BIO1003

2024-2025 Spring

May 2025

INSTRUCTOR: ZEYNEP TACER-CABA, PHD.

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Hereditary Similarity
Variation
Genetics
Living organisms
◦ are distinguished by their ability to reproduce their own kind

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Basic Concepts
Heredity
◦ the transmission of traits from one generation to the next
Variation
◦ shows that offspring differ somewhat in appearance from parents
and siblings
Genetics
◦ the scientific study of heredity and hereditary variation

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Inheritance of Genes
Offspring acquire genes by inheriting chromosomes
Genes
◦ the units of heredity
◦ segments of DNA
Gametes
A chromosome consists of
◦ a single DNA
◦ proteins associated with DNA

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Inheritance of Genes
Each gene in an organism’s DNA
◦ a specific locus on a certain chromosome
We inherit
–from our mother and our father

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Comparison of Asexual and Sexual
Reproduction
In asexual reproduction
◦ one parent produces genetically identical offspring by mitosis

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Comparison of Asexual and Sexual
Reproduction
In sexual reproduction
◦two parents give rise to offspring
◦unique combinations of genes inherited from the two
parents
◦variation occurs

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Fertilization and meiosis alternate in sexual life cycles
A life cycle
◦ the generation-to-generation sequence of stages in the
reproductive history of an organism

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Sets of Chromosomes in Human Cells
Somatic cells are any cells in the body that are not gametes.
Liver cells and neurons are somatic cells.
In humans
◦each somatic cell has 46 chromosomes
◦made up of two sets
◦one set of chromosomes comes from each parent

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A karyotype
an ordered, visual representation of the chromosomes in a cell

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 Homologous Chromosomes
the two chromosomes
composing a pair have
the same characteristics
autosomes

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 Homologous Chromosomes
similar in length
and centromere
position
the same staining
pattern

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Sex Chromosomes
distinct from each other in their
characteristics
represented as X and Y
determine the sex of the individual
◦XX being female
◦XY being male

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A diploid cell
two sets of each of its chromosomes
human has 46 chromosomes
◦2n= 46

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In a cell in DNA synthesis has occurred
◦ all the chromosomes are duplicated

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Unlike somatic cells
◦gametes, sperm and egg cells are haploid cells
◦containing only one set of chromosomes

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Behavior of Chromosome Sets in the
Human Life Cycle
At sexual maturity
◦ the ovaries and testes produce haploid gametes by meiosis
During fertilization
◦ these gametes, sperm and ovum, fuse, forming a diploid zygote
The zygote
◦ develops into an adult organism

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The Human Life Cycle

Multicellular diploid
adults (2n= 46)

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The Variety of Sexual Life Cycles
The three main types of sexual life cycles

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The Variety of Sexual Life Cycles

In animals
◦ meiosis occurs during
gamete formation
◦ gametes are the only
haploid cells

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 The Variety of Sexual Life Cycles
Plants and some algae
◦ an alternation of generations
◦ both diploid and haploid multicellular stages

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 The Variety of Sexual Life Cycles

In most fungi and some

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Meiosis
reduces the number of chromosome sets from diploid to
haploid
Meiosis
◦ takes place in two sets of divisions, meiosis I and meiosis II

https://www.jove.com/science-education/12070/what-is-meiosis?list=vHi6T8R6

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 The Stages of Meiosis
An overview of meiosis
Meiosis I
Meiosis II

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Interphase and Meiosis I

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Metaphase I
The homologous pairs of chromosomes are lined
up at the metaphase plate

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Anaphase I
move the chromosomes toward opposite ends of the cell

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Telophase I, cytokinesis, and meiosis II

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Meiosis II

https://www.jove.com/science-education/12072/meiosis-ii?list=vHi6T8R6

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Metaphase II
The chromosomes lined up on the metaphase plate

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Anaphase II
centromeres of the sister chromatids separate
individual chromosomes move to the opposite ends of the cell

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 Telophase II and Cytokinesis
chromatids have moved all the way to opposite ends of the cell
nuclei reappear
each of the four daughter cells has the haploid number of chromosomes
genetically different form the other daughter cells and from the parent cell

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A Comparison of Mitosis and Meiosis

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https://www.jove.com/science-education/12073/meiosis-vs-mitosis?list=vHi6T8R6

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Synapsis and crossing over
◦homologous chromosomes physically connect and
exchange genetic information
◦The tight pairing of the homologous chromosomes is
called synapsis.
◦An exchange of chromosome segments between non-
sister homologous chromatids occurs and is called
crossing over. This process is revealed visually after the
exchange as chiasmata.

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Tetrads on the metaphase plate
◦at metaphase I of meiosis, paired homologous
chromosomes (tetrads) are positioned on the metaphase
plates

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Separation of homologues
–At anaphase I of meiosis, homologous pairs move toward
opposite poles of the cell
–In anaphase II of meiosis, the sister chromatids separate

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A Comparison of Mitosis and Meiosis

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Genetic variation produced in sexual life cycles
contributes to evolution
Reshuffling of genetic material in meiosis
◦produces genetic variation

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Origins of Genetic Variation Among
Offspring
In species that produce sexually
◦the behavior of chromosomes during meiosis
◦fertilization
◦responsible for most of the variation

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Independent Assortment of
Chromosomes
Homologous pairs of chromosomes
◦orient randomly at metaphase I of meiosis

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Independent Assortment of
Chromosomes

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Crossing over

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Random Fertilization
The fusion of gametes
◦ will produce a zygote with any of about 64 trillion diploid combinations

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Evolutionary Significance of Genetic
Variation Within Populations
Genetic variation
◦ is the raw material for evolution by natural selection
Mutations
◦ are the original source of genetic variation
Sexual reproduction
◦ produces new combinations of variant genes, adding more genetic diversity

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Thanks for your listening
Questions??

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