Chromosomes

Chromosomes

Middle School

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Learning Objectives

• Define a chromosome and learn about its basic parts and structure.

• Explain how DNA, genes, and chromosomes are all related to each other.

• Differentiate between diploid and haploid cells and provide examples of each type.

• Describe how a karyotype can be used to determine an organism's biological sex.

Key Vocabulary

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Chromosome

A coiled, threadlike structure of DNA that carries genetic information within the cell's nucleus.

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Gene

A segment of DNA on a chromosome that codes for a specific protein, influencing a trait.

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Diploid

A cell containing a complete set of paired chromosomes, with one from each parent.

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Haploid

A cell, like a sex cell, that contains a single set of unpaired chromosomes.

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Karyotype

An organized visual profile of a person's chromosomes, used to check for chromosomal abnormalities.

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What Are Chromosomes?

• A chromosome is a thread-like structure inside the cell nucleus.

• It is made of DNA tightly coiled around proteins.

• Chromosomes carry genetic information in the form of genes.

• Humans have 23 pairs, inheriting one set from each parent.

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Multiple Choice

How many chromosomes are inherited from a human father?

• 46

• 23

• 48

• 22

The Structure of a Chromosome

• A chromosome has two identical copies called sister chromatids.

• A central point called the centromere joins the sister chromatids.

• The centromere divides the chromosome into a short and long arm.

Multiple Choice

What are the two identical copies of a replicated chromosome called?

• Sister Genes

• Sister Chromatids

• Centromeres

• Long Arms

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Genes on Chromosomes

• A gene is a specific segment of DNA with protein-making instructions.

• These proteins determine your physical traits, making genes the units of heredity.

• Each gene has a specific position on a chromosome called a locus.

• Homologous chromosomes are pairs carrying the same genes, one from each parent.

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Multiple Choice

What is the term for a specific segment of DNA that codes for a trait?

• Locus

• Centromere

• Gene

• Homologous

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Diploid vs. Haploid Cells

• Diploid cells (2n) have a full set of chromosomes in pairs.

• Most of your body’s somatic cells, like skin cells, are diploid.

• Haploid cells (n) have half the number of chromosomes, with no pairs.

• Gametes, or sex cells like sperm and eggs, are haploid.

Multiple Choice

Which of the following is an example of a haploid cell?

• Skin cell

• Muscle cell

• Nerve cell

• Egg cell

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Karyotypes and Sex Determination

Female Karyotype (XX)

• The sex chromosomes are the final pair shown in a karyotype.

• A biological female has two X chromosomes that are similar in size.

• This final pair is written as XX in a female karyotype.

Male Karyotype (XY)

• Sex chromosomes determine if an individual is biologically male or female.

• A biological male has one X chromosome and one smaller Y chromosome.

• This final pair is written as XY in a male karyotype.

Multiple Choice

A scientist observes a human karyotype and sees two X chromosomes in the 23rd pair. What can be concluded?

• The individual is male.

• The individual has a genetic defect.

• The individual is female.

• The karyotype is incomplete.

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Common Misconceptions about Chromosomes

Misconception	Correction
Chromosomes are always condensed and visible.	They are only visible during cell division and are usually less condensed.
More chromosomes mean a more complex organism.	Chromosome number does not determine an organism's complexity.
Genes and chromosomes are the same thing.	Genes are segments of DNA located on chromosomes.
All body cells have the same number of chromosomes.	Body cells are diploid, but sex cells (gametes) are haploid.

Multiple Choice

Why is it necessary for gametes (sex cells) to be haploid?

• So that when they fuse during fertilization, the resulting offspring has the correct diploid number of chromosomes.

• So that they are smaller and can move more easily.

• Because they do not have a nucleus.

• To prevent any genetic mutations from occurring during fertilization.

Multiple Choice

Which of the following best describes the relationship between DNA, genes, and chromosomes?

• Chromosomes are made of genes, which are made of DNA.

• Genes are made of DNA, and chromosomes are structures that package the genes.

• DNA is made of chromosomes, which are made of genes.

• Genes and chromosomes are both made of DNA and are separate structures.

Multiple Choice

A biologist is studying a newly discovered animal and wants to create a karyotype. What is the most significant reason for them to do this?

• To observe the animal's physical traits.

• To count the number of genes the animal has.

• To determine the animal's species, sex, and screen for chromosomal abnormalities.

• To see the shape of the animal's cell nucleus.

Multiple Choice

If an organism's diploid number is 60, how many chromosomes would be found in its somatic cells and gamete cells, respectively?

• 60 and 60

• 30 and 60

• 60 and 30

• 30 and 30

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Summary

• Chromosomes are condensed DNA structures that carry genes determining our traits.

• Human diploid body cells have 46 chromosomes, while haploid sex cells have 23.

• A duplicated chromosome consists of two sister chromatids joined by a centromere.

• A karyotype is a visual map of chromosomes used to determine sex.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4