UNIT 7: DNA Structure & Replication BIOLOGY I DAYSHEET 81-Introduction to DNA Replication

Name: Date: _ _Period:

Bellringer: Use your reading strategies to learn about the causes, symptoms, and treatment of the chromosomal disorder known as Down’s Syndrome.

Downs Syndrome: A Chromosomal Trisomy Event What is Down’s Syndrome?

Down’s syndrome, also known as Trisomy 21, occurs when an extra chromosome appears on the 21st pair.

This genetic disorder is associated with mild to moderate learning disabilities, developmental delays, characteristic facial features, and low muscle tone in early infancy. Many individuals with an extra 21st chromosome also have heart defects, leukemia, gastro-intestinal problems. There are a number of other health issues that can affect individuals with this genetic disorder. The symptoms for Down’s Syndrome can range from mild to severe.

The life expectancy for individuals with Trisomy 21 has dramatically increased over the past few decades as medical care and social inclusion have improved. A person with Down’s Syndrome in good health, will on average live to age 55 or beyond.

How common is Trisomy 21?

This genetic disorder occurs in approximately 1 of 800 live births. It is the leading cause of cognitive impairment (intellectual disability).

What are the genetic changes related to Trisomy 21?

Normally, humans contain just 23 pairs of different chromosomes. Each chromosome carries genes, which are needed for proper development and maintenance of our bodies. Proper chromosome numbers are maintained when an individual inherits 23 chromosomes from the mother (through the egg cell) and 23 chromosomes from the father (through the sperm cell).

Sometimes a person inherits an extra chromosome from one of the parents. In Trisomy 21, an individual receives 24 chromosomes from the mother, and 23 chromosomes from the father.

Can Down’s Syndrome be inherited?

Most cases of Down’s Syndrome are not inherited. The trisomy event occurs most often as a random event during the formation of reproductive cells (eggs). Affected people typically have no history of the disorder in their family. In all cases of the syndrome, the mother’s egg is always responsible for passing on the extra chromosome. The mothers age at conception can lead to an increase in the likelihood of having a child with this disorder: At 30 years 1 in 1,000 births will have this disorder. At 35 in 400 births will have this disorder. At 40 1 in 100, and at 45 1 in 30 births will have Down’s Syndrome.

How is Down’s Syndrome treated?

Although there is no real treatment for Trisomy 21, children with the disorder can go through therapy to improve their language skills, motor skills, and to help them develop as normally as possible.

How do doctors diagnose Trisomy 21?

If Down’s Syndrome is suspected after pre-labor tests, a diagnosis can be made via a chromosome analysis. This analysis is called a karyotype. Karyotypes are pictures of all the chromosomes in an individual’s cell being lines up by pairs and counted. This same technique can be used to determine if the individual is male or female. Can you determine the sex of the two individuals from the karyotypes pictured above?

Food for thought: What do you think it might be like to be the parent of a child with Down’s Syndrome? What would you be worried about? What do you think school might be like for your child? What do you think you would be able to share with your child? What might be different? Objective: I can explain how DNA replicates, or makes copies of itself. Homework: HW 81 is due next class! Your Unit 7 Assessment is next Tues/Wed!!!!!!

Activity 1: DNA Replication Notes Every cell in your body must have an EXACT copy of all 3 BILLION of your DNA Base Pairs…How does it make all of those copies?!

DNA Replication: The _________________ of making a ___________ of your DNA .

Steps to Replicate DNA: Step Description Picture

#1

Start with a parent molecule

of DNA

A ________________ molecule of DNA

is the original to be

_________________.

#2

Unzipping the DNA strands

An enzyme called ______________

comes in to _____________ and

separate the _____ strands of the

parent DNA. It is called “helicase”

because it unzips the double

___________.

#3

Adding the complementary

base pairs

Each strand can be used as a

________________ to make a new

strand. If the single strand has an

adenine, it would pair with a

________________, and if it has a

guanine, it would pair with a

_________________. The enzyme

that joins the new bases is

called________________________.

Remember poly=___________.

#4

Proofreading (DNA Spell

Check)

DNA _________________ checks

and fixes any ____________ it

finds. We now have 2

_________________ strands. Each

_______________ to the parent

strand but with one _______strand

and one ______ each.

Homework 50: Replication Reading Biology I

Name: __________________________ Period: ______