GENETICS: Meiosis and Reproduction

Meiosis

Cell division that produces reproductive cells in sexually reproducing organisms

Asexual reproduction

Process by which a single parent reproduces by itself

Sexual reproduction

A reproductive process that involves two parents that combine their genetic material to produce a new organism, which differs from both parents

Gene definition

individual unit of geneticinformation, a section of DNA thatcodes for something

Old gene definition

One Gene -> One Trait (eye color, height)

Current gene definition

One Gene -> One Protein (p53, Ras)

Allele definition

different versions of the same gene

How much genetic material of the parent is present in a gamete?

half

How many copies of each human chromosome do adults have?

Two, one from each of their parents (ignoring recombination)

Homologous chromosomes

Chromosomes that have the same sequence of genes, that have the same structure, and that pair during meiosis.

Are homologous chromosomes identical?

They contain the same genes, but are not identical and may not have the same alleles

Chromosome and cookbook analogy

- Each has a list of recipes in a specific order that tell you how to make something
- Chromosomes: DNA genes tell you how to make proteins
- Cookbooks: Recipes tell you how to make food
- Homologous Chromosomes are like two copies of the same cookbook inherited from each parent
- People often edit recipes, making slightly different versions of the same dish
- This is like the different alleles for a given gene that produce slightly different proteins

Ploidy

number of sets of chromosomes in a cell (number of chromosomes of each type a cell has)

Haploid cells

A cell containing only one copy each chromosome (n).

Diploid cells

A cell containing two copies of each chromosome (2n), one set inherited from each parent.

Gametes are diploid or haploid?

haploid

n =

the number of unique chromosomes a cell/organism has

n = ? in humans

23

What does meiosis create?

haploid gametes from diploid cells

Mitosis vs. Meiosis

Meiosis has two rounds of genetic separation and cellular division while mitosis only has one of each. In meiosis homologous chromosomes separate leading to daughter cells that are not genetically identical.

In mitosis the daughter cells are identical to the parent as well as to each other.

Meiosis 1 what separates

separates homologous chromosomes

Meiosis 2 what separates

sister chromatids separate

germline cells

cells in gonads (diploid) that form gametes (haploid)

Meiosis: Metaphase 1

homologous pairs (not sister chromatids) align on equator; each pair attaches to separate spindle fiber by its kinetochore

Meiosis: Anaphase 1

homologous pairs separate (not sister chromatids) and are pulled to opposite poles of cell; this is called disjunction when they are pulled apart; daughter cells get a mixture of mom and dads chromosomes

Meiosis: Metaphase 2

Chromosomes line up at the equator.

Meiosis: Anaphase 2

sister chromatids separate

Meiosis creates ___ daughter cells, each with _____ as many chromosomes as the parent cell

4; half

Independent assortment

Each chromosomepair splits during meiosis 1 randomly andindependently of other pairs

Crossing over

- Because they have similarDNA sequences, homologous chromosomes exchange part of their DNA during Meiosis 1
- This process also serves as a checkpoint to make sure each chromosome is paired up before the chromosomes divide

Crossing over is a form of homologous recombination

process that results in genetic exchange between homologous DNA from two different sources

random fertilization

source of genetic variation caused by the unlimited number of possible sperm & egg combinations

Genetic selection in sperm

may occur insperm due to unequal transcriptsharing during spermatogenesis

Are X and Y homologous?

No, semi-homologous

SRY gene

Y linked gene that triggers the male sexual development pathway in animals.

What if there is no SRY gene?

Female

Dosage compensation

Mechanism in which X chromosome inactivation equalizes gene expression between males and females.

X chromosome inactivation

Early in fetal development, each cell in a female randomly inactivates one of the X-chromosomes by forming a dense ball of heterochromatin called a Barr body

Calico cats as evidence as X-inactivation

Cells divide from the early embryo and x chromosomes are randomly inactivated. There are two populations in the adult cat, each with one of the two X chromosomes inactivated. One codes for black fur and one codes for orange fur, leading to the calico appearance.

What triggers X inactivation?

Xist long non-coding RNA(lncRNA) Triggers X-inactivation

Whichever is triggered first is the one that is inactivated

Non-disjunction

Error in meiosis in which homologous chromosomes fail to separate.

Aneuploidy

Abnormal number of chromosomes.

Nondisjunction in meiosis 1

All gametes affected,extra chromosome ishomologouschromosome (can carrydifferent alleles)

Nondisjunction in meiosis 2

2/4 gametes affected,extra chromosome issister chromatid(identical)

Errors in DNA replication and crossing over cause _________________________.

Chromosomal abnormalities

Aneuploidy and chromosome changes cause __________________________________________ for genes.

Copy number variation

Copy number variation

the phenomenon of duplication or deletion of segments of DNA within a genome, so that different individuals have different numbers of copies of a particular DNA segment

How to identify copy number variation?

FISH and sequencing

Extra or fewer sequence reads for an area indicates a mutation/nondisjunction that induced copy number variation

Germline vs somatic mutations and future generations

Germline mutations can be passed to future generations - half of gametes carry the mutation

Somatic mutations - none of gametes carry mutation