Reproduction

In this lesson, students focus on how genetic information is passed down from parents to offspring when cells divide. They model the two forms of cell division (mitosis and meiosis), and use their models to support an argument for how asexual reproduction phenomena is different from sexual reproduction phenomena.

• Almost all of the cells in a multicellular organism have the same DNA because of DNA replication and cell division.
• As organisms grow, their cells aren’t growing bigger and bigger. Instead, they grow because their cells are duplicating. They do this through cell division. Cell division refers to the splitting of a single cell into daughter cells, each with DNA from its parent cell.
• Right before a cell gets ready to divide, its DNA condenses into threadlike structures called chromosomes. Each chromosome is made up of a single DNA molecule, and it holds hundreds or thousands of genes on it.
• The cell has to undergo some structural changes in order for mitosis to take place.

• Why does DNA need to replicate before the cell divides?
• How is DNA replication different from transcription, which happens during protein synthesis?
• What is the relationship between the replication of DNA and heredity?
• Why do humans have chromosome pairs?
• What happens to the cell’s chromosomes after mitosis?
• Why does the nucleus have to change during mitosis?
• How does the cell ensure that the daughter cells will receive one copy of each chromosome?
• What causes the sister chromatids to separate?
• What has to happen once each side of the cell has the same number and kind of chromosomes as the parent cell?
• When does mitosis happen in the human body?

Misconception: In sexually reproducing organisms, one parent contributes genes for some characteristics while the other parent contributes genes for other characteristics.

Fact: Both parents contribute genes for every characteristic in sexually reproducing organisms.

Asexual reproduction : reproduction that requires only one parent (e.g., binary fission, budding, and fragmentation)

Cell division : the splitting of a single cell into daughter cells, each with DNA from its parent cell

Chromosome : a threadlike structure of DNA and protein; found in the nucleus of eukaryotic cells; a discrete package of genetic material

Daughter cell : a cell formed by the division of a parent cell

Heredity : the passing on of traits from parents to offspring

Meiosis : a form of cell division that results in four daughter cells, each with half the chromosome number of the parent cell

Mitosis : a form of cell division that results in two daughter cells with the same number and kind of chromosomes as the parent cell

Replicate : to make a copy of

Reproduction : the ability of a mature organism to have offspring

Sexual reproduction : the creation of a new individual from combined genetic information of two parents of different sexes

The Journey of a Sea Turtle

Every year, the leatherback sea turtle travels up to 16,093 kilometers (10,000 miles) and crosses the entire Pacific Ocean in search of jellyfish, which is its primary diet. It then returns to tropical waters to nest and breed. The leatherback is the largest of all sea turtles. An adult can weigh between 227 and 907 kilograms (500 and 2000 pounds).

The male sea turtle never leaves the ocean, while the female comes on land to lay her eggs in nests on sandy beaches. Once she lays her eggs, the female sea turtle leaves them and returns to the ocean. The temperature of the nest determines whether the eggs will become male or female turtles. If temperatures are between 28-29 degrees Celsius (83- 85 degrees Fahrenheit), the eggs will develop into a mix of male and female turtles. Temperatures warmer than this produce females, while temperatures lower than this produce males. Once the baby turtles hatch, they are on their own to make their way to the ocean. If they survive the journey, they will grow, becoming larger as they turn into adults.

Cell Division and Growth

As baby sea turtles grow and turn into adults, their cells aren’t growing bigger and bigger. Instead, they grow because their cells are duplicating. They do this through cell division. Cell division refers to the splitting of a single cell into daughter cells, each with DNA from its parent cell.

This is true for all multicellular organisms. For example, you started out as a single cell. That one cell divided, and then those daughter cells all divided, and then the daughter cells of those daughter cells divided. This continues until you reach adulthood, at which point your body will be made up of trillions of cells.

Before a cell can divide, it grows until it is double in size. This growth is important because the cell also has to replicate its DNA. To replicate means to make a copy of. Remember how in protein synthesis, part of the DNA double helix unzips and separates so that a template can be used for making an mRNA molecule?

The entire DNA molecule can also unzip. This is because each strand of the double helix runs in opposite directions. At a certain point, the twisted, tightly packed double helix unwinds and separates its two strands, unzipping down the middle. Each strand serves as a template for a new strand of DNA molecules.

The two new strands are both exact copies of the original DNA molecule because A nucleotides are added wherever there are Ts, and Cs where there are Gs.

Chromosomes

DNA replicates itself right before a cell gets ready to divide. Because of DNA replication, a copy of an organism’s DNA gets passed along to the daughter cells. This is how all of your cells contain the same DNA.

Right before a cell gets ready to divide, its DNA condenses into threadlike structures called chromosomes. Each chromosome is made up of a single DNA molecule, and it holds hundreds or thousands of genes on it. The genes are located on the chromosomes in a very specific way. Because of this, if scientists know where one gene is located, they can find it on anyone’s chromosomes.

Different kinds of organisms have different numbers and shapes of chromosomes. For example, prokaryotic cells don’t have a nucleus, so their DNA is spread throughout the entire cell. Most bacteria have one or two circular chromosomes. Plants and animals have linear chromosomes that are arranged in pairs. Fruit flies have four pairs of chromosomes, while lobsters have 50 pairs of chromosomes.

In humans, there are 23 pairs of chromosomes found in the nucleus of each of your cells (except for red blood cells, which don’t have a nucleus). Chromosomes are in pairs because one chromosome is inherited from each parent. The first 22 chromosome pairs are the autosomes, which are chromosomes that everyone has. They are numbered according to size. The gender-specific sex chromosomes are the final chromosome pair. Biological females have an XX combination, and biological males have an XY combination.

Both chromosomes in the chromosome pair contain genes that code for the same proteins. These are called alleles. An allele is a form of the same gene that has small differences in the sequence of DNA bases. For example, one allele might have the instructions for proteins that would result in blue eyes, while another allele might have the instructions for proteins that would result in brown eyes. Each parent contributes one allele.

After DNA replicates, there are two of each chromosome. The identical chromosomes are called sister chromatids, and they are linked together by the centromere. Once a cell has replicated its DNA, it gets ready to divide so it can pass along its genetic information to its offspring.

There are several reasons why cells divide. One important reason is growth. The more cells an organism has, the larger that organism is. Cell division also allows cells to repair damaged cells or replace dead cells. For example, human skin cells constantly divide so they can replace damaged or dead skin cells. Muscle cells also divide frequently to replace cells damaged by exercise or injury. Finally, cell division allows cells to reproduce. Reproduction is the ability of a mature organism to have offspring.

For the main activity of this lesson, students develop and use models to show how asexual reproduction results in offspring with identical genetic information, and how parent cells in sexually reproducing organisms use the process of meiosis to produce four daughter cells that are genetically different.