Sexual Reproduction & Meiosis

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

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 sperm egg Woo hoo!

Zygote

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

What does DNA do? DNA is a nucleic acid containing 4 different nitrogenous bases (A, T, G, C) 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

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

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

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

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

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

Naming the DNA Chromosome copy copied during cell division

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

46 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 See the problem? Chromosomes!!

Sorting out the DNA 20 Question: Answer: If we combine the chromosomes from each parent during sexual reproduction, why doesn’t the chromosome number double? example (not human) Answer: Gametes must have half the number of chromosomes as body cells egg sperm 10 10 20

Sorting out the DNA Try these 46 78 24 How many chromosomes in each gamete? 46 Human 78 Chicken 24 Rice Gametes Zygote 23 39 12 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!!!!

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

Homologous Chromosomes Gamete from mother Gamete from father

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

Meiosis I Prophase I

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

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

Down Syndrome

Nondisjunction – Turner Syndrome

Nondisjunction – Turner Syndrome