Unit 2 – Reproduction and DevelopmentUnit 2 – Reproduction and Development

Cellular ReproductionCellular Reproduction

Human KaryotypeHuman Karyotype

Key features of a Key features of a chromosome:chromosome:

centromere (where spindle attaches)centromere (where spindle attaches) telomeres (special structures at the telomeres (special structures at the

ends)ends) arms (the bulk of the DNA). arms (the bulk of the DNA).

ChromatinChromatin: the long fibers that form : the long fibers that form chromosomes and contain DNA, RNA and chromosomes and contain DNA, RNA and various proteins. Found in the nucleus of cells.various proteins. Found in the nucleus of cells.

ChromosomeChromosome: condensed chromatin structure : condensed chromatin structure formed when cells replicate (divide) (p.578)formed when cells replicate (divide) (p.578)

ChromatidChromatid: one half of a chromosome. Two : one half of a chromosome. Two sister chromatids are joined by a centromere sister chromatids are joined by a centromere to form a chromosometo form a chromosome

Chromatin and Chromatin and ChromosomesChromosomes

Chromosomes come in 2 forms depending Chromosomes come in 2 forms depending on the stage of the cell cycle:on the stage of the cell cycle:

The The monadmonad form consists of a single chromatid, a form consists of a single chromatid, a single piece of DNA containing a centromere and single piece of DNA containing a centromere and telomeres at the ends. telomeres at the ends.

The The dyaddyad form consists of 2 identical chromatids form consists of 2 identical chromatids (sister chromatids) attached together at the (sister chromatids) attached together at the centromere. centromere.

Chromosomes are in the dyad form before mitosis, Chromosomes are in the dyad form before mitosis, and in the monad form after mitosis.and in the monad form after mitosis.

The dyad form is the result of DNA replication: a The dyad form is the result of DNA replication: a single piece of DNA (the monad chromosome) single piece of DNA (the monad chromosome) replicated to form 2 identical DNA molecules (the 2 replicated to form 2 identical DNA molecules (the 2 chromatids of the dyad chromosome). chromatids of the dyad chromosome).

More ChromosomesMore Chromosomes Diploid organisms have 2 copies of each Diploid organisms have 2 copies of each

chromosome, one from each parent. The chromosome, one from each parent. The two members of a pair of chromosomes two members of a pair of chromosomes are called are called homologueshomologues..

Each species has a characteristic number Each species has a characteristic number of chromosomes, its of chromosomes, its haploid numberhaploid number n. n. Humans have n=23, that is, we have 23 Humans have n=23, that is, we have 23 pairs of chromosomes. Drosophila have pairs of chromosomes. Drosophila have n=4, 4 pairs of chromosomes.n=4, 4 pairs of chromosomes.

Cell cycleCell cycle

Cell cycleCell cycle a continuous sequence of cell growth and divisiona continuous sequence of cell growth and division

The cell cycle consists of two main stages:The cell cycle consists of two main stages:

1. 1. InterphaseInterphase – growth phase; includes G1, S – growth phase; includes G1, S phase, and G2phase, and G2

G1 (gap 1): cell carries out metabolic activities G1 (gap 1): cell carries out metabolic activities and prepares for cell divisionand prepares for cell division

S phase: DNA is replicatedS phase: DNA is replicated G2 (gap 2): centrioles replicate and cell prepares G2 (gap 2): centrioles replicate and cell prepares

for divisionfor division

Cell cycle (continued)Cell cycle (continued) 2. division stage – includes mitosis 2. division stage – includes mitosis

and cytokinesis; shortest stageand cytokinesis; shortest stage

Different cells have different timing for Different cells have different timing for their cells cycles; some take longer their cells cycles; some take longer than others to go thorough their cycle, than others to go thorough their cycle, and they also spend different amounts and they also spend different amounts of time in each stage.of time in each stage.

Cell Cycle Cell Cycle M = mitosis, where the cell divides M = mitosis, where the cell divides

into 2 daughter cells. The into 2 daughter cells. The chromosomes go from the dyad (2 chromosomes go from the dyad (2 chromatid) form to the monad (1 chromatid) form to the monad (1 chromatid) form. chromatid) form.

G1 = “gap”; where the cell spends G1 = “gap”; where the cell spends most of its time, performing its tasks most of its time, performing its tasks as a cell. Monad chromosomesas a cell. Monad chromosomes

S = DNA synthesis. Chromosomes go S = DNA synthesis. Chromosomes go from monad to dyad.from monad to dyad.

G2 = Dyad chromosomes, cell getting G2 = Dyad chromosomes, cell getting ready for mitosis.ready for mitosis.

G1, S, and G2 are collectively called G1, S, and G2 are collectively called “interphase”, the time between “interphase”, the time between mitosis mitosis

MitosisMitosis

Parent cell and daughter cells

MitosisMitosis division of the cell’s nucleus where the division of the cell’s nucleus where the

daughter cells receive the exact number of daughter cells receive the exact number of chromosomes and genetic makeup as the chromosomes and genetic makeup as the parent cell parent cell

In order for an organism to grow, repair, and In order for an organism to grow, repair, and maintain its function new cells are needed to maintain its function new cells are needed to replace old ones. replace old ones.

Each cell that undergoes mitosis produces 2 Each cell that undergoes mitosis produces 2 new cells. Mitosis allows the regeneration of new cells. Mitosis allows the regeneration of damaged tissue (like cuts) and to replace damaged tissue (like cuts) and to replace worn out cells (like red blood cells)worn out cells (like red blood cells)

Mitosis ensures that the same amount of Mitosis ensures that the same amount of genetic information is in each type of cell.genetic information is in each type of cell.

MitosisMitosis Mitosis is division of Somatic cells (body cells); Mitosis is division of Somatic cells (body cells);

not germ (sex) cell not germ (sex) cell

Mitosis is ordinary cell division among the Mitosis is ordinary cell division among the cells of the body. cells of the body.

During mitosis the chromosomes are divided During mitosis the chromosomes are divided evenly, so that each of the two daughter cells evenly, so that each of the two daughter cells ends up with 1 copy of each chromosome.ends up with 1 copy of each chromosome.

For humans: start with 46 dyad chromosomes For humans: start with 46 dyad chromosomes in 1 cell, end with 46 monads in each of 2 in 1 cell, end with 46 monads in each of 2 cells.cells.

CytokinesisCytokinesis separation of the cytoplasm and the formation separation of the cytoplasm and the formation

of two new daughter cells; cytokinesis occurs of two new daughter cells; cytokinesis occurs after telophase of mitosisafter telophase of mitosis

Parent cellParent cell – the original cell that divides – the original cell that divides during mitosis to form two new daughter cells during mitosis to form two new daughter cells

Daughter cellsDaughter cells – the cells produced during – the cells produced during mitosis of a parent cellmitosis of a parent cell

Stages of MitosisStages of Mitosis Before mitosis begins, DNA is Before mitosis begins, DNA is

replicated during interphase.replicated during interphase. Stages:Stages:

ProphaseProphase MetaphaseMetaphase AnaphaseAnaphase TelophaseTelophase

MitosisMitosis

All Phases Of Mitosis

InterphaseInterphase The first part of the interphase The first part of the interphase

stage is called gap 1 (G1); cells stage is called gap 1 (G1); cells carry out metabolic activities carry out metabolic activities to prepare for cell division. to prepare for cell division.

S phase; DNA gets replicated. S phase; DNA gets replicated. gap 2 (G2); cells prepare to gap 2 (G2); cells prepare to

undergo division.undergo division. The division stage includes 2 The division stage includes 2

processes: mitosis – the processes: mitosis – the division of the nucleus, and division of the nucleus, and cytokinesis – the division of the cytokinesis – the division of the cytoplasm. These two cytoplasm. These two processes are the shortest processes are the shortest events in the cell cycle. events in the cell cycle.

ProphaseProphase

During prophase, the During prophase, the chromatin coils and forms chromatin coils and forms thick condensed thick condensed chromosomes. chromosomes.

Each chromosome is made Each chromosome is made up of two sister chromatids up of two sister chromatids (two DNA molecules that are (two DNA molecules that are identical to each other). identical to each other). These chromatids are held These chromatids are held together by a centromere. together by a centromere.

ProphaseProphase

--chromosomes condense --chromosomes condense --nuclear envelope disappears--nuclear envelope disappears --centrioles move to opposite --centrioles move to opposite

ends of ends of the cellthe cell --spindle forms--spindle forms

MetaphaseMetaphase During metaphase, the spindle During metaphase, the spindle

fibers attach to the centromere of fibers attach to the centromere of the two replicated chromosomes. the two replicated chromosomes.

The chromatids are then guided The chromatids are then guided by the spindle fibers to the by the spindle fibers to the middle of the cell – the cells middle of the cell – the cells equator. equator.

A spindle fiber from one pole is A spindle fiber from one pole is then attached to one chromatid then attached to one chromatid and another from the opposite and another from the opposite pole is attached to the other pole is attached to the other chromatids at the centromere. chromatids at the centromere.

MetaphaseMetaphase chromosomes are lined up on cell chromosomes are lined up on cell

equator, attached to the spindle at equator, attached to the spindle at the centromeresthe centromeres

AnaphaseAnaphase During anaphase, the During anaphase, the

centromere splits apart and centromere splits apart and the chromatids are pulled the chromatids are pulled to the opposite poles of the to the opposite poles of the cell by the spindle fibers. cell by the spindle fibers.

The pull is caused by the The pull is caused by the shortening of the shortening of the microtubules that make up microtubules that make up the spindle fibers.the spindle fibers.

AnaphaseAnaphase Centromeres divide (chromosomes are Centromeres divide (chromosomes are

monads)monads)

The monad chromosomes are pulled to The monad chromosomes are pulled to opposite poles by the spindle.opposite poles by the spindle.

TelophaseTelophase Telophase begins when the Telophase begins when the

chromatids have all reached the chromatids have all reached the opposite poles within the cell. opposite poles within the cell.

At this point each of the At this point each of the chromatids is a “single” – a non-chromatids is a “single” – a non-replicated chromosome. replicated chromosome.

The chromosomes begin to The chromosomes begin to unwind; the spindle fibers break unwind; the spindle fibers break down and disappear because they down and disappear because they are no longer needed at this stage.are no longer needed at this stage.

The nucleus reappears and a The nucleus reappears and a nuclear membrane forms around nuclear membrane forms around each new set of chromosomes.each new set of chromosomes.

TelophaseTelophase

cytokinesis: cytoplasm divided into 2 cytokinesis: cytoplasm divided into 2 separate cellsseparate cells

chromosomes de-condensechromosomes de-condense nuclear envelope re-formsnuclear envelope re-forms spindle vanishesspindle vanishes

Meiosis Meiosis Meiosis is a special type of cell division Meiosis is a special type of cell division

that can occur only in reproductive that can occur only in reproductive organs.organs.

Meiosis produces reproductive cells called Meiosis produces reproductive cells called gametes (either eggs or sperm). gametes (either eggs or sperm).

These gametes are haploid cells, which These gametes are haploid cells, which means they contain only one copy of each means they contain only one copy of each type of chromosome that the diploid type of chromosome that the diploid parent cell contains parent cell contains

* Haploid – only one * Haploid – only one * Diploid – contains two* Diploid – contains two

MeiosisMeiosis Takes 2 cell divisions, Meiosis 1 and Takes 2 cell divisions, Meiosis 1 and

Meiosis 2, with no DNA synthesis Meiosis 2, with no DNA synthesis between.between.

In humans, start with 46 chromosomes In humans, start with 46 chromosomes (23 pairs) in dyad state. After M1, (23 pairs) in dyad state. After M1, there are 2 cells with 23 dyad there are 2 cells with 23 dyad chromosomes each. After M2 there are chromosomes each. After M2 there are 4 cells with 23 monad chromosomes 4 cells with 23 monad chromosomes each.each.

Meiosis Meiosis

First Meiotic Division (M1)First Meiotic Division (M1) Prophase of M1 is very long, with a number Prophase of M1 is very long, with a number

of sub-stages. of sub-stages. Main event in prophase of M1 is “Main event in prophase of M1 is “crossing crossing

overover”, also called “recombination”. ”, also called “recombination”. In crossing over, homologous chromosomes In crossing over, homologous chromosomes

pair up, and exchange segments by pair up, and exchange segments by breaking and rejoining at identical locations. breaking and rejoining at identical locations.

Several crossovers per chromosome, with Several crossovers per chromosome, with random positions. This is the basis for random positions. This is the basis for linkage mapping.linkage mapping.

More M1More M1 In metaphase of M1, pairs of homologous In metaphase of M1, pairs of homologous

chromosomes line up together. In mitosis chromosomes line up together. In mitosis and M2, chromosomes line up as single and M2, chromosomes line up as single individuals.individuals.

Anaphase of M1: the spindle pulls the two Anaphase of M1: the spindle pulls the two homologues to opposite poles. However, homologues to opposite poles. However, the centromeres don’t divide, and the the centromeres don’t divide, and the chromosomes remain dyads. chromosomes remain dyads.

Telophase of M1: cytoplasm divided into 2 Telophase of M1: cytoplasm divided into 2 cells, each of which has 1 haploid set of cells, each of which has 1 haploid set of dyad chromosomesdyad chromosomes

Prophase IProphase IDuring prophase I, each pair During prophase I, each pair of chromosomes that carry of chromosomes that carry the same gene (homologous the same gene (homologous chromosomes) become chromosomes) become aligned. These homologous aligned. These homologous pairs are called tetrads. pairs are called tetrads. During the pairing process, During the pairing process, crossing over of crossing over of chromosomes can occur, chromosomes can occur, where non-sister chromatids where non-sister chromatids exchange segments of exchange segments of chromosomes.chromosomes.

Metaphase IMetaphase IMetaphase I follows prophase I. Metaphase I follows prophase I. This is when the spindle fiber This is when the spindle fiber attaches to the centromere of attaches to the centromere of each chromosome. A spindle each chromosome. A spindle fiber from one pole attaches to fiber from one pole attaches to one pair of sister chromatids and one pair of sister chromatids and a spindle fiber from the opposite a spindle fiber from the opposite pole attaches to the other pole attaches to the other pair.The spindle fibers pull each pair.The spindle fibers pull each tetrad to the equator, but they tetrad to the equator, but they do not line up like in mitosis. do not line up like in mitosis.

Anaphase IAnaphase I During anaphase I, the During anaphase I, the

homologous chromosomes homologous chromosomes separate and move to the separate and move to the opposite poles of the cell. opposite poles of the cell. They are then pulled apart by They are then pulled apart by the shortening of the spindle the shortening of the spindle fibers. However, here they do fibers. However, here they do not split as they did in mitosis not split as they did in mitosis and so the sister chromatids and so the sister chromatids stay together. This means stay together. This means that only one chromosome that only one chromosome from each pair will move to from each pair will move to each pole of the cell. each pole of the cell.

Telophase ITelophase IWhen telophase I occurs, cell When telophase I occurs, cell division goes directly to meiosis division goes directly to meiosis II. If telophase does not occur, II. If telophase does not occur, the homologous chromosomes the homologous chromosomes begin to uncoil and then the begin to uncoil and then the spindle fibers disappear. The spindle fibers disappear. The cytoplasm gets divided, and cytoplasm gets divided, and nuclear membrane forms nuclear membrane forms around each group and two around each group and two cells are formed.cells are formed.

* Telophase I doesn’t occur in all cells * Telophase I doesn’t occur in all cells

Meiosis IIMeiosis IIThe phases of meiosis II are the same as The phases of meiosis II are the same as mitosis. The two cells from telophase I go mitosis. The two cells from telophase I go through prophase II , Metaphase II, anaphase through prophase II , Metaphase II, anaphase II and telophase II. Each of these cells II and telophase II. Each of these cells beginning meiosis II is haploid but they do beginning meiosis II is haploid but they do consist of replicated chromosomes. At the consist of replicated chromosomes. At the end of meiosis II, the daughter cells are still end of meiosis II, the daughter cells are still haploid but each cell contains single, un-haploid but each cell contains single, un-replicated chromosomes. The daughter cells replicated chromosomes. The daughter cells will develop into gametes in animals and will develop into gametes in animals and either gametes of spores in plants.either gametes of spores in plants.

Second Meiotic Division Second Meiotic Division (M2)(M2)

Meiosis 2 is just like mitosis.Meiosis 2 is just like mitosis. In prophase, the chromosomes condense In prophase, the chromosomes condense

and the spindle forms.and the spindle forms. Metaphase of M2: dyad chromosomes line Metaphase of M2: dyad chromosomes line

up singly on the cell equator.up singly on the cell equator. Anaphase of M2: centromeres divide, Anaphase of M2: centromeres divide,

chromosomes are now monads which get chromosomes are now monads which get pulled to opposite poles.pulled to opposite poles.

Telophase: cytoplasm divided into 2 cells.Telophase: cytoplasm divided into 2 cells. After M2: total of 4 cells from the original After M2: total of 4 cells from the original

cell. Each contains one haploid set of cell. Each contains one haploid set of monad chromosomesmonad chromosomes

Identify the StageIdentify the Stage

Gametogenesis in MammalsGametogenesis in Mammals Gametogenesis is the creation of the Gametogenesis is the creation of the

sperm and egg cells from the products sperm and egg cells from the products of meiosis, through changes in the of meiosis, through changes in the cytoplasm.cytoplasm.

In male mammals, sperm production is In male mammals, sperm production is continuous from puberty until death. continuous from puberty until death.

All 4 meiotic products remodel their All 4 meiotic products remodel their cytoplasm and grow a long flagellum to cytoplasm and grow a long flagellum to become spermatozoans, or sperm cells.become spermatozoans, or sperm cells.

Gametogenesis in Female Gametogenesis in Female MammalsMammals

In female mammals, ovarian cells start In female mammals, ovarian cells start meiosis 1 before birth, but the process is meiosis 1 before birth, but the process is stopped in prophase of M1. stopped in prophase of M1.

Meiosis resumes after puberty, under Meiosis resumes after puberty, under hormonal control. A small number of oocytes hormonal control. A small number of oocytes (cells undergoing meiosis) are shed from the (cells undergoing meiosis) are shed from the ovary during a female’s menstrual cycle. ovary during a female’s menstrual cycle. Usually only 1 oocyte is shed in humans, but Usually only 1 oocyte is shed in humans, but other mammals produce higher numbers.other mammals produce higher numbers.

After ovulation, the oocyte finishes meiosis 1.After ovulation, the oocyte finishes meiosis 1. Meiosis 2 only occurs after fertilization.Meiosis 2 only occurs after fertilization.

During both meiotic divisions, the division of the During both meiotic divisions, the division of the cytoplasm is asymmetric: one cell gets nearly all of cytoplasm is asymmetric: one cell gets nearly all of the cytoplasm. This cell becomes the egg. The the cytoplasm. This cell becomes the egg. The other cell in both divisions is called a “polar body”. other cell in both divisions is called a “polar body”.

One polar body is created in M1, and another in One polar body is created in M1, and another in M2. In some mammals, the first polar body divides M2. In some mammals, the first polar body divides so there are a total of 4 meiotic products, 1 egg so there are a total of 4 meiotic products, 1 egg plus 3 polar bodies. plus 3 polar bodies.

In humans, the first polar body never undergoes In humans, the first polar body never undergoes M2, so the final meiotic products in human females M2, so the final meiotic products in human females are a haploid egg, a haploid polar body, and a are a haploid egg, a haploid polar body, and a diploid polar body. diploid polar body.

Angiosperm Life CycleAngiosperm Life Cycle Angiosperms are flowering plants.Angiosperms are flowering plants. All eukaryotes alternate between a diploid All eukaryotes alternate between a diploid

phase and a haploid phase. In animals, the phase and a haploid phase. In animals, the haploid phase is a single cell, the sperm or haploid phase is a single cell, the sperm or the egg, and there is no haploid cell division.the egg, and there is no haploid cell division.

In plants, there is a distinct haploid organism In plants, there is a distinct haploid organism which has cell divisions and a life of its own.which has cell divisions and a life of its own.

The plant diploid phase is called the The plant diploid phase is called the sporophytesporophyte. In angiosperms and most other . In angiosperms and most other land plants, the sporophyte is the large land plants, the sporophyte is the large visible plant body that we see. visible plant body that we see.

The plant haploid phase is called the The plant haploid phase is called the gametophytegametophyte. . In lower plants, such as club mosses, this phase is In lower plants, such as club mosses, this phase is prominent. But in angiosperms, the gametophyte prominent. But in angiosperms, the gametophyte stage is quite short and small.stage is quite short and small.

Specifically, the male gametophyte, the pollen Specifically, the male gametophyte, the pollen grain, consists of 3 haploid nuclei. These nuclei grain, consists of 3 haploid nuclei. These nuclei are derived from one haploid meiotic product, by are derived from one haploid meiotic product, by mitosis. Two of the nuclei are “sperm nuclei” and mitosis. Two of the nuclei are “sperm nuclei” and the other controls the metabolism of the pollen the other controls the metabolism of the pollen grain.grain.

The female gametophyte, the ovule, consists of 8 The female gametophyte, the ovule, consists of 8 haploid nuclei. These 8 nuclei are derived from haploid nuclei. These 8 nuclei are derived from one of the meiotic products.one of the meiotic products.

Double FertilizationDouble Fertilization All angiosperms undergo “double fertilization”. It All angiosperms undergo “double fertilization”. It

is a major defining characteristic of angiosperms.is a major defining characteristic of angiosperms. In double fertilization, 2 sperm (pollen) nuclei In double fertilization, 2 sperm (pollen) nuclei

fertilize the ovule. fertilize the ovule. When the pollen grain lands on the stigma of a When the pollen grain lands on the stigma of a

flower, it germinates a long “pollen tube”, which flower, it germinates a long “pollen tube”, which grows down the style to the ovary, which contains grows down the style to the ovary, which contains the ovules. The 2 sperm nuclei migrate down the the ovules. The 2 sperm nuclei migrate down the pollen tube into the ovule. pollen tube into the ovule.

Each ovule has a cell at one end that is pollinated Each ovule has a cell at one end that is pollinated by one of the sperm nuclei. This fertilized cell is by one of the sperm nuclei. This fertilized cell is diploid, and it grows into the embryo and diploid, and it grows into the embryo and ultimately into the sporophyte plant body.ultimately into the sporophyte plant body.

The ovule also has 2 nuclei in the center, which join The ovule also has 2 nuclei in the center, which join with the other sperm nuclei to form a triploid tissue, with the other sperm nuclei to form a triploid tissue, the endosperm. The endosperm thus contains 2 the endosperm. The endosperm thus contains 2 maternal haploid genomes plus one paternal haploid maternal haploid genomes plus one paternal haploid genome. The endosperm develops into a nutritive genome. The endosperm develops into a nutritive tissue used by the germinating seed. tissue used by the germinating seed.

After fertilization, both embryo and endosperm grow After fertilization, both embryo and endosperm grow and develop into a seed. After a while, development and develop into a seed. After a while, development arrests and the seed dries out and forms a hard arrests and the seed dries out and forms a hard coat. The seed contains a multicellular embryo and coat. The seed contains a multicellular embryo and a multicellular endosperm. The seed is a resting a multicellular endosperm. The seed is a resting stage. When conditions are right, the seed stage. When conditions are right, the seed germinates: the embryo eats the endosperm until germinates: the embryo eats the endosperm until photosynthesis begins. photosynthesis begins.