1. cell division and reproduction 2 © zanichelli editore 2015

Cell division and reproduction

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© Zanichelli editore 2015

Cell duplication

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Cell division and organism reproduction

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Cell division causes a cell to divide into two daughter cells.

In unicellular organisms, it is used to reproduce, while in multicellular organisms it is the basis for growth, development and replacement of damaged cells.


Three events in cell division

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• DNA replication – the genetic material duplicates.

• DNA segregation – the two copies of genetic material are separated.

• Cytokinesis – the cytoplasm divides to form two daughter cells.

The mechanisms are different in prokaryotic cells, which have a single circular chromosome, and eukaryotic cells which have multiple linear molecules of DNA.


Prokaryotes and binary fission

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Prokaryotes, like bacteria, divide by binary fission, a form of asexual reproduction that produces two cells which are genetically identical to the mother cell.


Somatic cells and gametes

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Somatic cells make up the body of an organism.They divide by mitosis and all have the same genetic information.

Gametes are the cells used in reproduction.They are produced by a process called meiosis.


Cell cycle and mitosis

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The life cycle of a somatic cell is divided byinterphase and mitosis.

During interphase, the cell grows (G1), replicates the DNA (S) and prepares for division (G2).The mitotic phase includes the mitosis and the cytokinesis.


Chromosomes, chromatids and chromatin

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During normal metabolic activities, DNA molecules are dispersed in the nucleus as chromatin.

Each chromosome is made up of a single chromatid - a DNA molecule - in phase G1. After replication, it is made of two sister chromatids. After condensation, chromosomes become visible through the light microscope.


Mitosis produces two identical nuclei /1

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Interphase Chromosomes replicate before cell division.

Prophase Nuclear membrane breaks down.Sister chromatids condense.Spindle apparatus is formed.

MetaphaseSister chromatids line up at the equatorial plate.



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AnaphaseSister chromatids come apart and are pulled towardsthe spindle poles.

TelophaseDaughter cells form.There is an indentation on the cell’s surface atthe equatorial plate.

CytokinesisThe cytoplasm, organelles and other cellularstructures are divided into the two cells.



Cytokinesis

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In animal cells, cytokinesis occurs with the formation of a cleavage furrow.

In plant cells, it requires the formation of a cell plate from Golgi vesicles and a cell wall.


Mitosis and apoptosis

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In the body of an organism, mitosis is necessary for development and growth and for replacing damaged cells.

In the human body, the rate of cell division varies in different tissues and organs.

Apoptosis is a process of programmed cell death.


Meiosis and sexual reproduction

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Asexual and sexual reproduction

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Asexual reproduction produces clones with the same genetic material of the parent.

Sexual reproduction requires two processes:

• meiosis, which halves the number of chromosomes and produces gametes;

• fertilization, which combines two gametes and produces a zygote.


Cells can be haploid or diploid

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In most animal species, gametes are haploid (they have a single set of chromosomes) and somatic cells are diploid (they have two complete sets of chromosomes).

Homologous chromosomes possess the same types of genes, often present in different forms (called alleles): one is inherited from the mother and one comes from the father.


Meiosis

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Meiosis involves two successive divisions of a diploid cell, but DNA replicates only before the first division.

The result of meiosis is the production of four haploid cells which are genetically different because they can have different alleles.

Daughter haploid cells (gametes)

Diploid cell

Sister chromatids

Homologouschromosomes


Meiosis I

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Meiosis I includes DNA replication, the crossing-over and the subsequent separation of homologous chromosomes.

It produces two cells with a single set of chromosomes, but each chromosome is composed of two chromatids.


Meiosis II

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During meiosis II, the two cells divide through a process similar to mitosis, , but which is not preceded by DNA replication.

Meiosis II produces four haploid cells, with chromosomes formed by a single chromatid.


Each gamete is unique

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Any individual within a species produces gametes during meiosis which are different from each other due to two events:

•crossing-over – it allows the recombination of genetic material through the exchange of segments of DNA between the chromatid of homologous pairs;

•independent assortment – during anaphase I, homologous chromosomes are randomly divided into different cells, which leads to the causal combination of maternal and paternal chromosomes in the daughter cells.


Sexual reproduction generates variability

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In species that reproduce sexually, offspring are different from each other and different from the parents; they inherit half of the chromosomes from the mother and half from the father. Furthermore, the chromosomes are mixed during meiosis.


The karyotype and the genetic identity

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Sexual reproduction can mix alleles but does not create new ones; despite variability, all individuals within a species have a similar karyotype.

The karyotype is the characteristic set of homologous chromosomes, which occur in a species-specific number, shape and size.


Human kariotype

Sex determination in humans

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The human katyotype includes 22 pairs of autosomal chromosomes and one pair of sexual chromosomes, which is different in males (XY) and females (XX).

The sex of the offspring depends on the male gamete, which can carry the X or Y chromosome.

XX XY

X X X Y

X X

XX

X Y

XY

Female Male

Fertilization

Female Male


Errors during meiosis can cause problems

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During meiosis, errors can occur during the separation of homologous chromosomes or chromatids: a phenomenon called nondisjunction.

It causes chromosomal anomalies, with the formation of zygotes that have trisomy or monosomy conditions.

Homologous chromosomes do not separate

Both gametes have an additional chromosome

Both gametes lack a chromosome


1. cell division and reproduction 2 © zanichelli editore 2015

Documents

zanichelli editore

cell plate

somatic cells

cell cycle

cell duplication

cell wall

mother cell

eukaryotic cells