?

homologous pair before crossing-over

homologous pair after crossing-over

Metaphase, Anaphase, Telophase, Cytokinesis (I)

• These stages occur the same way they occur in Mitosis except in Meiosis I, the homologous chromosomes separate and end up in different cells (segregated)

Metaphase I

Anaphase I

Telophase I and Cytokinesis I

RULE OF INDEPENDENT ASSORTMENT– Homologs (chromosomes that are homologous to each

other) move in a random fashion to either end of the cell. – The two resulting cells end up with a random assortment

of the mother and father’s chromosomes.

• How are the daughter cells resulting from meiosis I different from their parent cell?– Genes have been recombined by crossing over– Each daughter may have some chromosomes from

mom and some from dad (independent assortment)

– The resulting cells have a haploid # of double armed chromosomes.

The Second Meiotic Division2. Meiosis II: The division of the daughter cells made in

meiosis I- Unlike the first division, chromosomes are NOT replicated during Interphase II

- Prophase II- no tetrads form because homologous chromosomes have already been separated

Metaphase, Anaphase, Telophase, Cytokinesis (II)

• Similar to the stages of Meiosis I, only sister chromatids are separated this time

• This gives each daughter only half the chromosomes of the original parent cell– 4 daughters are produced– The daughters have a haploid

number of single armed chromosomes

Metaphase II

Anaphase II

Telophase II and Cytokinesis II

The products of Meiosis

• The female gamete = egg cell• The male gamete = sperm cell• Each is Haploid (has ½ the normal # of

chromosomes)• During fertilization, egg and sperm randomly

combine making a Diploid zygote• The zygote grows into an organism through

Mitosis

Sources of Genetic Variation in Offspring

1. Crossing over2. Independent Assortment3. Random combination of sperm and egg.These all contribute to why siblings are all different despite having the same parents!

Summary

• Meiosis I-First division –diploid number of double-arm chromosomes is reduced to a haploid number of double-arm chromosomes

• Meiosis II- Second division –haploid number of double-arm chromosomes is divided to produce cells with a haploid number of single-arm chromosomes

Why do you think Meiosis is called a reduction division process?

Egg and Sperm Production

GAMETOGENESIS- the production of gametes. • The process is slightly different in males and

females 1. Oogenesis- production of eggs2. Spermatogenesis- production of sperm

1. Oogenesis• In the ovaries of the female• Produces one egg for every parent cell• Occurs in human females during embryonic development and

continues until menopause.• Note that the cytoplasm is NOT equally divided between the

eggs during this process. The result is one large, functional egg with a better chance of survival.

2. Spermatogenesis

• in the male testes• produces 4 viable (functioning) sperm for

every beginning cell

Chromosomal Disorders

• Occur mainly due to errors in meiosis (rare)

• Nondisjunction- when homologous chromosomes fail to separate during meiosis I

• Or when sister chromatids fail to separate during meiosis II

• Gametes will be made with the wrong number of chromosomes

Example

• An extra chromosome in a gamete will lead to an offspring with 3 copies of one chromosome

• This is called trisomy• The most common form of

trisomy involves chromosome 21

• Three copies of chromosome 21 leads to downs syndrome

Other Chromosomal DisordersChromosome Affected

Is nondisjunction Involved?

Symptoms Other interesting details?

Turner’s SyndromeJacob’s SyndromeKlinefelter SyndromeEdward’s SyndromeCri-du-chat

Metafemale

Compare

during asexual reproduction.

MITOSIS MEIOSIS

Kinds of Cells Produced

Number of Cells Produced

Chromosome # of parent cell

Chromosome # of daughter cells

Number of times DNA is replicated

Number of cell divisions

Function 1) To form new cells during

growth or to repair damaged

cells.

2) To produce a new organism

during asexual reproduction

To form gametes for sexual

reproduction