Sexual Reproduction & Meiosis SBI3U Mr. Kleiman

What is Sexual Reproduction? Sexual Reproduction –Occurs when two different parents reproduce to form unique offspring (different from their parents and siblings). Mr. Kleiman

What determines the TRAITS of the offspring? DNA codes for all traits The code for each trait is called a GENE Offspring receive a randomized combination of genes from their parents Some genes will be expressed, while others will not All genes that are passed on from one generation to another are called HEREDITARY

What determines the TRAITS of the offspring? Even traits that are not expressed in either parent, but are coded for in their DNA, are hereditary blond??

Which traits do you get? Before we examine how and why some traits are expressed while others are not, we must first understand how traits are passed on from one generation to another Let’s review sexual reproduction and the process of meiosis

Gametes Woo hoo! egg sperm

Zygote

Sexual Reproduction Gametes join to form a Zygote Gametes Zygote egg sperm develops into body cells DNA Combined DNA

What does DNA do? The sequence of bases is called the GENETIC CODE! –this code tells your cells what to do and contributes to all of your physical traits Eg. hair/eye colour, shoe size, determines if you are male or female DNA is a nucleic acid containing 4 different nitrogenous bases (A, T, G, C)

What does DNA do? Scientists also believe that your DNA has an impact on how you think and how you behave.

The code S G PS A PS T PS C PS G PS G PS A PS C PS C P S G PS G PS T PS C PS C PS G PS A PS T PS C P Template Strand S G PS A PS T PS C P Base Sugar Phosphate P S base Nucleotide Genetic Code

Naming the DNA Chromatin DNA with associated proteins as it is found in the nucleus. Chromosome

Naming the DNA Chromosome One particular bundle of DNA (every species has it’s own number of chromosomes)

Naming the DNA Chromosome One particular bundle of DNA (every species has it’s own number of chromosomes) Can be coiled up during cell division. (easier to move around without getting tangled up)

Naming the DNA copycopied during cell division Chromosome

Naming the DNA Sister Chromatids Identical chromosomes bound together by a centromere copy

Sorting out the DNA Chromosomes are bundles of DNA Every species has a different number of chromosomes How many chromosomes do humans have? 46 Chromosomes

Sorting out the DNA If you got 46 chromosomes from your mother and you got 46 chromosomes from your father you would have… 92 Chromosomes!!

Sorting out the DNA If you then reproduced, your offspring would get 92 chromosomes from you and 92 chromosomes from your partner… that’s… 184 Chromosomes!! See the problem?

Sorting out the DNA Question: If we combine the chromosomes from each parent during sexual reproduction, why doesn’t the chromosome number double? Answer: Gametes must have half the number of chromosomes as body cells egg sperm example (not human)

Sorting out the DNA Try these –How many chromosomes in each gamete? 46 Human 78 Chicken 24 Rice Gametes Zygote This means that EACH cell in your body has one copy of DNA from your mother and one copy of DNA from your father… BUT ONLY HALF OF THEIR GENES!!!!

How? How do gametes get half the number of chromosomes? Meiosis By the process of

Homologous Chromosomes Gamete from mother Gamete from father

Homologous Chromosomes Zygote Homologous chromosomes Homologous chromosomes are NOT identical, but code for the same traits

Meiosis I Prophase I

Meiosis I Metaphase I Crossing Over Completely unique DNA created Line up homologous chromosomes

Meiosis I Metaphase I

Meiosis I Anaphase I

Meiosis I Telophase I

Meiosis II Prophase II DNA is not copied

Meiosis II Metaphase II

Meiosis II Anaphase II

Meiosis II Telophase II

Meiosis… the end result 4 unique cells created from one body cell Each cell has half the normal number of chromosomes (gametes)

Key Features of Meiosis Meiosis I –Copy chromosomes –Line up and separate homologous chromosomes –Crossing over between homologous chromosomes Meiosis II –Separate sister chromatids (normal cell division)

Why are you random? There are BUILT IN MECHANISMS in meiosis to create variation: Crossing over events which generate an infinite number of new random chromosomes Random assortment of your parents homologous chromosomes in their gametes during meiosis I and sister chromatids during meiosis II

Why are you random? Your specific combination of genes is a result of… One of many sexual encounters on a particular day at a particular time One of millions of sperm which meet with one of thousands of eggs

Why are you random? DNA doesn’t define you… Your environment, choices and random chance all play a large role in your –Gene expression –Physical Development –Experiences –Health –Abilities

Identical?

Errors in Meiosis

Nondisjunction – Down Syndrome

Down Syndrome

Nondisjunction – Turner Syndrome