AP Biology Unit 5 notes

Meiosis & Sexual Reproduction

Cell division / Asexual reproduction

mitosis:

• produce cells with same information

  • identical daughter cells

  • exact copies (clones)

• same amount of DNA

  • same number of chromosomes

  • same genetic information

Asexual reproduction:

• single-celled eukaryotes

  • yeast (fungi)

  • protists

    • paramecium

    • amoeba

• simple multicellular eukaryotes

  • hydra

• disadvantages? no natural variation

Homologous Chromosomes:

• paired chromosomes

  • both chromosomes of a pair carry “matching” genes

    • control same inherited characters

    • homologous = same information

/

how do we make sperm and eggs?

• must reduce 46 chromosomes → 23

  • must reduce the number of chromosomes by half

Meiosis: production of gametes

• alternating stages

  • chromosome number must be reduced

    • diploid → haploid

    • 2n → n

      • humans: 46 → 23

    • meiosis reduces chromosome number

    • makes gametes

  • fertilization restores chromosome number

    • haploid → diploid

    • n → 2n

Meiosis

• reduction division

  • special cell division for sexual reproduction

  • reduce 2n → 1n

  • diploid → haploid

    • “two” → “half”

  • makes gametes

    • sperm, eggs

Preparing for meiosis

• 1st step of meiosis

  • duplication of DNA

  • why bother?

    • meiosis evolved after mitosis

    • convenient to use. “machinery” of mitosis

    • DNA replicated in S phase of interphase of meiosis (just like in mitosis)

Steps of meiosis

• meiosis 1

  • interphase

  • prophase 1

  • metaphase 1

  • anaphase 1

  • telophase 1

    • 1st division of meiosis separates homologous pairs ( 2n → 1n ) “reduction division”

• meiosis 2

  • prophase 2

  • metaphase 2

  • anaphase 2

  • telophase 2

    • 2nd division of meiosis separates sister chromatids (1n → 1n) “just like mitosis”

Meiosis 1

• 1st division of meiosis

• seperates homologous pairs

Trading pieces of DNA

• crossing over

  • during prophase 1, sister chromatids interwine

    • homologous pairs swap pieces of chromosome

      • DNA breaks & re-attaches

Crossing over

• 3 steps

1. cross over

2. breakage of DNA

3. re-fusing of DNA

• new combination of traits

Meiosis 2

• 2nd division of meiosis

• seperates sister chromatids

mitosis vs meiosis

The value of sexual reproduction

• sexual reproduction introduces gentic variation

  • genetic recombination

    • independent assortment of chromosomes

    • random alignment of homologous chromosomes in Metaphase 1

  • crossing over

    • mixing of alleles across homologus chromosomes

  • random fertilization

    • which sperm fertilizes which egg?

• dividing evolution

  • providing variation for natrual selection

Variation for genetic recombination

• independent assortment for chromosomes

  • meiosis introduces genetic variation

  • gametes of offspring do not have same combination of genes as gametes from parents

    • randdom assortment in humans produces 8,388,608 different combination in agmetes

Variation from crossing over

• crossiing over creates completely new combination of traits on ech chromosome

  • creates an infinate variety in gametes

Variation from random fertilization

• sperm + egg = ?

  • any 2 parents will produce a zygote with over 70 trillion possible diploid combination

Sexual reproduction creates variability

• sexual reproduction allows us to maintain both genetic similarity and diffrences

Sperm production

• spermatogenesis

• continuois & prolific process

• each ejaculation = 100-600 million sperm

Egg Production

• Oogensis

  • eggs in ovaries halted before Anaphase 1

  • Meiosis 1 completed during maturation

  • Meiosis 2 completed after fertilization

  • 1 egg + 2 polar bodies

Diffrences across kingdoms

• not all organisms use haploid & diploid stages in same way

  • which on is dominant (2n or n) differs

  • but still alternate between haploid & diploid

    • must for sexual reproduction

Mendelian Genetics

Common Ancestry

• DNA and RNA carry genetic information

• The genetic code is shared by all living systems

• Gregor Mendel studied inheritance and created two laws that can be applied to the study of genetics

Gregor Mendel

• Mendel was an Austrian monk who experimented on pea plants and discovered the basic principles of heredity.

• why pea plants?

  • many varieties

  • controlled mating

  • relatively short generation time

Pea Plant Traits

• Mendel only tracked characteristics that came in two distinct forms:

• examples:

  • color (purple or white)

  • seed shape (round or wrinkled)

• to help control his experiments, he used true breeding plants

  • true breeding: organisms that produce offspring of the same variety over many generations of self-pollination.

    • example: true-breeding purple pea plants will only produce purple offspring with self-pollination.

Generations

• P generation: true-breeding parental generation

• F1 generation: (first filial) hybrid offspring of P generation

• F2 generation: (second Filial) offspring of the F generation

  

Punnett Squares

• diagrams used to predict the allele combinations of offspring from a cross with known genetic compositions

• capital letter denote dominant traits

• lower case letters denote recessive traits

Genetics Vocabulary

• Homozygous: an organism that has a pair of identical allels for a character

  • example:

  • homozygous dominant: AA

  • homozygous recessive: aa

• Heterozygous: an organism has two different alleles for a gene

  • example:

  • Aa

• Genotype: the genetic makeup (alleles) for an organism

• Phenotype: an organisms appearance, which is determined by the genotype

Testcross

• helps to determine if the dominant trait is homozygous dominant or heterozygous

Principles of Heredity

• Mendels expirements allowed him to develop two fundamental princples of heredity:

  1. The law of segregation

  2. The law of independent assortment

Discoveries

• Mendel noticed that the cross between purple and white true breeding pea plants produced only purple F1 offspring

  • Did the white characteristic disappear?

    • No, because the white pea flower chracteristic came back in the F2 generation

Dominant vs Recessive

• Mendel hypothesized that the purple must be a dominant trait to the white flower, which is a recessive trait

• Mendel performed the same crosses for each of the seven characteristics of pea plants and found the same results

  • he found that the F2 genereation was always a 3:1 ratio

Mendel’s Model

• to explain the 3:1 ratio he observed in the F2 generation, mendel created a model with four concepts:

1. Alternative versions of genes (alleles) account for variations in inherited charactistics

2. For each character, an organism inherits two copies (two alleles) of a gene, one from each parent

3. If two alleles at a locus differ, then the dominant allele determines the appearance and the recessive alleles has no noticeable effect

4. Law of segreation: the two alleles for the same trait seperate during gamete fornation and end up in different gametes

Alleles: A closer look

• Alleles: Different versions of the same gene.

• somatic cells are diploid

  • they contain two copies of each chromosome

  • Alleles: alternative versions of a gene

    

The Law of Segregation:

• During gamete formation, the two alleles for each gene segregate (separate) from each other.

• This means that each gamete receives only one allele for each gene.

• Key concepts:

• Alleles: Different versions of the same gene.

• Homologous Chromosomes: Pairs of chromosomes, one from each parent, that carry genes for the same traits.

• Meiosis: The process of cell division that produces gametes (sperm and egg cells).

• Significance:

  • Explains how genetic variation is maintained in populations.

  • Provides a foundation for understanding the inheritance of traits.

• Visual Representation: Often depicted using Punnett squares to predict the possible offspring genotypes and phenotypes from a cross between two individuals.

Monohybrid Crosses

• the law of segregation was determined by doing crosses between true-breeding plants which produced F1 hybrids, known as monohybrids

  • examples: BB x bb produce F1 that are all Bb

  • Monohybrid Crosses: a cross between the F1 hybrids

  • BB x Bb

The law of Independent Assortment

• Mendels second principle is the law of independent assortment: genes for one trait are not inherited eith genes of another trait

  • instead of following one trait in his crosses, this time Mendel followed Two triats (i.e. pea pod color and pea pod shape)

• This law only applies to: genes that are located on different chromosomes (not homologous) OR genes that are very far apart on the same chromosome

Dihybrid Crosses

• the law of independent assortment was determined by doing crosses between plants that were true bredding for two traits, which produces F1 hybrids known as dihybrids

• example: YYRR x yyrr

  • all F1 dihydrids would be YyRr

• Dihybrid cross: a cross betwen F1 dihybrids

  • YyRr x YyRr

How to solve Genetics Problems

1. Write down the symbols for the alleles (sometimes they are given to you)

2. Write down the genotypes given

   a. if phenotypes are given, then write down the possible genotypes

3. determine what the problem is asking, and write out the cross as: [genotype] x [genotype]

4. set up the Punnett square

Law of Probability

• the law of segregation and independent assortment reflect rules of probability.

• The multiplication rule: the probability that two or more independent events will occur together in some specific combination

• example: if you flip a coin twice, what is the probability that it will land heads up both times?

  • ½ x ½ = ¼

• example: what is the probability of having 3 girls in a row?

  • ½ x ½ x ½ = 1/8

• The addition rule: the probability that two or more mutually exclusive events will occur

• example: what is the chance of rolling a dice and it lands on a 1 or 6?

  • 1/6 + 1/6 = 1/3

Pedigrees

• many human traits follow Mendelian patterns of genetics

• Pedigrees: family trees that give a visual of inheritance patterns of particular traits.

Reading Pedigrees

• if a trait is dominant, one parent must have the trait

  0 dominant traits do not skip a generation

• if a trait is X-linked, then males are more commonly affected than males.