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Reproduction and Chromosome Transmission
Chapter 02 Reproduction and Chromosome Transmission Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
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Reproduction and Chromosome Transmission are Key to Genetics
Reproduction – the process by which new cells or organisms are produced Requires the transmission of chromosomes from parent to offspring When eukaryotic cells divide, they must sort their chromosomes so each cell receives the correct number
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2.1 General Features of Chromosomes
Definition of the term chromosome Key differences between prokaryotic and eukaryotic cells Procedure for making a karyotype Similarities and differences between homologous chromosomes
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Chromosomes Chromosomes are structures within living cells that contain the genetic material - the genes Chromosomes are composed of DNA, the genetic material Proteins, to provide an organized structure In eukaryotes the DNA-protein complex is called chromatin Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Two Types of Cells Cells are classified as one of two types:
Prokaryotes - Bacteria and archaea Eukaryotes - Protists, fungi, plants and animals Protists and fungi may be single-celled, but are still eukaryotes because they have a membrane-bound nucleus Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Prokaryotes No nucleus No membrane-bound organelles
Usually contain a single type of circular chromosome Found in the nucleoid Outside the membrane is a rigid cell wall May contain other structures Outer membrane Flagellum Outer membrane Cell wall Nucleoid (where bacterial chromosome is found) Ribosomes in cytoplasm Flagellum Plasma (also known as inner membrane) 1 mm
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Eukaryotes Have a nucleus – DNA surrounded by membrane
Two or more linear chromosomes May have flagella and other structures May have cell wall, different than prokaryotes Membrane-bounded organelles such as: Mitochondria – have DNA Chloroplasts – have DNA Lysosomes Golgi apparatus Golgi body Nuclear envelope Chromosomal DNA Nucleus Nucleolus Polyribosomes Ribosome Rough endoplasmic reticulum Cytoplasm Membrane protein Plasma membrane Smooth endoplasmic Mitochondrion Mitochondrial DNA Centriole Microtubule Microfilament Lysosome
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Chromosomes are Examined Cytogenetically to Produce a Karyotype
Cytogenetics – The field of genetics that involves the microscopic examination of chromosomes A cytogeneticist typically examines the chromosomal composition of a particular cell or organism This allows the detection of individuals with abnormal chromosome number or structure Provides a way to distinguish between two closely-related species
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Animal cells are of two types
Somatic cells Body cells, other than gametes Ex: Blood cells Gametes (germ cells) Sperm and egg cells Precursor cells that give rise to sperm and egg To get a complete karyotype, the cytogeneticist examines somatic cells Usually blood cells
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During cell division chromosomes can be seen with a light microscope.
Each chromosome has a unique size, shape and banding pattern (light and dark areas) A karyotype is a set of images of the chromosomes
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Eukaryotic Chromosomes Are Inherited in Sets
Most eukaryotic species are diploid Two sets of chromosomes For example: Humans – 46 total chromosomes (23 per set) Dogs – 78 total chromosomes (39 per set) Fruit fly – 8 total chromosomes (4 per set) Tomato – 24 total chromosomes (12 per set) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Members of a pair of chromosomes are called homologs
The two homologs form a homologous pair The two chromosomes in a homologous pair Are nearly identical in size Have the same banding pattern Have the same centromere location Have the same genes But not necessarily the same alleles Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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The DNA sequences on homologous chromosomes are also very similar
There is usually less than 1% difference between homologs Nevertheless, these slight differences in DNA sequence provide the allelic differences in genes Eye color gene Blue allele vs. brown allele Homologous pair of chromosomes Gene loci (location) A b c B AA Bb cc Genotype: Homozygous for the dominant allele Heterozygous recessive The physical location of a gene on a chromosome is called its locus ( plural: loci ).
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The sex chromosomes (X and Y) are not homologous
They differ in size and genetic composition They do have short regions of homology, though
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2.2 Cell Division The process of binary fission in bacteria
Phases of the eukaryotic cell cycle Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Cell Division One purpose of cell division is asexual reproduction
How some unicellular organisms make new individuals Examples: Bacteria Amoeba Baker’s Yeast (Saccharomyces cerevisiae) Cell division also allows multicellularity Plants, animals and some fungi derive from a single cell that has undergone repeated cell divisions Ex: Humans Start as a fertilized egg, grow to several trillion cells Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Bacteria Reproduce Asexually by Binary Fission
The capacity of bacteria to divide is really quite astounding Escherichia coli can divide every minutes Prior to division, the bacterial cell replicates its chromosome Then the cell divides into two daughter cells by a process called binary fission Binary fission is asexual – does not involve genetic contributions from two different gametes Mother cell Bacterial chromosome Septum Two daughter cells FtsZ protein Replication of bacterial
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Eukaryotic Cell Cycle Eukaryotic cells must use a more complex process
Each daughter cell must receive the right number of each type of chromosome Series of phases is called the cell cycle Cell cycle: G1 phase – Gap 1 S phase – Synthesis G2 phase – Gap 2 M phase – Mitosis G1 G0 S Formation of two daughter cells (Nondividing cell) Chromosome Restriction point Mother cell Nucleolus M Interphase Cytokinesis Telophase Anaphase Metaphase Prometaphase Prophase G2 Mitosis Interphase
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G1 phase S phase G2 phase The cell prepares to divide
Restriction point – the point at which molecular changes have accumulated to commit the cell to proceed through cell division S phase Chromosomes are replicated After replication the copies are called chromatids The two sister chromatids are joined at the centromere to form a dyad Kinetochore proteins on the centromere help with sorting G2 phase The cell accumulates the material for nuclear and cell division Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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M phase When mitosis occurs
Distributes replicated chromosomes to produce two identical daughter cells Cytokinesis – the process that divides the cell into two daughters A pair of sister chromatids (a dyad) Kinetochore (proteins attached to the centromere) Centromere (DNA that is hidden beneath the kinetochore proteins) One chromatid chromatid (a monad)
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A cell may remain for long periods of time in the G0 phase
A cell in G0 phase has either Postponed making a decision to divide Or made the decision to never divide again Ex: Terminally differentiated cells like neurons Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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2.3 Mitosis and Cytokinesis
Structure and function of the mitotic spindle Phases of mitosis Key differences in cytokinesis between plants and animals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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The Mitotic Spindle Apparatus
Organizes and sorts eukaryotic chromosomes Forms from microtubule-organizing centers (MTOCs) In animals, the two MTOCs are called centrosomes Centrosomes lie at each spindle pole A pair of centrioles is within each centrosome Plants do not have centrosomes The nuclear envelope functions as an MTOC Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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The mitotic spindle has three types of microtubules
Aster microtubules From the centrosome to the plasma membrane Help position the spindle Polar microtubules Project from the centrosomes to the middle Help to push the spindle poles away from each other Kinetochore microtubules Attach to the kinetochore on the centromere of the chromosomes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Mitosis Mitosis is the process of organizing and sorting the chromosomes into two daughter cells during the cell cycle Mitosis takes place in five phases Prophase Prometaphase Metaphase Anaphase Telophase Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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In these diagrams, the original mother cell is diploid (2n)
It contains a total of 6 chromosomes Three per set (n = 3) Before mitosis begins, the cell is in Interphase Refer to Figure 2.8 for the phases of mitosis Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Cytokinesis In most cases, mitosis is quickly followed by cytokinesis
to produce the separate daughter cells In animals Formation of a cleavage furrow In plants Formation of a cell plate S G1 G2 Cytokinesis Cleavage furrow 150 mm
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The two daughter cells are genetically identical
Mitosis and cytokinesis ultimately produce two daughter cells with the same number of chromosomes as the mother cell The two daughter cells are genetically identical Except for rare mutations Thus, mitosis ensures genetic consistency from one cell to the next The development of multicellularity relies on the repeated processes of mitosis and cytokinesis
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2.4 Meiosis Phases of meiosis
Key differences between mitosis and meiosis
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Meiosis Meiosis produces haploid cells from a cell that was originally diploid Like mitosis, the cell has progressed through G1, S, and G2 Unlike mitosis, meiosis involves two successive divisions called Meiosis I and Meiosis II, each subdivided into Prophase Prometaphase Metaphase Anaphase Telophase Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Prophase of Meiosis I Prophase I is further subdivided into periods known as Leptotene Zygotene Pachytene Diplotene Diakinesis Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Prophase of Meiosis I Leptotene Zygotene Pachytene
Replicated chromosomes begin to condense Zygotene Via the process of synapsis, homologous chromosomes recognize each other and align, forming the synaptonemal complex Pachytene Homologs are aligned Pairs of sister chromatids – four total! – are called bivalents or tetrads Crossing over occurs Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Crossing Over During pachytene of Prophase I
Crossing over involves a physical exchange of chromosome pieces that result in exchange of genetic information On each chromosome, may occur a couple times or a couple dozen times, depending on size and species -- About twice per chromosome in human sperm During crossing over, a chiasma forms (plural: chiasmata) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Prophase of Meiosis I Diplotene Diakinesis
The synaptonemal complex starts to dissociate, allowing the bivalent to separate slightly Diakinesis The synaptonemal complex has disappeared Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Prometaphase of Meiosis I
Pairing and crossing over are completed at the end of prophase In prometaphase I, the spindle apparatus is formed Chromatids are attached to the spindle via kinteochore microtubules Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Metaphase of Meiosis I At metaphase, the bivalents (or tetrads) are organizaed along the metaphase plate Pairs of sister chromatids form a double row (not a single row as in mitosis) Sister chromatids may be aligned in a very large number of possible ways 2n possible alignments Humans would have 223 – more than 8 million possible arrangements!
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Anaphase of Meiosis I and Cytokinesis
The two pairs of sister chromatids within a bivalent separate from one another However, the connection between the sister chromatids does not break The joined pair of sister chromatids moves to the pole In other words, the two dyads within a tetrad separate and move to opposite poles
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Telophase of Meiosis I and Cytokinesis
The dyads have separated to opposite poles The chromatids may decondense and the nuclear membrane may re-form at this point Meiosis I ends with two cells, each with three pairs (in this example) of sister chromatids This is a reduction division, and the cells are considered haploid, because they only carry one set of homologous chromosomes
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Meiosis II The sorting events that occur during meiosis II are similar to those that occur during mitosis, however the starting point is different For a diploid organism with six chromosomes Mitosis begins with 12 chromatids joined as six pairs of sister chromatids Meiosis II begins with 6 chromatids joined as three pairs of sister chromatids There is no chromosomal replication prior to meiosis II Meiosis II proceeds through prophase, prometaphase, metaphase, anaphase, and telophase Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Mitosis vs. Meiosis Mitosis produces two diploid daughter cells
Meiosis produce four haploid daughter cells Mitosis produces cells that are genetically identical Meiosis produces cells that are not genetically identical The daughter cells contain only one homologous chromosome from each pair The daughter cells contain many different combinations of the single homologs
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2.5 Sexual Reproduction Definition of sexual reproduction
How animals make sperm and egg cells How plants alternate between haploid and diploid generations Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Sexual Reproduction The most common way for eukaryotic organisms to produce offspring Parents make gametes with half the amount of genetic material These gametes fuse with each other during fertilization to begin the life of a new organism The process of forming gametes is called gametogenesis Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Some simple eukaryotic species are isogamous
They produce gametes that are morphologically similar Example: Many species of fungi and algae Most eukaryotic species are heterogamous These produce gametes that are morphologically different Sperm cells Relatively small and mobile Egg cell or ovum Usually large and nonmobile Stores a large amount of nutrients, in animal species Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Gametes are typically haploid
They contain a single set of chromosomes Gametes are 1n, while diploid cells are 2n A diploid human cell contains 46 chromosomes A human gamete only contains 23 chromosomes During meiosis, haploid cells are produced from diploid cells Chromosomes must be correctly distributed Each gamete must receive one chromosome from each pair Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Spermatogenesis The production of sperm
In male animals, it occurs in the testes A diploid spermatogonial cell divides mitotically to produce two cells One remains a spermatogonial cell The other becomes a primary spermatocyte The primary spermatocyte progresses through meiosis I and II Primary spermatocyte (diploid) Spermatids Sperm cells (haploid) MEIOSIS I MEIOSIS II Secondary spermatocyte
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Each spermatid matures into a haploid sperm cell
Meiois I yields two haploid secondary spermatocytes Meiois II yields four haploid spermatids Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Primary spermatocyte (diploid) (a) Spermatogenesis Spermatids Sperm cells (haploid) MEIOSIS I MEIOSIS II Secondary spermatocyte
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The structure of a sperm includes
A long flagellum A head The head contains a haploid nucleus Capped by the acrosome In human males, spermatogenesis is a continuous process A mature human male produces several hundred million sperm per day Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Oogenesis The production of egg cells
In female animals, it occurs in the ovaries Early in development, diploid oogonia produce diploid primary oocytes In humans, for example, about 1 million primary oocytes per ovary are produced before birth Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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The primary oocytes initiate meiosis I
However, they enter into a dormant phase They are arrested in prophase I until sexual maturity At puberty, primary oocytes are periodically activated to progress through meiosis I In humans, one oocyte per month is activated The division in meiosis I is asymmetric producing two haploid cells of unequal size A large secondary oocyte and a small polar body Secondary oocyte Primary oocyte (diploid) First polar body
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The secondary oocyte enters meiosis II but is quickly arrested in it
It is released into the oviduct An event called ovulation If the secondary oocyte is fertilized Meiosis II is completed A haploid egg and a second polar body are produced Second polar body Egg cell (haploid)
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The haploid egg and sperm nuclei then fuse to create the diploid nucleus of a new individual
Note that only one of the four cells produced in this meiosis becomes an egg Secondary oocyte Second polar body Primary oocyte (diploid) First polar body Egg cell (haploid)
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Plants Alternate Between Haploid and Diploid Generations
The life cycles of plant species alternate between two generations Haploid generation is called the gametophyte Diploid generation is called the sporophyte Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Meiosis produces haploid cells called spores
Spores divide by mitosis to produce the gametophyte In simpler plants, like mosses: Spores develop into gametophytes that have large numbers of cells In flowering plants: Spores develop into gametophytes that have only a few cells – they are tiny What we see as “the plant” is the sporophyte Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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Meiosis occurs within two different structures of the sporophyte
Anthers Produce the male gametophyte – the pollen grain Ovaries Produce the female gametophyte – the embryo sac
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Animals vs. Plants Animals produce gametes by meiosis
Plants produce spores by meiosis The spores develop into gametophytes The haploid gametophyte becomes multicellular by mitotic cell divisions The mutlicellular gametophyte then goes on to produce certain specialized cells as gametes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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